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GLADE User's Guide

1. 59 A 1 CORBA Architecture 59 A 2 Interface Definition Language 59 A 3 Network Communication HBubevstem 62 AGI SDSACP EE 62 A 3 CORBA ORP sese 63 A 4 Distributed Application Development 63 AAT DSA Application Development 63 A A7 CORBA Application Development 63 A 5 Some Elements of Compaison eue 64 Appendix B GNU Free Documentation License Pacts hate arene eae tiene aes a awe M 67
2. Get Balance all C P end BankClient Chapter 2 The Distributed Systems Annex 13 2 4 4 Remote Access to Class Wide Types RACW A bank client is now connected to a bank through a terminal The bank wants to notify a connected client bu means of a message on its terminal when another client transfers a given amount of money to his account In the following example a terminal is designed as a distributed object Each bank client will register its terminal object to the bank server for further use In the code below Term_Type is the root type of the distributed terminal hierarchy with Types use Types package Terminal is pragma Pure type Term_Type is abstract tagged limited private procedure Notify MyTerm access Term Type Payer in Customer Type Amount in Integer is abstract private type Term Type is abstract tagged limited null record end Terminal In the code below the RCI unit RACWBank defines Term Access a remote access to class wide type Term Access becomes a reference to a distributed object In the next section we will see how to derive and extend Term Type how to create a distributed object and how to use a reference to it with Terminal Types use Terminal Types package RACWBank is pragma Remote Call Interface type Term Access is access all Term Type Class procedure Register MyTerm in Term Access Customer in Customer Type Password in Password Type
3. By contrast a remote subprogram labeled with pragma Asynchronous allows statically and dynamically bound remote calls to it to be executed asynchronously A call to an asynchronous procedure doesn t wait for the completion of the remote call and lets the caller continue its execution The remote procedure must have only in parameters and any exception raised during the execution of the remote procedure is lost When pragma Asynchronous applies to a regular subprogram with in parameters any call to this subprogram will be executed asynchronously The following declaration of AsynchronousRCI Asynchronous gives an example package AsynchronousRCI is pragma Remote Call Interface procedure Asynchronous X Integer pragma Asynchronous Asynchronous procedure Synchronous X Integer type AsynchronousRAS is access procedure X Integer Chapter 2 The Distributed Systems Annex 21 pragma Asynchronous AsynchronousRAS end AsynchronousRCI package AsynchronousRT is pragma Remote Types type Object is tagged limited private type AsynchronousRACW is access all Object Class pragma Asynchronous AsynchronousRACW procedure Asynchronous X Object procedure Synchronous X in out Object function Create return AsynchronousRACw private type Object is tagged limited null record end AsynchronousRT A pragma Asynchronous applies to a Remote Access_to_Subprogram RAS An asyn chronous RAS can be both async
4. DSA server to an ORB This would also allow 66 GLADE User s Guide applications written in other languages to invoke DSA features We are also seeking to use this approach to offer a DII mechanism for DSA Appendix B GNU Free Documentation License 67 Appendix B GNU Free Documentation License Version 1 1 March 2000 Copyright 2000 Free Software Foundation Inc 59 Temple Place Suite 330 Boston MA 02111 1307 USA Everyone is permitted to copy and distribute verbatim copies of this license document but changing it is not allowed 0 PREAMBLE The purpose of this License is to make a manual textbook or other written docu ment free in the sense of freedom to assure everyone the effective freedom to copy and redistribute it with or without modifying it either commercially or noncommercially Sec ondarily this License preserves for the author and publisher a way to get credit for their work while not being considered responsible for modifications made by others This License is a kind of copyleft which means that derivative works of the document must themselves be free in the same sense It complements the GNU General Public License which is a copyleft license designed for free software We have designed this License in order to use it for manuals for free software because free software needs free documentation a free program should come with manuals providing the same freedoms that the software does But th
5. Declaration page undefined and undefined Partition Command Line Options page undefined Most commonly this would be dfs lt directory gt 3 6 4 Partition Option nolaunch This feature is useful when the configuration has been built with an Ada starter When this feature is activated the main partition does not launch the other partitions anymore The user has to launch them by hand Environment Variable Command Line Option Type NOLAUNCH nolaunch None 3 6 5 Partition Option detach This option is not intended to be specified by the user When this feature is activated the process forks itself and the child closes its standard input output and error descriptors This feature is always activated when a partition is launched from the main partition using a remote shell with a starter Ada or Shell Chapter 3 Getting Started With GLADE 53 The only case where it can be useful is for a configuration built with a Shell starter In this case the user can edit the shell script to pipe the output of a partition into a file To do so the detach feature has to be removed Environment Variable Command Line Option Type DETACH detach None 3 6 6 Partition Option slave This feature is not supposed to be used by the user When this feature is activated this partition cannot be a boot server partition anymore This is useful when a partition has been configured as a boot partition and when the user does not want it to be a mai
6. GLADE will evolve so will this configuration language Most of the attributes and pragmas can be overloaded at run time by command line arguments or environment variables 3 5 1 Language Keywords All the Ada keywords are reserved keywords of the GLADE configuration language gnatdist generates full Ada code in order to build the different executables To avoid naming conflicts between Ada and GLADE configuration language all the Ada keywords have been reserved even if they are not used in the configuration language Chapter 3 Getting Started With GLADE 35 There are three new keywords e configuration to encapsulate a configuration e Partition that is a predefined type to declare partitions e Channel that is a predefined type to declare channels between partitions 3 5 2 Pragmas and Representation Clauses It is possible to modify the default behavior of the configuration via a pragma definition PRAGMA pragma PRAGMA NAME PRAGMA ARGUMENTS It is also possible to modify the default behavior of all the partitions or channels via an attribute definition clause applied to the predefined type Partition or Channel REPRESENTATION CLAUSE for Partition ATTRIBUTE NAME use ATTRIBUTE ARGUMENTS for Channel ATTRIBUTE_NAME use ATTRIBUTE ARGUMENTS It is also possible to modify the default behavior of a given partition or channel via an attribute definition clause applied to the partition or channel itself REP
7. Main use Another Main for Partition 3 Reconnection use Block Until Restart for Partition 4 Command Line use v for Partition 4 Termination use Local Termination pragma Starter Method gt Ada pragma Boot_Server Protocol_Name gt tcp Protocol Data gt hostname unused port pragma Version False Channel 1 Channel Partition 1 Partition 4 Channel 2 Channel Partition 2 Partition 3 for Channel 1 Filter use ZIP for Channel 2 Filter use My Own Filter for Partition Filter use ZIP Chapter 3 Getting Started With GLADE 49 41 pragma Registration Filter Some Filter 42 43 begin 44 Partition 2 RCI B2 RCI B4 Normal 45 Partition 3 RCI B3 46 end MyConfig 1 Line 01 Typically after having created the following configuration file the user types gnatdist myconfig cfg If the user wants to build only some partitions then he will list the partitions to build on the gnatdist command line as follows gnatdist myconfig cfg partition 2 partition 3 The name of the file prefix must be the same as the name of the configuration unit in this example myconfig cfg The file suffix must be cfg For a given distributed application the user can have as many different configuration files as desired 2 Line 04 Partition 1 contains no RCI package However it will contain the main pro cedure of the distributed application called Master Procedure in this example If the
8. Or for Partition Filter use ZIP The latter sets the default filter for all partitions of the application the former only sets the default filter for the partition Partition_1 It is also possible to apply a default filter and to override this default for specific channels My Channel Channel Partition 1 Partition 2 for My Channel Filter use ZIP for Partition 1 Filter use Some Other Filter This makes Partition_1 use Some Other Filter for all remote calls except for any com munication with Partition 2 where the filter ZIP is applied gnatdist takes care of consistency checking of a filter definition For instance multiple filter definitions for the same channel are not allowed Filtering is only active if specified explicitly in the configuration file REPRESENTATION CLAUSE for CHANNEL IDENTIFIER Filter use STRING LITERAL for PARTITION IDENTIFIER Filter use STRING LITERAL 3 5 24 Pragma Registration Filter Some filtering algorithms require that some parameters be sent to the receiver first to enable it to correctly de filter the data If this is the case it may be necessary to filter these parameters as well For such purposes it is possible to install a global filter for all partitions which will then be used to filter the parameters of other filters This filter is called the registration filter It can be set by a pragma because a pragma applies to the configuration Chapter 3 Getting Started W
9. Other services end RACWBank 2 4 5 Summary on Pragma Remote Call Interface Remote call interface units e Allow subprograms to be called and executed remotely 14 GLADE User s Guide e Allow statically bound remote calls remote subprogram Allow dynamically bound remote calls remote access types Forbid variables and non remote access types e Prevent specification from depending on normal units 2 5 Pragma Remote_Types 2 5 1 Overview of Pragma Remote_Types Unlike RCI units library units categorized with this pragma can define distributed objects and remote methods on them Both RCI and RT units can define a remote access type as described above RACW A subprogram defined in a RT unit is not a remote subprogram Unlike RCI units a RT unit can be duplicated on several partitions in which case all its entities are different with each other This unit is different on each partition in which it is defined 2 5 2 Distributed Object If we want to implement the notification feature proposed in the previous section we have to derive Term_Type Such a derivation is possible in a remote types unit like NewTerminal see below Any object of type New_Term_Type becomes a distributed object and any reference to such an object becomes a fat pointer or a reference to a distributed object see Term_Access declaration in undefined Remote Access to Class Wide Types RACW page undefined with Types Terminal use Types Termin
10. all the shared passive units of a given partition to be mapped on the same storage support When the attribute data location applied to a partition is a list of locations all the storage support units needed for this partition are linked in the executable By default only the first one is activated The user can choose to change the activated support by another one specified in the location list This can be done using the partition option data location see undefined Partition Option data location page undefined Chapter 3 Getting Started With GLADE 41 As passive partitions cannot be activated it is not possible to provide a location list as a data_location attribute It is not possible to change dynamically its location either 3 5 13 Partition Attribute Allow_Light_PCS On some circumstances GLADE can detect that a partition does not need the full PCS functionalities This occurs in particular when the partition does use any task any RCI unit or any RACW object Therefore the partition does not receive any message that is not a reply to a previous request In this case the PCS does not drag in the tasking library and a light PCS is linked in the partition executable This specific configuration is automatically determined by GNATDIST with the ALI file information This optimization can be inappropriate especially when the user wants to use the Dis tributed Shared Memory storage support which runs Li and Hudak s algorithm In
11. call whose controlling argument is an access to class wide operand re mote access on class wide types RACW These remote access types can be declared in a RCI package as well 10 GLADE User s Guide A remote access type RAS or RACW can be viewed as a fat pointer that is to say a structure with a remote address and a local address like an URL protocol lt remote machine gt lt local directory gt The remote address must denote the host of the partition on which the entity has been created the local address describes the local memory address within the host It is very unlikely that RCI units can be duplicated in the distributed system An implementation may allow separate copies of a RCI unit as long as it ensures that the copies present a consistent state to all clients In the general case preserving consistency is very costly For this reason the implementation may require a RCI unit to be unique in the distributed system 2 4 2 Regular Remote Subprograms RCI In the following example a RCIBank offers several remote services Balance Transfer Deposit and Withdraw On the caller side the bank client uses the stub files of unit RCIBank On the receiver side the bank receiver uses the skeleton files of unit RCIBank including the body of this package package Types is pragma Pure type Customer_Type is new String type Password_Type is new String end Types with Types use Types package RCIBank is prag
12. eee cece cree 47 3 5 26 Partition Attribute Task_Pool 4T 3 5 27 A Complete Example 000 48 3 6 Partition Command Line Options 4 51 3 6 1 Partition Option boot_location 51 3 6 2 Partition Option self Jocaton 52 3 6 3 Partition Option data location 52 3 6 4 Partition Option nolaunch 52 3 6 5 Partition Option detach 0 05 52 3 6 6 Partition Option slave sisse eesssess 53 3 6 7 Partition Option boot_mirror 53 3 6 8 Partition Option mirror_expected 53 3 6 9 Partition Option connection_hits 53 3 6 10 Partition Option reconnection 53 3 6 11 Partition Option termination 54 3 6 12 Partition Option trace sseesseee esee 54 3 6 13 Partition Option replay 0 54 3 7 Debugging Facilities 0 0 0 0 ccc eee 55 3 8 GLADE File Hierarchy 0 00 c eee eee eee 55 3 9 GLADE Internal 55 3 9 1 The GARLIC POH 55 3 9 2 The PolyORB PC 56 3 9 8 Heterogeneous System 0 0 eee ee eee 56 3 9 4 Allocating Partition Idee 56 3 9 5 Executing Concurrent Remote Calls 56 3 9 6 Priority Inheritance 00 e eee ee 57 3 9 7 Remote Call Abhortion 57 3 9 8 User Filter Implementation 57 3 10 Remote Shell Notes 57 Appendix A DSA and CORBA
13. partition trace file includes the absolute path as well If a file name with a relative path is passed on the command line following the trace file argument then the home user s directory is concatenated to the trace file argument 3 6 13 Partition Option replay In order to replay a partition whose execution has been previously traced the command line argument replay is required In addition the special boot server location replay has to be specified i e by using the boot location replay command line argument To replay a traced execution of partition whose executable is named part we start it with the command part nolaunch slave replay boot location replay possibly under the control of a debugger such as gdb Since the exact contents of the messages received is recorded differences in input from external sources such as standard input during replay will most likely give unexpected results Also replay of applications whose behavior is inherently non deterministic for example if they use tasking will be problematic Chapter 3 Getting Started With GLADE 55 N B It is important that the same executable is used for replay as when the trace file was generated otherwise strange behavior can be expected 3 7 Debugging Facilities To trace his application the user sets the following two environment variables to true The variable S_RPC provides information on what is going on the execution
14. storage support name like dfs for Distributed File System and a storage support data like a directory dfs glade LOCATION Support Name gt STRING LITERAL Support Data gt STRING LITERAL LOCATION LIST LOCATION LOCATION Note that a location may have an undefined or incomplete support data In this case the support is free to compute a support data For instance tcp specifies that the protocol is used but that the protocol data machine port is to be determined by the protocol itself A location or a list of locations can be can be concatenated into a single string to be used as a command line option or an environment variable see undefined Partition Command Line Options page undefined If a partition wants to communicate with another partition once the location list of the latter is known the caller will use the first location of the callee whose protocol is locally available For instance if a callee exports three locations N1 D1 N2 D2 and N3 D3 a caller with protocols N2 and N3 locally available will try to communicate with the callee using the protocol of name N2 and of specific data D2 3 5 6 Partition Attribute Main Basically the distributed system annex DSA helps the user in building a distributed application from a non distributed application Of course this is not the only possible model offered by DSA The user can design implement and test his appli
15. that depends on normal units 2 6 Pragma Shared Passive 2 6 1 Overview of Pragma Shared Passive The entities declared in such a categorized library unit are intended to be mapped on a virtual shared address space file memory database When two partitions use such a 18 GLADE User s Guide library unit they can communicate by reading or writing the same variable in the shared unit This supports the conventional shared variables paradigm Entryless protected objects can be declared in these units to provide an atomic access to shared data thus implementing a simple transaction mechanism When the address space is a file or a database the user can take advantage of the persistency features provided by these storage nodes 2 6 2 Shared and Protected Objects In the code below we define two kinds of shared objects External Synchronization requires that the different partitions updating this data synchronize to avoid conflicting operations on shared objects Internal Synchronization provides a way to get an atomic operation on shared objects Note that only entry less subprograms are allowed in a shared passive unit package SharedObjects is pragma Shared Passive Max Positive 10 type Index Type is range 1 Max type Rate Type is new Float type Rates Type is array Index Type of Rate Type External Synchronization Rates Type protected Internal Synchronization is procedure Set Index in Index Type Rate
16. this case messages are exchanged without being replies to previously sent requests and the nor mal PCS should be linked instead of the light one Note also that GNATDIST cannot know for sure that the DSM storage support assigned at configuration time is used at run time The user can configure this optimization with the following attribute REPRESENTATION_CLAUSE for PARTITION IDENTIFIER Allow Light PCS use BOOLEAN_LITERAL 3 5 14 Pragma Priority It might be necessary for real time applications to get control over the priority at which a remote procedure call is executed By default the PCS sends the priority of the client to the server which sets the priority of an anonymous task to this value The pragma Priority allows to decide which priority policy should apply in the distributed application PRIORITY POLICY LITERAL Server Declared Client Propagated PRAGMA pragma Priority Policy gt PRIORITY POLICY LITERAL e The default policy Client Propagated consists in propagating the client priority to the server e The policy Server Declared consists in executing the remote procedure call at a priority specific to the partition This priority can be set using the partition attribute Priority 3 5 15 Partition Attribute Priority This attribute allows to set the priority at which level a remote procedure call is executed on a server when the priority policy is Server Declared By default the default priority of the
17. time and porting the application to several platforms to uncover subtle differences between various network interfaces Of course this communication code can be encapsulated in wrappers to reduce its com plexity but it is clear that most of it can be automatically generated Message passing diverts developer s attention from the application domain The query and reply scenario is a classical scheme in distributed applications using message passing for such a scheme can be compared to using a goto mechanism in a non distributed application This is consid ered unacceptable methodology in modern software engineering A more robust design is be to use a structured approach based on procedure calls In some respects network programming can be compared to the multi threading pro gramming issue The user can decide to split his code into several pieces and to multiplex the thread executions himself using a table driven model The scheduling code ends up embedded into the user code This solution is error prone and fragile in regard to any future modification Relying on an implementation of threads such as provided in POSIX is a better solution Relying on language primitives that support concurrency such as Ada tasks is best 1 2 Using a Middleware Environment A middleware environment is intended to provide high level abstractions in order to easily develop user applications Environments like CORBA or Distributed Computing Environme
18. AS 2 4 4 Remote Access to Class Wide Types RACW 2 4 5 Summary on Pragma Remote Call Interface 2 5 Pragma Remote Types 0 cee cee cee eee eee 2 5 1 Overview of Pragma Remote_Types 2 5 2 Distributed Objet 2 5 8 Transmitting Dynamic Structure 2 5 4 Summary on Remote Types Units 2 6 Pragma Ghared Dasaive 0c eee e eee eee eee 2 6 1 Overview of Pragma Shared_Passive 2 6 2 Shared and Protected Objects 2 6 8 Summary on Pragma Shared_Passive 2 7 More About Categorization Pragmas 2 7 1 Variables and Non Remote Access Types 2 7 2 RPC Failures eo pis ek eas ea ee eee 25 39 Exceptions mee RI RE 2 7 4 Pragma Asynchronous uusssesllslsessss 2 7 5 Pragma All_Calls_Remote 2 7 6 Generic Categorized Unts 2 7 7 Categorization Unit Dependencies 2 8 Partition Communication Subsystem 2 8 1 Marshaling and Unmarshaling Operations 2 8 2 Incorrect Remote Dispatching 28 3 Partition Ile pe cone tee She eke tes 2 8 4 Concurrent Remote Call 2 8 5 Consistency and Elaboration 2 8 6 Abortion and Termination 2 9 Most Features in One sample ii GLADE User s Guide 3 Getting Started With GLADE 33 3 1 Introduction to GLADE 0 0 ccc
19. GLADE User s Guide GLADE GNAT Library for Ada Distributed Environment GLADE Version glade gpl 2006 Laurent Pautet Samuel Tardieu Document revision level Revision 1 54 2 1 Date Date 2006 06 23 10 51 49 Copyright 1997 2006 Free Software Foundation Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License Version 1 1 or any later version published by the Free Software Foundation with the Invariant Sections being GNU Free Documentation License with the Front Cover Texts being GLADE User s Guide GNAT Library for Ada Distributed Environment and with no Back Cover Texts A copy of the license is included in the section entitled GNU Free Documentation License About This Guide 1 About This Guide What This Guide Contains This guide contains the following chapters undefined Introduction to Distributed Systems page undefined describes different ways to develop systems that must run on networks of computers undefined The Distributed Systems Annex page undefined discusses the features presented in Annex E of the Ada 95 language reference This chapter provides a tutorial for beginners and includes several useful examples for more advanced programmers undefined Getting Started With GLADE page undefined describes how to use the configuration tool gnatdist It also describes in detail the default GLADE Partition C
20. ID is not tight in any way to a specific protocol or to a specific location 3 9 5 Executing Concurrent Remote Calls When multiple remote subprogram calls occur on the same partition they are handled by several anonymous tasks The number of tasks in the anonymous tasks pool can be con figured by three figures see undefined Partition Attribute Task Pool page undefined Therefore the user may have to synchronize global data in the Remote_Call_Interface or Remote_Types unit to preserve concurrent access on data If the user want to suppress the Chapter 3 Getting Started With GLADE 57 multiple requests features he can force the configuration of the anonymous tasks pool to 0 1 0 1 1 That means that there will be at most one anonymous task running at a time 3 9 6 Priority Inheritance It is compiler dependent whether the caller priority is preserved during a remote pro cedure call In fact it can be unsafe to rely on priorities because two partitions may have different priority ranges and policies Nevertheless GLADE preserves the caller prior ity This priority is marshaled and unmarshaled during the remote procedure call and the priority of the anonymous task on the server is set to the caller priority This default policy can be modified by using pragma Priority undefined Pragma Pri ority page undefined and partition attribute Priority undefined Partition Attribute Priority page undefined 3 9 7 Remot
21. NITS DEFINING IDENTIFIER LIST DEFINING IDENTIFIER DEFINING IDENTIFIER STATEMENT IDENTIFIER ENUMERATION OF ADA UNITS SEQUENCE OF STATEMENTS STATEMENT STATEMENT Once declared a partition is an empty list of Ada units The operator adds the Ada units list on the right side to the current list of Ada units that are already mapped to the partition This is a non destructive operation Whether a unit is a relevant Ada unit or not is checked later on by the back end of gnatdist These assignments can occur in the declarative part as well as in the body part ENUMERATION_OF_ADA_UNITS ADA UNIT ADA_UNIT 3 5 5 Location Declaration There are several kinds of location in the GLADE configuration language We shall present them in the next subsections but here is a short overview of these locations e Boot Location defines the network locations to use to communicate with the the boot server during the boot phase Chapter 3 Getting Started With GLADE 37 e Self Location defines the network locations to use by others to communicate with the current partition e Data Location defines the data storage location used by the current partition to map its shared passive units A location is composed of a support name and a specific data for this support For instance a network location is composed of a protocol name like tcp and a protocol data like machine port A storage location is composed of a
22. Partition Option trace GLADE has a facility for trace replay based debugging If trace mode is turned on GLADE will record into a trace file all messages received by a partition The trace file can then be used to replay the execution of the partition in isolation To get a partition to generate a trace file it has to be given the command line argument trace This is most easily done by using a command line option see undefined Partition Attribute Command Line page undefined in the configuration file to add trace to the command lines of the partitions whose executions are to be replayed When the application has been built and started using the starter as usual the trace files will be generated It is also possible to build the distributed application with the None starter and then to include the trace argument on the command line As a default the file name of the trace file is the name of the partition s executable i e the string returned by the standard procedure Ada Command Line Command Name with a suffix ptf ptf stands for Partition Trace File It contains all the incoming requests delivered to the current partition The file name can be changed with the trace file lt othername gt command line argument Note that when a remote partition is launched with rsh under Unix GLADE invokes the executable s name of this partition with its absolute path included Therefore when trace is passed on the command line the
23. RESENTATION CLAUSE for PARTITION IDENTIFIER ATTRIBUTE NAME use ATTRIBUTE ARGUMENTS When an attribute definition clause is applied to a given object of a predefined type this overrides any attribute definition of the predefined type In the next sections attributes apply to a given object rather than to the predefined type 3 5 3 Configuration Declaration The distribution of one or several Ada programs is described by a single configuration unit This configuration unit has a specification part and an optional body part A con figuration unit is declared as an Ada procedure would be The keyword configuration is reserved for this purpose CONFIGURATION UNIT configuration IDENTIFIER is DECLARATIVE PART begin SEQUENCE OF STATEMENTS end IDENTIFIER 36 GLADE User s Guide 3 5 4 Partition Declaration In the declarative part the user declares his partitions and can change their default behavior gnatdist provides a predefined type Partition The user can declare a list of partitions and can also initialize these partitions with an initial list of Ada units DECLARATIVE PART DECLARATIVE_ITEM DECLARATIVE ITEM PARTITION DECLARATION CHANNEL DECLARATION REPRESENTATION CLAUSE SUBPROGRAM DECLARATION PRAGMA SUBPROGRAM DECLARATION MAIN PROCEDURE DECLARATION PROCEDURE DECLARATION FUNCTION DECLARATION PARTITION DECLARATION DEFINING IDENTIFTER LIST Partition ENUMERATION OF ADA U
24. a set of hosts 3 5 17 Pragma Import Two kinds of subprograms are allowed in the GLADE configuration language main procedure is used as a partition Main attribute and a function is used as a partition Host attribute SUBPROGRAM DECLARATION procedure MAIN PROCEDURE IDENTIFIER is in PARTITION NAME procedure PROCEDURE IDENTIFIER function FUNCTION IDENTIFIER PARAMETER IDENTIFIER in String return String The function can be an Ada function default or a shell script To import a shell script the pragma Import must be used PRAGMA pragma Import Chapter 3 Getting Started With GLADE 43 Entity gt FUNCTION_IDENTIFIER Convention gt CONVENTION_LITERAL External_Name gt STRING_LITERAL pragma Import Best Node Shell best node In this case the GLADE PCS invokes the shell script with the partition name as a command line argument The shell script is supposed to return the partition host name see undefined Partition Attribute Host page undefined 3 5 18 Partition Attribute Directory Directory allows the user to specify in which directory the partition executable is stored This can be useful in heterogeneous systems when the user wants to store executables for the same target in a given directory Specifying the directory is also useful if the partition executable is not directly visible from the user environment For instance when a remote command like rsh is invoked the
25. al package NewTerminal is pragma Remote Types type New Term Type is new Term Type with null record procedure Notify MyTerm access New Term Type Payer in Customer Type Amount in Integer function Current return Term Access end NewTerminal In the code below a client registers his name and his terminal with RACWBank There fore when any payer transfers some money to him RACWBank is able to notify the client of the transfer of funds with NewTerminal RACWBank Types use NewTerminal RACWBank Types procedure TermiClient is Chapter 2 The Distributed Systems Annex 15 MyTerm Term Access Current Customer Customer Type poor Password Password Type yyyy begin Register MyTerm Customer Password Execute other things end TermiClient In the code below a second client the payer registers his terminal to the bank and executes a transfer to the first client with NewTerminal RACWBank Types use NewTerminal RACWBank Types procedure Term2Client is MyTerm Term Access Current Payer Customer Type rich Password Password Type xxxx Payee Customer Type poor begin Register MyTerm Payer Password Transfer Payer Password 100 Payee end Term2Client In the code below we describe the general design of Transfer Classical operations of Withdraw and Deposit are performed Then RACWBank retrieves the terminal of the payee if pres
26. and Line Options page undefined Note pragma Boot Server is now obsolete It is recommended to use pragma Boot Location This wording is more consistent with the rest of the configuration lan guage see Self Location undefined Partition Option self location page undefined and Data Location undefined Partition Option data location page undefined 3 5 10 Partition Attribute Self Location Except for the boot partition on which the boot server is located a partition is reachable through a dynamically computed location for instance the partition looks for a free port when the protocol is tcp The user may want such a partition to be reachable from a given location especially if the user wants to make this partition a boot mirror To do so he can force the partition location with self location feature 40 GLADE User s Guide REPRESENTATION CLAUSE for PARTITION IDENTIFIER Self Location use LOCATION for PARTITION IDENTIFIER Self Location use LOCATION LIST If the attribute definition clause applies to the predefined type Partition the locations have to be incomplete Otherwise all the partitions would be reachable through the same locations which is definitively not recommended When an attribute self location definition clause applies to a given partition the pro tocol units needed for this partition are linked in the executable By default when the self location attribute is not redefined the default prot
27. anonymous task is the default task priority REPRESENTATION CLAUSE 42 GLADE User s Guide for PARTITION_IDENTIFIER Priority use INTEGER_LITERAL 3 5 16 Partition Attribute Host Logical nodes or partitions can be mapped onto physical nodes The host name can be either a static or dynamic value In case of a static value the expression is a string literal In case of a dynamic value the representation clause argument is a function that accepts a string as parameter and that returns a string value When the function is called the partition name is passed as parameter and the host name is returned FUNCTION DECLARATION function FUNCTION IDENTIFIER PARAMETER IDENTIFIER in String return String REPRESENTATION CLAUSE for PARTITION IDENTIFIER Host use STRING LITERAL for PARTITION IDENTIFIER Host use FUNCTION IDENTIFIER The signature of the function must be the following it takes a string parameter which corresponds to a partition name It returns a string parameter which corresponds to the host name The function that returns the host name can be an Ada function default or a shell script A pragma Import is used to import a function defined in Ada or in Shell see undefined Pragma Import page undefined This function is called on the main partition by the GLADE PCS to launch a given partition on a given logical node In case of load balancing the function can return the most appropriate among
28. at least one year after the last time you distribute an Opaque copy directly or through your agents or retailers of that edition to the public It is requested but not required that you contact the authors of the Document well before redistributing any large number of copies to give them a chance to provide you with an updated version of the Document 4 MODIFICATIONS You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above provided that you release the Modified Version under precisely this License with the Modified Version filling the role of the Document thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it In addition you must do these things in the Modified Version A Use in the Title Page and on the covers if any a title distinct from that of the Document and from those of previous versions which should if there were any be listed in the History section of the Document You may use the same title as a previous version if the original publisher of that version gives permission B List on the Title Page as authors one or more persons or entities responsible for authorship of the modifications in the Modified Version together with at least five of the principal authors of the Document all of its principal authors if it has less than five C State on the Title page the name of the publisher of the Modified Version as
29. cation in a non distributed environment and then should be able to switch from the non distributed case to a distributed case As mentioned before this two phase design approach has several advantages In a non distributed case the user executes only one main executable possibly with a name corresponding to the main unit name of his application With gnatdist in a distributed case a main executable with a name corresponding to the main unit name is responsible for starting the entire distributed application Therefore the user can start his application the same way he used to do in the non distributed case For this reason the configuration language provides a way to declare the main procedure of the non distributed application MAIN PROCEDURE DECLARATION procedure MAIN PROCEDURE IDENTIFIER is in PARTITION IDENTIFIER 38 GLADE User s Guide In this case the partition in which the main procedure has been mapped is called the main partition It includes in its code a call to this main procedure The main partition has an additional specific role because the boot server is located on it see undefined GLADE Internals page undefined The main procedures for the other partitions have a null body However the user can also modify this behavior by providing an alternate main procedure To do this an alternate main subprogram has to be declared and assigned to the partition Main attribute PROCEDURE_DECLARATION proced
30. cumstances a partition for instance Partition_2 can be duplicated To be duplicated Unit 2 and Unit 3 which are configured on Partition 2 have to provide only dynamically bound remote subprograms Otherwise a partition calling a remote subprogram on Unit 2 would not be able to statically determine where to perform the remote call between the two instances of Unit 2 xe arch fig pdf Chapter 2 The Distributed Systems Annex 9 2 2 Categorization Pragmas Library units can be categorized according to the role they play in a distributed program A categorization pragma is a library unit pragma that restricts the kinds of declarations that can appear in a library unit and possibly in its child units as well as the legal semantic dependences that the categorized unit can have There are several categorization pragmas e Remote Call Interface e Remote Types e Shared Passive e Pure The following paragraphs do not present the detailed semantics of these pragmas formal details will be found in the Ada95 Reference Manual Their purpose is to give the reader an intuitive overview of the purpose of these pragmas If a library unit is not categorized this unit is called a normal unit and plays no special role in the distributed application Such a unit is duplicated on any partition in which it is mentioned A parenthetical remark to avoid the need for specific run time libraries for the DSA the notion of remote rendezvous has not been intro
31. d by the GLADE PCS randomly computed during GLADE installation is already assigned Line 32 It is a bounded error to elaborate a partition of a distributed program that contains a compilation unit that depends on a different version of the declaration of an RCI library unit than the one included in the partition to which the RCI library unit was assigned When the pragma Version is set to False no consistency check is performed Line 335 Declare two channels Other channels between partitions remain unknown Line 37 Use transparent compression decompression for the arguments and results of any remote calls on channel Channel_1 i e between Partition 1 and Partition 4 Line 38 Use filter My Own Filter on any declared channel ie Channel_1 amd Channel 2 As Channel 1 filter attribute is already assigned this applies only to Channel_2 This filter must be implemented in a package System Garlic Filters My Own Filter Line 39 For all data exchanged between partitions use the filter ZIP Le for both arriving remote calls as well as for calls made by a partition Line 41 Some_Filter wil be used to exchange a filter s parameters between two partitions Some_Filter itself must be an algorithm that doesn t need its own pa rameters to be filtered again This filter must be implemented in a package Sys tem Garlic Filters Some_Filter Line 43 The configuration body is optional The user may have fully described his configuration in th
32. d legibly all these Cover Texts Front Cover Texts on the front cover and Back Cover Texts on the back cover Both covers must also clearly and legibly identify you as the publisher of these copies The front cover must present the full title with all words of the title equally prominent and visible You may add other material on the covers in addition Copying with changes limited to the covers as long as they preserve the title of Appendix B GNU Free Documentation License 69 the Document and satisfy these conditions can be treated as verbatim copying in other respects If the required texts for either cover are too voluminous to fit legibly you should put the first ones listed as many as fit reasonably on the actual cover and continue the rest onto adjacent pages If you publish or distribute Opaque copies of the Document numbering more than 100 you must either include a machine readable Transparent copy along with each Opaque copy or state in or with each Opaque copy a publicly accessible computer network location containing a complete Transparent copy of the Document free of added material which the general network using public has access to download anonymously at no charge using public standard network protocols If you use the latter option you must take reasonably prudent steps when you begin distribution of Opaque copies in quantity to ensure that this Transparent copy will remain thus accessible at the stated location until
33. d use Read for List Write use Write end StringArrayStream Non remote access types cannot be declared in the public part of a remote types unit However it is possible to define private non remote access types as long as the user provides its marshaling procedures that is to say the mechanism needed to place a value of the type into a communication stream The code below describes how to transmit a linked structure Chapter 2 The Distributed Systems Annex 17 The package declaration provides a type definition of single linked lists of unbounded strings An implementation of the marshaling operations could be the following package body StringArrayStream is procedure Read S access Root_Stream_Type Class L out List is begin if Boolean Input S then L new Node L Content new String String Input S List Read S L Next else L null end if end Read procedure Write S access Root Stream Type Class L in List is begin if L null then Boolean Qutput S False else Boolean Output S True String Output S L Content all List Write S L Next end if end Write Other services end StringArrayStream 2 5 4 Summary on Remote Types Units Remote types units e Support the definition of distributed objects e Allow dynamically bound remote calls via remote access types e Allow non remote access type with marshaling subprograms e Cannot have a specification
34. ding unknown excep tions When an exception is raised as a result of the execution of a remote subprogram call it is propagated back to the caller However the caller may not have any visibility over the Appendix A DSA and CORBA 63 exception declaration but may still catch it with a when others clause However if the caller does not catch it and let it be propagated upstream maybe in another partition and if the upstream caller has visibility over this exception it must be able to catch it using its name That means that the PCS must recognize that a previously unknown exception maps onto a locally known one for example by being able to dynamically register a new exception into the runtime A 3 2 CORBA ORB In CORBA a much more fragmented approach to communication services was adopted they are essentially defined externally For example the naming service which maps object names to object references is a distributed object with a standard IDL interface While this approach seems more pure it has performance drawbacks Being itself a distributed object the naming service cannot be optimized for the needs of a specific ORB A special case is also required in the ORB for it to be able to locate the naming service itself chicken and egg problem in order to get a reference on a distributed object an IOR Interface Object Reference to start with the programmer needs to have an IOR for the naming service This IOR can be retrieved
35. duced in Ada95 tasks cannot be invoked directly from one partition to another Therefore declarations of task types and general protected types with entries are not allowed in categorized Ada library units 2 3 Pragma Declared Pure This pragma is not specific to the Distributed Systems Annex A pure package can appear in the context of any package categorized or not pure package is a preelaborable package that does not contain the declaration of any variable or named access type It is particularly useful to define types constants and subprograms shared by several categorized packages In contrast normal packages cannot appear in the context of categorized package declarations Because a pure package has no state it can be duplicated on several partitions 2 4 Pragma Remote Call Interface 2 4 1 Overview of Pragma Remote Call Interface Library units categorized with this pragma declare subprograms that can be called and executed remotely An RCI unit acts as a server for remote calls There is no memory space shared between server and clients A subprogram call that invokes one such subprogram is a classical RPC operation it is a statically bound operation because the compiler can determine the identity of the subprogram being called Dynamically bound calls are provided through two mechanisms e The dereference of an access to subprogram value i e a value whose type is a re mote access to subprogram RAS e A dispatching
36. e Call Abortion When a remote procedure call is aborted GLADE will abort the calling task on the caller side It will also try to abort the remote anonymous task performing the remote call This task will be aborted without being requeued in the anonymous tasks pool 3 9 8 User Filter Implementation As has been briefly mentioned above a filter with aname NAME must be implemented in a package called System Garlic Filters Name The user may write his own filters which must implement their filtering of data in the primitive operations of a type derived from the type System Garlic Filters Filter Type His filter package must then register an instance of his newly derived type with GLADE by calling System Garlic Filters Register From that on his filter is ready to be used For more information on how to write filter packages see the sample implementation of a ZIP filter in files s gafizi ad bs in the distribution The user might also want to look at the example in the Filtering directory of the GLADE distribution 3 10 Remote Shell Notes To start a partition the main partition executes a remote shell except when the dis tributed application is built with a None starter Thus the user has to make sure that he is authorized to execute a remote shell on the remote machine In this case a first step would be to add into his SHOME rhosts file a line like lt remote machine gt lt user name gt If he is not authorized at all h
37. e Program Error end Read procedure Write S access Root Stream Type Class X in T is begin raise Program Error end Write end ACRRT with ACRRT use ACRRT package ACRRCI is pragma Remote Call Interface pragma A11 Calls Remote procedure P X 7T end ACRRCI package body ACRRCI is procedure P X T is begin null end P end ACRRCI with ACRRCI ACRRT procedure ACRMain is X ACRRT T begin ACRRCI P X end ACRMain 2 7 6 Generic Categorized Units generic package GenericRCI is pragma Remote Call Interface Chapter 2 The Distributed Systems Annex 25 procedure P end GenericRCI with GenericRCI package RCIInstantiation is new GenericRCI pragma Remote Call Interface RCIInstantiation with GenericRCI package NormalInstantiation is new GenericRCI Any of these categorized units can be generic Instances do not automatically inherit the categorization of their generic units and they can be categorized explicitly If they are not instances are normal compilation units Like any other categorized unit a categorized instance must be at the library level and regular restrictions of categorized units apply on instantiation in particular on generic formal parameters 2 7 7 Categorization Unit Dependencies Each categorization pragma has very specific visibility rules As a general rule RCI gt RT gt SP gt Pure where the comparison indicates allowed semantic dependencies This mean
38. e can bypass this problem All he has to do is e Open a session on each machine listed in his configuration file e If MAIN PART is the partition that includes the main procedure and if he wants to start MAIN PART on host MAIN HOST e Choose a TCP port number PORT NUM e Then for each partition PART start manually the corresponding executable on the corresponding host as follows 58 GLADE User s Guide PART nolaunch boot location tcp MAIN HOST PORT NUM The nolaunch parameter must be included for the main partition it means that this partition is not in charge of launching others e If he wants to kill the distributed application before it terminates kill MAIN_PART Appendix A DSA and CORBA 59 Appendix A DSA and CORBA A 1 CORBA Architecture CORBA is an industry sponsored effort to standardize the distributed object paradigm via the CORBA Interface Definition Language IDL The use of IDL makes CORBA more self describing than any other client server middleware The Common Object Request Broker Architecture and Specification revision 2 2 describes the main features of CORBA which are Interface Definition Language Language Mappings Stubs Skeletons and Object Adapters ORB Interface Repository Dynamic Invocation ORB protocols and CORBA services corba arch fig pdf The IDL specifies modules constants types and interfaces An object interface defines the operations exceptions and public attributes a c
39. e declaration part Line 44 Partition 2 contains two RCI packages RCI_B2 and RCI_B4 and a normal package A normal package is not categorized Line 45 Partition 3 contains one RCI package RCI_B3 Chapter 3 Getting Started With GLADE 51 3 6 Partition Command Line Options Most of the previous attributes and pragmas can be modified at run time The user can redefine some of the configuration options by defining shell environment variables or by passing arguments on the command line of a partition executable In general for a given feature Aa_Bb_Cc there is a corresponding environment variable AA BB_CC and a corresponding command line option aa_bb _cc The environment variable AA BB CC can be set to a value of the expected type When a partition is executed from such a shell the value assigned in the configuration file is replaced by the value of the environment variable If the user shell is sh bash or zsh type AA BB CC x export AA BB CC If the user shell is csh or tcsh type setenv AA BB CC x where lt x gt is a value of the expected type When the partition is launched with a command line option aa_bb_cc x the value assigned in the configuration file or by the shell environment variable is replaced by x For some environment variables the value of the environment variable may be irrelevant For some command line options no extra argument is needed In the following type None means that e
40. e remote services are defined in packages categorized by three kinds of pragmas RCI RT SP The distributed boundaries are more transparent as the application is not split into IDL and host language sources In DSA any Ada type can be used except access types but this can be solved by provid ing the marshaling operations for such a type The exception model is entirely preserved Overloading is allowed in DSA not in CORBA The user can also define generic packages and use mixin mechanism to obtain some kind of multiple inheritance The DSA user can design implement and test his application in a non distributed en vironment and then switch to a distributed situation With this two phase design ap proach the user always works within his favorite Ada95 environment The use of pragma AII Calls Remote also facilitates debugging of a distributed application in a non distributed context Appendix A DSA and CORBA 65 To work on client stubs or server skeletons the CORBA user will have to deal with generated code In any case understanding the host language mapping is always very useful It can be required for some languages like C An Ada programmer should avoid using forward declaration or multiple inheritance and in some respects sequence The CORBA user has to re adapt his code to the code generated by the translator from the IDL file any time the latter is modified He also has to use the predefined CORBA types instead of Ada sta
41. ecific partitions This concerns particularly remote call interface and remote types packages Specify the main procedure of the distributed application see undefined Partition Attribute Main page undefined e Type gnatdist lt configuration file gt e Start the distributed application by invoking the start up shell script or default Ada program depending on the Starter option see undefined Pragma Starter page un defined 3 3 Gnatdist Command Line Options gnatdist switches configuration file list of partitions The switches of gnatdist are for the time being exactly the same as those of gnatmake with the addition of PCS which allows the user to override the default selection of distri bution runtime library PCS By default gnatdist outputs a configuration report and the 34 GLADE User s Guide actions performed The switch n allows gnatdist to skip the first stage of recompilation of the non distributed application The names of all configuration files must have the suffix cfg There may be several configuration files for the same distributed application as the user may want to use different distributed configurations depending on load and other characteristics of the computing environment If a list of partitions is provided on the command line of the gnatdist command only these partitions will be built In the following configuration example the user can type gnatdist configuration lt
42. econnection policy filters The RCI name server is used to register newly elaborated RCI packages This RCI package registration occurs once the partition has been allocated a partition id The par tition registers its RCI and SP packages with their names their version ids and internal information As described previously the boot server partition can be replicated on boot mirrors in order to prevent this partition from being a single point of failure A partition has always to connect to a boot server or a boot mirror in order to get a minimal information set on the other existing partitions 56 GLADE User s Guide The boot server is the first boot mirror of the system A new partition declared as a boot mirror joins the group of boot mirrors The group of boot mirrors operates as a token ring any request from a new partition to a boot mirror is sent on the ring through a token A request can traverse the ring once or twice before being approved by all the other boot mirrors When the boot server dies a new boot server is elected among the remaining boot mirrors A boot server is responsible for the global termination detection That is why a new boot server has to be elected 3 9 2 The PolyORB PCS As an alternative to GARLIC PolyORB can be used as the PCS for GLADE This is achieved using the PCS polyorb command line switch for gnatdist PolyORB can also be configured as the default PCS at GLADE build time Using PCS pol
43. ed Heterogeneous System page unde fined The user can overload these operations except for predefined types Overloading with a textual version provides the user with a way to debug its application even outside of the Distributed Systems Annex with Ada Streams use Ada Streams package New_Integers is pragma Pure type New_Integer is new Integer procedure Read S access Root_Stream_Type Class V out New Integer procedure Write S access Root Stream Type Class V in New Integer for New Integer Read use Read for New Integer Write use Write end New Integers package body New Integers is procedure Read S access Root Stream Type Class V out New Integer is B String String Input S begin V New Integer Value B end Read procedure Write S access Root Stream Type Class V in New Integer is begin String Output S New Integer Image V end Write end New Integers Chapter 2 The Distributed Systems Annex 27 The language forces the user to provide read and write operations for non remote ac cess types Transmitting an access value by dumping its content into a stream makes no sense when it is going to be transmitted to another partition different memory spaces To transmit non remote access types see undefined Transmitting Dynamic Structure page undefined 2 8 2 Incorrect Remote Dispatching When a remote subprogram takes a class wide argument there
44. ed on an object desig nated by such a remote access a remote call is performed transparently on the partition on which the object resides The types of these distributed objects are declared in li brary units categorized as remote types RT e Shared objects Global data can be shared between active partitions providing a repos itory similar to a shared memory a shared file system or a database Entry less pro tected objects allow safe concurrent access and update shared objects This feature is orthogonal to the notion of distributed objects which are only accessed through exported services These shared objects are declared in library units categorized as shared passive SP The remotely called subprograms declared in a library unit categorized as remote call interface RCI or remote types RT may be either statically or dynamically bound The partition on which a statically bound remote subprogram is executed can be determined before the call This is a static remote subprogram call In contrast A remote method or a dereference of an access to remote subprogram are dynamically bound remote calls because the partition on which the remote subprogram is executed is determined at runtime by the actuals of the call In the following example Data l and Data_2 are shared passive SP library units Data l is configured on a passive partition mapped on a storage node Partition_1 and Partition 2 are active partitions Note that under some cir
45. ed system or as a single program on a non distributed system The DSA has been designed to minimize the source changes needed to convert an ordinary non distributed program into a distributed program The simplest way to start with DSA is to develop the application on a non distributed system Of course the design of the application should take into account the fact that some units are going to be accessed remotely In order to write an Ada95 distributed program it is necessary for the user to label by means of categorization pragmas some of library level compilation units of the application program The units which require categorization are typically those that are called remotely and those that provide the types used in remote invocations In order to insure that distributed execution is possible these units are restricted to contain only a limited set of Ada constructs For instance if the distributed system has no shared memory shared variables must be forbidden To specify the nature of these restrictions the DSA provides different categorization pragmas each of which excludes some language constructs from the categorized package Of course the user can develop the non distributed application with his usual software engineering environment It is critical to note that the user needs no specialized tools to develop his her distributed application For instance he can debug his application with the usual debugger Note that a non distrib
46. eens 33 3 2 How to Configure a Distributed Application 33 3 39 Gnatdist Command Line Options sssaaaarssssnueua 33 3 4 Gnatdist Behind the Scene 34 3 5 The Configuration Language 34 3 5 1 Language Keywords 0 0 c cece e ee eee 34 3 5 2 Pragmas and Representation Clauses 35 3 5 8 Configuration Declaration lilius 35 3 5 4 Partition Declaration 0 000 eee 36 3 5 5 Location Declaraton esses 36 3 5 6 Partition Attribute Mam 37 o Pragma Start r srona rele ld uerb 38 3 5 8 Pragma Remote Shell 38 3 5 9 Pragma Boot Location 39 3 5 10 Partition Attribute Self Location 39 3 5 11 Partition Attribute Passive aaaaa00a 40 3 5 12 Partition Attribute Data Location 40 3 5 13 Partition Attribute Allow Light PCS 41 3 5 14 Pragma Priority oo eeaeee e ee a ee 41 3 5 15 Partition Attribute Priority 41 3 5 16 Partition Attribute Host 04 42 3 5 17 Pragma Import 42 3 5 18 Partition Attribute Directory 43 3 5 19 Partition Attribute Command Line 43 3 5 20 Partition Attribute Termination 43 3 5 21 Partition Attribute Reconnection 44 3 5 22 Channel Declaration lsllils esee 45 3 5 23 Partition and Channel Attribute Filter 45 3 5 24 Pragma Registration_Filter 46 3 5 25 Pragma Versio rsss
47. ent and invokes a dispatching operation by dereferencing a distributed object Term The reference is examined at run time and the execution of this operation takes place on the partition on which the distributed object resides with Types use Types package body RACWBank is procedure Register MyTerm in Term Access Customer in Customer Type Password in Password Type is begin Insert In Local Table MyTerm Customer end Register procedure Transfer Payer in Customer Type Password in Password Type Amount in Positive Payee in Customer Type Find Customer terminal Term Term Access Find In Local Table Payee begin 16 GLADE User s Guide Withdraw Payer Amount Deposit Payee Amount if Term null then Notify on Payee terminal Notify Term Payer Amount end if end Transfer Other services end RACWBank 2 5 3 Transmitting Dynamic Structure with Ada Streams use Ada Streams package StringArrayStream is pragma Remote_Types type List is private procedure Append L access List 0 in String function Delete L access List return String private type String Access is access String type Node type List is access Node type Node is record Content String_Access Next List end record procedure Read S access Root_Stream_Type Class L out List procedure Write S access Root_Stream_Type Class L in List for List Rea
48. ent restriction has several useful consequences The language can provide more powerful features because it is not constrained by the common features available in all host languages In Ada95 the user will define a specification of remote services and implement them exactly as he would for ordinary non distributed services His Ada95 environment will compile them to produce a stub file on the caller side and a skeleton file that automatically includes the body of the services on the receiver side Creating objects obtaining or registering object references or adapting the object skeleton to the user object implementation are made transparent because the language environment has a full control on the development process Comparing with multi thread programming once again the language extension solution is equivalent to the solution adopted for tasking facilities in Ada Writing a distributed application is as simple as writing a concurrent application there is no binding consideration and no code to wrap The language and its run time system take care of most issues that would divert the programmer s attention from the application domain GLADE User s Guide Chapter 2 The Distributed Systems Annex 7 2 The Distributed Systems Annex A critical feature of the Distributed Systems Annex DSA is that it allows the user to develop his application the same way whether this application is going to be executed as several programs on a distribut
49. erator boolean next_n in unsigned long how_many out BindingList bl Other declarations not shown A method is defined by a unique name no overloading is allowed and its signature the types of its formal parameters Each parameter can be of mode in out or inout whose meanings are comparable to their Ada homonyms Every exception that can be raised by a method must also be declared as part of the method signature The oneway attribute can be applied to a subprogram giving it at most once semantics instead of the exactly once default This precludes a method from having output param eters a return value or from raising an exception It is not portable to assume that the caller resumes its execution once the input parameters are transmitted Most CORBA data types map in a straightforward way onto predefined Ada types with the exception of any and sequence any that can designate any CORBA type is mapped onto a stream type with read and write operations A sequence holds a list of items of a given type and is represented in Ada using a pair of lengthy generic packages One may note that the CORBA string type is mapped onto the Unbounded_String Ada95 type The IDL does not provide an equivalent to unconstrained arrays The Ada95 mapping provides special mechanisms to implement two difficult to map CORBA features First it provides a translation of multiple inheritance As described above an Ada95 package defines a type deri
50. es called skeletons These files are specific to a vendor and product as they make calls to a proprietary communication subsystem but their structure and interface are supposed to follow a standard canvas The client stubs convert user queries into requests to the ORB which transmits these requests through an object adapter to the server skeleton A 2 Interface Definition Language In DSA the IDL is a subset of Ada95 The user identifies interface packages at com pile time Some library level packages are categorized using pragmas and these interface packages have to be library units 60 GLADE User s Guide In CORBA the IDL is a description language it supports C syntax for constant type and operation declarations From IDL descriptions a translator can directly generate client header files and server implementation skeletons An IDL file can start by defining a module This provides a name space to gather a set of interfaces This is a way to introduce a level of hierarchy lt module gt lt interface gt lt operation gt The Ada95 binding maps this element into a child package include will make any other namespaces visible A module can define interfaces An interface defines a set of methods that a client can invoke on an object An interface can also define exceptions and attributes An exception is like a C exception a data component can be attached to it An attribute is a compo nent field For each Attribute t
51. executable directory has to be present in the user path If the Directory attribute has been specified the executable full name is used REPRESENTATION CLAUSE for PARTITION IDENTIFIER Directory use STRING LITERAL 3 5 19 Partition Attribute Command Line The user may want to pass arguments on the command line of a partition However when a partition is launched automatically by the main partition the partition command line includes only GLADE arguments To add arguments on the command line the user can take advantage of the following attribute REPRESENTATION CLAUSE for PARTITION IDENTIFIER Command Line use STRING LITERAL 3 5 20 Partition Attribute Termination The Ada95 Reference Manual does not provide any specific rule to handle global termi nation of a distributed application see undefined Abortion and Termination page un defined In GLADE by default a set of partitions terminates when each partition can terminate and when no message remains to be delivered A distributed algorithm that checks for this global condition is activated periodically by the main boot server TERMINATION LITERAL Global Termination Local Termination Deferred Termination REPRESENTATION CLAUSE 4A GLADE User s Guide for PARTITION_IDENTIFIER Termination use TERMINATION_LITERAL e When a partition is configured with the global termination policy it terminates as soon as the main boot server sends a signal t
52. figured on a partition such a partition can be launched several times without any problem When one or more RCI packages are configured on a partition such a partition cannot be launched more than once If this partition were to be launched repeatedly it would not be possible to decide which partition instance should execute a remote procedure call When a partition crashes or is stopped one may want to restart this partition and possibly restore its state with Shared_Passive packages for instance In such a situation the partition is already known to other partitions and possibly marked as a dead partition Several policies can be selected RECONNECTION_LITERAL Reject_On_Restart Fail_Until_Restart Block_Until_Restart REPRESENTATION CLAUSE for PARTITION IDENTIFIER Reconnection use RECONNECTION LITERAL e When this partition is configured with the Reject On Restart reconnection policy the dead partition is kept dead and any attempt to restart it fails Any remote call to a subprogram located on this partition results in a Communication Error exception The Reject On Restart policy is the default policy Chapter 3 Getting Started With GLADE 45 e When this partition is configured with the Fail_Until_Restart reconnection policy the dead partition can be restarted Any remote call to a subprogram located on this partition results in an exception Communication_Error as long as this partition has not been restarted As s
53. from the command line from a file or by invoking the ORB Interface depending on the CORBA version Regarding exception propagation an ORB is not able to propagate an exception that has not been declared in the IDL interface This restriction although annoying because it restricts the usage of exceptions is understandable given the multi language CORBA approach what should be done for example when a C exception reaches a caller writ ten in Ada Note that an implementation may provide more information in the CORBA exception message such as the C or Ada exception name A 4 Distributed Application Development A 4 1 DSA Application Development The DSA does not describe how a distributed application should be configured It is up to the user using a partitioning tool whose specification is outside the scope of the annex to define what the partitions in his program are and on which machines they should be executed GLADE provides a Configuration Tool and a Partition Communication Subsystem to build a distributed application The gnatdist tool and its configuration language have been specially designed to let the user partition his program and specify the machines where the individual partitions will be executing The Generic Ada Reusable Library for Interparti tion Communication GARLIC is a high level communication library that implements the interface between the Partition Communication Subsystem defined in the Reference Manual and t
54. he implementation automatically creates the subprograms Get_Attribute and Set_Attribute Only Get is provided for readonly attributes An interface can derive from one or more interfaces multiple inheritance The Ada95 binding maps this element into a package or a child package For the client stub the implementation will automatically create a tagged type named Ref which is derived from CORBA Object Ref or from another Ref type defined in another interface in a package whose name matches the one of the interface For the server skeleton the implementation will automatically create a tagged type named Object which is derived from an implementation defined private tagged type Object in a package named Impl which is a child package of a package named after the interface name interface Impl module CosNaming 1 typedef string Istring struct NameComponent 1 Istring id Istring kind typedef sequence lt NameComponent gt Name enum BindingType nobject ncontext struct Binding Name binding_name BindingType binding type typedef sequence Binding BindingList interface BindingIterator interface NamingContext 1 exception CannotProceed 1 NamingContext cxt Name rest of name I void bind in Name n in Object obj raises CannotProceed void list in unsigned long how_many out BindingList bl Appendix A DSA and CORBA 61 out BindingIterator bi Other declarations not shown interface BindingIt
55. he maximum number of anonymous tasks in the GLADE PCS In other words it provides a way to limit the number of remote calls in the PCS When a RPC request is received if the number of active remote calls is greater than the task pool maximum size then the request is kept pending until an anonymous task completes its own remote call and becomes available REPRESENTATION_CLAUSE for PARTITION_IDENTIFIER Task_Pool use TASK_POOL_SIZE_ARRAY TASK_POOL_SIZE_ARRAY NATURAL LITERAL Task Pool Minimum Size NATURAL LITERAL Task Pool High Size NATURAL LITERAL Task Pool Maximum Size In order to have only one active remote call at a time the task pool configuration is declared as follows 48 GLADE User s Guide for Partition Task Pool use 0 0 1 3 5 27 A Complete Example Almost every keyword and construct defined in the configuration language has been used in the following sample configuration file 01 configuration MyConfig is 02 03 Partition 1 Partition procedure Master Procedure is in Partition 1 Partition 2 Partition 3 Partition for Partition 2 Host use foo bar com function Best Node Partition Name String return String pragma Import Shell Best Node best node for Partition 3 Host use Best Node Partition 4 Partition RCI B5 for Partition 1 Directory use usr you test bin for Partition Directory use bin procedure Another Main for Partition 3
56. he network communication layer with object oriented techniques A 4 2 CORBA Application Development The ORB provides a core of basic services All other services are provided by objects with IDL The OMG has standardized a set of useful services like Naming Trading Events 64 GLADE User s Guide Licensing Life Cycle Events A CORBA vendor is free to provide an implementation of these services The Naming Service allows the association binding of an object reference with user friendly names A name binding is always defined relative to a naming context wherein it is unique A naming context is an object itself and so can be bound to a name in another naming context One thus creates a naming graph a directed graph with naming contexts as vertices and names as edge labels Given a context in a naming graph a sequence of names can thus reference an object This is very similar to the naming hierarchies that exist in the Domain Name System and the UNIX file system A typical scenario to start working with the Name Service consists in providing a well known remote reference that defines the root of a naming and naming context hierarchy Then many naming operations can be executed on this hierarchy The Trading Service provides a higher level of abstraction than the Naming Service If the Naming Service can be compared to the White Pages the Trading Service can be compared to the Yellow Pages The Events service provides a way for ser
57. hronous and synchronous depending on the designated subprogram For instance in the code below remote call 1 is asynchronous but remote call 2 is synchronous A pragma Asynchronous applies to a RACW as well In this case the invocation of any method with in parameters is always performed asynchronously Remote method invocation 3 is asynchronous but remote method invocation 4 is synchronous with AsynchronousRCI AsynchronousRT use AsynchronousRCI AsynchronousRT procedure AsynchronousMain is RAS AsynchronousRAS RACW AsynchronousRACW Create begin Asynchronous Dynamically Bound Remote Call 1 RAS AsynchronousRCI Asynchronous Access RAS 0 Abbrev for RAS all 0 Synchronous Dynamically Bound Remote Call 2 RAS AsynchronousRCI Synchronous Access RAS 0 Asynchronous Dynamically Bound Remote Call 3 Asynchronous RACW all Synchronous Dynamically Bound Remote Call 4 Synchronous RACW all end AsynchronousMain 22 GLADE User s Guide This feature supports the conventional message passing paradigm The user must be aware that this paradigm and asynchronous remote calls in particular has several draw backs e It violates original remote procedure semantics e It allows the equivalent of a remote GOTO mechanism e It prevents easy development and debugging in a non distributed context e It can introduce potential race conditions To illustrate the latter le
58. hysical location of the partition Partition_ID s can be used to check whether a RCI package is configured locally with RCI with Ada Text IO procedure Check PID is begin if RCI Partition ID Check PID Partition ID then Ada Text IO Put Line package RCI is configured locally else Ada Text IO Put Line package RCI is configured remotely end if end Check PID 2 8 4 Concurrent Remote Calls It is not defined by the PCS specification whether one or more threads of control should be available to process incoming messages and to wait for their completion But the PCS 28 GLADE User s Guide implementation is required to be reentrant thereby allowing concurrent calls on it to service concurrent remote subprogram calls into the server partition This means that at the im plementation level the PCS manages a pool of helper tasks This apart from performance is invisible to the user 2 8 5 Consistency and Elaboration A library unit is consistent if the same version of its declaration is used in all units that reference it This requirement applies as well to a unit that is referenced in several partitions of a distributed program If a shared passive or RCI library unit U is included in some partition P It is a bounded error to elaborate another partition P1 of a distributed program that that depends on a different version of U As a result of this error Program_Error can be raised in one or both partitions during elaborat
59. ilable GLADE But it is a validation requirement to permit the user to replace his current PCS with a third party PCS We can note this issue was not resolved in CORBA until revision 2 2 For the same reasons we can expect future DSA implementations to ensure PCS compatibility Using its IDL the OMG has described a number of Common Object Services COS that are frequently needed in distributed systems Unfortunately these specifications are limited to IDL descriptions and most of the semantics are up to the vendor The DSA misses such user level libraries including basic distributed software components More generally the lack of component libraries has always been a problem for Ada Implementing CORBA services as native Ada95 distributed objects taking advantage of the standard language features yields a simpler easy to understand and use specification We have already implemented the Naming service the Events service and a service close to the Concurrency one with DSA Developing the CORBA services was an interesting experience We realized that although those services are nicely specified by an IDL file their semantics is quite vague in such a way portability is dramatically broken This work will be described in a future paper Another major goal of the GLADE team is to export DSA services to the CORBA world The idea is to translate all DSA features to equivalent IDL features using ASIS This would allow the DSA user to connect his
60. ill note that this information is close to what the Ada Semantic Interface Specification ASIS can provide The interface repository allows a client to discover the signature of a method which it did not know at compile time It can subsequently use this knowledge together with values for the method s parameters to construct a complete request and invoke the method The set of functions that permits the construction of a method invocation request at run time is the Dynamic Invocation Interface DII The IR API allows the client to explore the repository classes to obtain a module defini tion tree From this tree the client extracts subtrees defining constants types exceptions and interfaces From an interface subtree the client can select an operation with its list of parameters type name and mode and exceptions A client has then three ways to make a request As in the static case he can send it and wait for the result he can also do a one way call and discard the result With dynamic requests a third mechanism is offered the client can send the request without waiting for the result and obtain it later asynchronously The DII has a server side counterpart called Dynamic Skeleton Interface DSI Both mechanisms are powerful but very complex and tedious to use In some respects they also violate the Ada95 philosophy because strong typing is not preserved Most users will keep working with static invocations A 3 Network Comm
61. in Rate Type procedure Get Index in Index Type Rate out Rate Type private Rates Rates Type end Internal Synchronization end SharedObjects 2 6 3 Summary on Pragma Shared Passive Allow direct access to data from different partitions e Provide support for shared distributed memory Support memory protection by means of entryless protected objects e Prevent specification from depending on normal units 2 7 More About Categorization Pragmas Chapter 2 The Distributed Systems Annex 19 2 7 1 Variables and Non Remote Access Types In RT or RCI package declarations variable declarations are forbidden and non remote access types are allowed as long as their marshaling subprograms are explicitly provided see undefined Transmitting Dynamic Structure page undefined 2 7 2 RPC Failures Calls are executed at most once they are made exactly one time or they fail with an exception When a communication error occurs System RPC Communication_Error is raised 2 7 3 Exceptions Any exception raised in a remote method or subprogram call is propagated back to the caller Exceptions semantics are preserved in the regular Ada way package Internal is Exc exception end Internal package RemPkg2 is pragma Remote Call Interface procedure Subprogram end RemPkg2 package RemPkg1 is pragma Remote Call Interface procedure Subprogram end RemPkg 1 Let us say that RemPkg2 Internal and RemExcMain
62. ing tools are not generally available and the machine generated HTML produced by some word processors for output purposes only The Title Page means for a printed book the title page itself plus such following pages as are needed to hold legibly the material this License requires to appear in the title page For works in formats which do not have any title page as such Title Page means the text near the most prominent appearance of the work s title preceding the beginning of the body of the text 2 VERBATIM COPYING You may copy and distribute the Document in any medium either commercially or noncommercially provided that this License the copyright notices and the license notice saying this License applies to the Document are reproduced in all copies and that you add no other conditions whatsoever to those of this License You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute However you may accept compensation in exchange for copies If you distribute a large enough number of copies you must also follow the conditions in section 3 You may also lend copies under the same conditions stated above and you may publicly display copies 3 COPYING IN QUANTITY If you publish printed copies of the Document numbering more than 100 and the Doc ument s license notice requires Cover Texts you must enclose the copies in covers that carry clearly an
63. ion U Version yields a string that identifies the version of the unit declaration and any unit declaration on which it depends U Version_Body yields a string that identifies the version of the unit body These attributes are used by the PCS to verify the consistency of an application After elaborating the library units but prior to invoking the main subprogram the PCS checks the RCI unit versions and then accept any incoming RPC To guarantee that it is safe to call receiving stubs any incoming RPC is kept pending until the partition completes its elaboration 2 8 6 Abortion and Termination If a construct containing a remote call is aborted the remote subprogram call is cancelled Whether the execution of the remote subprogram is immediately aborted as a result of the cancellation is implementation defined An active partition terminates when its environment task terminates In other terms a partition cannot terminate before the Ada program itself terminates The standard termi nation mechanism applies but can be extended with extra rules see undefined Partition Attribute Termination page undefined for examples 2 9 Most Features in One Example The example shown on the following figure highlights most of the features of DSA The system is based on a set of factories and workers and a storage Each entity is a partition itself A factory hires a worker from a pool of workers hire 1 and assigns a job query 2 to hi
64. is License is not limited to software manuals it can be used for any textual work regardless of subject matter or whether it is published as a printed book We recommend this License principally for works whose purpose is instruction or reference 1 APPLICABILITY AND DEFINITIONS This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License The Document below refers to any such manual or work Any member of the public is a licensee and is addressed as you A Modified Version of the Document means any work containing the Document or a portion of it either copied verbatim or with modifications and or translated into another language A Secondary Section is a named appendix or a front matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document s overall subject or to related matters and contains nothing that could fall directly within that overall subject For example if the Document is in part a textbook of mathematics a Secondary Section may not explain any mathematics The relationship could be a matter of historical connection with the subject or with related matters or of legal commercial philosophical ethical or political position regarding them The Invariant Sections are certain Secondary Sections whose titles are desig
65. is a risk of using an object of a derived type that will not be clean enough to be transmitted For example given a type called Root_Type if a remote procedure takes a Root_Type Class as an argument the user can call it with an instance of Derived_Type that is Root Type enriched with a field of a task type This will lead to a non communicable type to be transmitted between partitions To prevent this paragraph E 4 18 of the reference manual explains that any actual type used as parameter for a remote call whose formal type is a class wide type must be declared in the visible part of a Pure or Remote_Types package This property also holds for remote functions returning class wide types To summarize the actual type used should have been eligible for being declared where the root type has been declared If a bad object is given to a remote subprogram Program_Error will be raised at the point of the call 2 8 3 Partition Ids U Partition_ID identifies the partition where the unit U has been elaborated For this purpose the PCS provides an integer type Partition_ID to uniquely designate a partition Note that a Partition_ID is represented as a universal integer and has no meaning outside of the PCS The RM requires that two partitions of a distributed program have different Partition_ID s at a given time A Partition ID may or may not be assigned statically at compile or link time A Partition ID may or may not be related to the p
66. ith GLADE 4T PRAGMA pragma Registration_Filter Filter gt STRING_LITERAL 3 5 25 Pragma Version A library unit is consistent if the same version of its declaration is used throughout see undefined Consistency and Elaboration page undefined It can be useful to deactivate these checks especially when the user wants to be able to update a server without updating a Client PRAGMA pragma Version Check gt BOOLEAN_LITERAL 3 5 26 Partition Attribute Task_Pool When multiple remote subprogram calls occur on the same partition they are handled by several anonymous tasks These tasks can be allocated dynamically or re used from a pool of preallocated tasks When a remote subprogram call is completed the anonymous task can be deallocated or queued in a pool in order to be re used for further remote subprogram calls The number of tasks in the anonymous tasks pool can be configured by means of three independent parameters e The task pool minimum size indicates the number of anonymous tasks preallocated and always available in the GLADE PCS Preallocating anonymous tasks can be useful in real time systems to prevent task dynamic allocation e The task pool high size is a ceiling When a remote subprogram call is completed its anonymous task is deallocated if the number of tasks already in the pool is greater than the ceiling If not then the task is queued in the pool e The task pool maximum size indicates t
67. lient can invoke or access CORBA offers a model based only on distributed objects In some respects it can be compared to Java as this language provides only an object oriented programming model and discards the classical structured programming model An IDL translator generates client stubs and server skeletons in a host language C C Java Smalltalk Ada95 a language mapping specifies how IDL entities are implemented in the host language Depending on the features available in the host language the mapping can be more or less straightforward When an IDL feature is not defined in the host language the mapping provides a standardized but complex way of simulating the missing feature Although the user works with the generated code a good understanding of the language mapping is often necessary When the host language does not provide object oriented features the user has to deal with a complex simulation of those functions A C programmer has to follow several rules related to parameters passed by reference Defining whether the callee or the caller is responsible for parameter memory allocation can be regarder as an issue of C programming conventions The most difficult parts of the Ada mapping which an Ada programmer should avoid whenever possible are multiple inheritance and forward declarations The IDL translator produces several host language source files depending on the language mapping client files called stubs and server fil
68. line procedure Master Procedure is in Partition 1 was missing Partition 1 would be completely empty This is forbidden because a partition has to contain at least one library unit gnatdist produces an executable with the name of Master Procedure which will start the various partitions on their host machines in the background The main partition is launched in foreground Note that by killing this main procedure the whole distributed application is terminated 3 Line 08 Specify the host on which to run partition 2 4 Line 12 Use the value returned by a program to figure out at execution time the name of the host on which partition 3 should execute For instance execute the shell script best node which takes the partition name as parameter and returns a string giving the name of the machine on which partition 3 should be launched 5 Line 14 Partition 4 contains one RCI package RCI_B5 No host is specified for this partition The startup script will ask for it interactively when it is executed 6 Line 16 Specify the directory in which the executable of partition partition_1 will be stored 7 Line 17 Specify the directory in which all the partition executables will be stored except partition_1 see undefined Pragmas and Representation Clauses page undefined Default is the current directory 8 Line 20 Specify the partition main subprogram to use in a given partition 9 Line 22 Specify a reconnection policy in case of a crash of Pa
69. m The worker performs the job and saves the result reply 3 in a storage common to all the factories The worker notifies the factory of the end of his job notify 4 full ex fig pdf When a worker has completed his job the result must be saved in a common storage To do this we define a protected area in SP package Storage see following code An entry less protected object ensures atomic access to this area Chapter 2 The Distributed Systems Annex 29 package Storage is pragma Shared_Passive protected Queue is procedure Insert Q R Integer procedure Remove Q in Integer R out Integer private Other declarations end Queue end Storage Common is a Remote Types package that defines most of the remote services of the above system see following code First we define a way for the workers to signal the completion of his job This callback mechanism is implemented using RAS Notify with Storage use Storage package Common is pragma Remote Types type Notify is access procedure Q Integer pragma Asynchronous Notify type Worker is abstract tagged limited private procedure Assign W access Worker Q in Integer N in Notify is abstract type Any Worker is access all Worker Class pragma Asynchronous Any Worker private type Worker is abstract tagged limited null record end Common We define an abstract tagged type Worker which is intended to be the root type of the wh
70. ma Remote Call Interface function Balance Customer in Customer Type Password in Password Type return Integer procedure Transfer Payer in Customer Type Password in Password Type Amount in Positive Payee in Customer Type procedure Deposit Customer in Customer Type Amount in Positive procedure Withdraw Customer in Customer Type Password in Password Type Chapter 2 The Distributed Systems Annex 11 Amount in out Positive end RCIBank with Types use Types with RCIBank use RCIBank procedure RCIClient is B Integer C Customer Type rich P Password Type xxxx begin B Balance C P end RCIClient 2 4 3 Remote Access to Subprograms RAS In the following example several mirroring banks offer their services through the same database Each bank registers a reference to each of its services with a central bank A client of the central bank requests a service from one of the mirroring banks To satisfy requests the RCI unit RASBank defines Balance Type a remote access to subprogram Recall that an access type declared in a remote unit has to be either remote access to subprogram or remote access to class wide type Note that to obtain a remote access to subprogram the subprogram that delivers the remote access must be remote itself Therefore MirrorBank is a RCI library unit with Types use Types package RASBank is pragma Remote Call Interface
71. n partition anymore Environment Variable Command Line Option Type SLAVE slave None 3 6 7 Partition Option boot mirror By default a partition is not a boot mirror except for a boot partition on which the boot server is located The user can force a partition to be a boot mirror Environment Variable Command Line Option Type BOOT_MIRROR boot mirror None 3 6 8 Partition Option mirror_expected This option suspends the execution of the distributed application until there is at least one boot mirror partition available excluding the boot server Environment Variable Command Line Option Type MIRROR_EXPECTED mirror expected None 3 6 9 Partition Option connection hits This option sets the number of times a partition tries to connect to the boot server before raising a Communication Error exception Environment Variable Command Line Option Type CONNECTION HITS connection hits Natural 3 6 10 Partition Option reconnection This option sets the reconnection policy see undefined Partition Attribute Reconnec tion page undefined for details Environment Variable Command Line Option Type RECONNECTION reconnection Reconnection Type 54 GLADE User s Guide 3 6 11 Partition Option termination This option sets the termination policy see undefined Partition Attribute Termina tion page undefined for details Environment Variable Command Line Option Type TERMINATION termination Termination Type 3 6 12
72. nated as being those of Invariant Sections in the notice that says that the Document is released under this License 68 GLADE User s Guide The Cover Texts are certain short passages of text that are listed as Front Cover Texts or Back Cover Texts in the notice that says that the Document is released under this License A Transparent copy of the Document means a machine readable copy represented in a format whose specification is available to the general public whose contents can be viewed and edited directly and straightforwardly with generic text editors or for images composed of pixels generic paint programs or for drawings some widely available drawing editor and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent A copy that is not Transparent is called Opaque Examples of suitable formats for Transparent copies include plain ASCII without markup Texinfo input format LaTeX input format SGML or XML using a publicly available DTD and standard conforming simple HTML designed for human modification Opaque formats include PostScript PDF proprietary formats that can be read and edited only by proprietary word processors SGML or XML for which the DTD and or process
73. nd which sets V to 1 2 7 5 Pragma All Calls Remote A pragma All Calls Remote in a RCI unit forces remote procedure calls to be routed through the communication subsystem even for a local call This eases the debugging of an application in a non distributed situation that is very close to the distributed one because the communication subsystem including marshaling and unmarshaling procedures can be exercised on a single node In some circumstances a non distributed application can behave differently than an ap plication distributed on only one partition This can happen when both ATI Calls Remote and Asynchronous features are used at the same time see undefined Pragma Asyn chronous page undefined for an example Another circumstance occur when the mar shaling operations raise an exception In the following example when unit ACRRCI is a All Calls Remote package the program raises Program Error When unit ACRRCI is no longer a All Calls Remote package then the program completes silently with Ada Streams use Ada Streams package ACRRT is pragma Remote Types type T is private private type T is new Integer procedure Read S access Root Stream Type Class X out T procedure Write S access Root Stream Type Class X in T for T Read use Read for T Write use Write end ACRRT 24 GLADE User s Guide package body ACRRT is procedure Read S access Root_Stream_Type Class X out T is begin rais
74. ndard types he has to call ORB functions or a naming service to obtain remote object references As Ada95 is its own IDL the user does not deal with any generated stub or skeleton code The configuration environment takes care of updating object stub and skeleton files when sources have been updated The system automatically provides some naming functions like declaring RCI services It also takes care of aborting remote procedure calls detecting distributed termination checking version consistency between clients and servers and preserving and propagating any remote exception Note that none of these features are immediately available in CORBA The RM does not require a DSA implementation to work on heterogeneous systems but GLADE like any reasonable implementation provides default XDR like marshaling operations This feature can be inhibited for performance reasons An ORB is required to implement a Common Data Representation CDR to ensure safe communications between heterogeneous systems CORBA is a very rich but very complex standard Its drawbacks include the high learning curve for developing and managing CORBA applications effectively performance limitations as well as the lack of portability and security These drawbacks are the price to pay for language interoperability a facility the Ada95 oriented DSA does not provide Interoperability between compilers is not yet an issue with DSA because there is only one implementation ava
75. nt DCE provide a framework to develop client server applications based on the Remote Procedure Call model RPC The RPC model is inspired from the query and reply scheme In rough analogy with a regular procedure call arguments are pushed onto a stream along with some data specifying the remote procedure to be executed The stream is transmitted over the network to the server The server decodes the stream performs the regular subprogram call locally and then puts the output parameters into another stream along with the exception if any raised by the subprogram execution The server then A GLADE User s Guide sends this stream back to the caller The caller decodes the stream and raises locally the exception if needed CORBA provides the same enhancements to the remote procedure model that object oriented languages provide to classical procedural languages These enhancements in clude encapsulation inheritance type checking and exceptions These features are offered through an Interface Definition Language IDL The middleware communication framework provides all the machinery to perform some what transparently remote procedure calls or remote object method invocations For in stance each CORBA interface communicates through an Object Request Broker ORB A communication subsystem such as an ORB is intended to allow applications to use objects without being aware of their underlying message passing implementation In addition
76. o do so The main boot server checks periodically whether the application can terminate When all partitions are ready to terminate the main boot server sends to each partition a termination request The global termination policy is the default policy e The deferred termination policy is very similar to the global termination The only difference is that when a partition with a deferred termination policy receives a termi nation request it just ignores it This policy allows a partition to run forever without preventing a set of partitions from terminating This policy is not yet implemented e When a partition is configured with the local termination policy it terminates as soon as the classical Ada termination is detected by the partition It means that this partition does not wait for the termination request of the main boot server In any case when the boot partition dies and when no alternate boot partition can elected see undefined The GARLIC PCS page undefined all the partitions die what ever their termination policy might be Note first that a partition cannot execute without a boot partition Second when the user wants to kill his non distributed application he kills the main program Enforcing the mechanism described above ensures that killing the main partition automatically kills all the partitions that is to say the whole distributed application 3 5 21 Partition Attribute Reconnection When no RCI package is con
77. ocol used by the partition and loaded in its executable is the tcp protocol 3 5 11 Partition Attribute Passive By default a partition is an active partition This attribute allows to define a passive partition In this case gnatdist checks that only shared passive units are mapped on the partition As this partition cannot register itself its location is hard coded in all the partitions that depend on its shared passive units REPRESENTATION CLAUSE for PARTITION IDENTIFIER Passive use BOOLEAN LITERAL 3 5 12 Partition Attribute Data Location Shared passive units can be mapped on passive or active partitions In both cases it is possible to choose the data storage support and to configure it with the specific data of a location REPRESENTATION CLAUSE for PARTITION IDENTIFIER Data Location use LOCATION for PARTITION IDENTIFIER Data Location use LOCATION LIST When an attribute data location definition clause applies to a given partition the data storage support units needed for this partition are linked in the executable By default when the data location attribute is not redefined the default storage support used by the partition and loaded in its executable is the dfs support dfs Distributed File System is a storage support available as soon as files can be shared between partitions It is not possible to map the different shared passive units of a given partition on different data storage locations GLADE requires
78. of remote proce dure calls resolved in System RPC s rpc adb The variable S PARINT provides informa tion on partitions and units status resolved in System Partition Interface s parint adb For instance using sh bash or zsh type S RPC true export S RPC S PARINT true export S PARINT 3 8 GLADE File Hierarchy All GLADE intermediate files object files etc are stored under a common directory named dsa The user may remove this whole directory and its content when he does not intend to rebuild his distributed applications 3 9 GLADE Internals The default GLADE PCS is called GARLIC which stands for Generic Ada Reusable Library for Interpartition Communication Most of the previous features like filtering trace replay termination reconnection version consistency and remote launching are provided via gnatdist specific features Some of these features are not configurable by the user 3 9 1 The GARLIC PCS When a partition starts executing one of the first elaboration steps is a registration with the partition id server and with the RCI name server These two servers are located on a boot server The partition id server is used to allocate a unique partition id when a new partition registers The id server also replies to information queries from other partitions This information includes the ip address the port on which the partition is waiting for requests and all its configuration parameters termination policy r
79. ole distributed objects hierarchy Assign allows a factory to specify a job to a worker and a way for the worker to signal its employer the completion of this job Any Worker is a remote access to class wide type RACW In other words it is a reference to a distributed object of any derived type from Worker class Note that the two remote access types Any Worker and Notify are declared as asynchronous Therefore any override of Assign 30 GLADE User s Guide will be executed asynchronously To be asynchronous an object of type Notify has to be a reference to an asynchronous procedure NewWorker is derived from type Worker and Assign is overridden with Common Storage use Common Storage package NewWorkers is pragma Remote_Types type NewWorker is new Worker with private procedure Assign W access NewWorker Q Integer N Notify private type NewWorker is new Worker with record NewField Field Type Other fields end record end NewWorkers The following code shows how to derive a second generation of workers NewNew Worker from the first generation NewWorker As mentioned above this RT package can be du plicated on several partitions to produce several types of workers and also several remote workers with Common Storage NewWorkers use Common Storage NewWorkers package NewNewWorkers is pragma Remote Types type NewNewWorker is new NewWorker with private procedure Assign W access NewNe
80. ommand Line Options page undefined Most commonly this would be tcp lt machine gt lt port gt 3 6 2 Partition Option self_location This option sets the current partition location see undefined Partition Attribute Self_Location page undefined for details Environment Variable Command Line Option Type SELF_LOCATION self_location Formatted String Environment Variable Command Line Option Type SELF_LOCATION self_location Formatted String The formatted string must conform to the location notation see undefined Loca tion Declaration page undefined and undefined Partition Command Line Options page undefined Most commonly this would be tcp lt machine gt lt port gt 3 6 3 Partition Option data_location This option sets the location of the data storage on which the shared passive units of the current partition are mapped see undefined Partition Attribute Data_Location page undefined for details This location has to be compatible with one of locations provided in the configuration file that means the partition option data_location must have a support name of one of the storage locations specified in the configuration file Environment Variable Command Line Option Type DATA LOCATION data location Formatted String Environment Variable Command Line Option Type DATA LOCATION DATA_location Formatted String The formatted string must conform to the location notation see undefined Loca tion
81. ommunication Subsystem GARLIC undefined DSA and CORBA page undefined is a detailed comparison between the capabilities of CORBA and those of the Distributed System Annex undefined GNU Free Documentation License page undefined contains the text of the license under which this document is being distributed GLADE User s Guide Chapter 1 Introduction to Distributed Systems 3 1 Introduction to Distributed Systems A distributed system architecture comprises a network of computers and the software components that execute on those computers Such architectures are commonly used to improve the performance reliability and reusability of complex applications Typically there is no shared address space available to remotely located components that is to say components running on different nodes of the network and therefore these components must communicate using some form of message passing 1 1 Using OS Network Services There are several programming techniques for developing distributed applications These applications have traditionally been developed using network programming interfaces such as sockets Programmers explicitly have to perform calls to operating system services a task that can be tedious and error prone This includes initializing socket connection and determining peer location marshaling and unmarshaling data structures sending and receiving messages debugging and testing several programs at the same
82. ompilation is called an aggregate and this License does not apply to the other self contained works thus compiled with the Document on account of their being thus compiled if they are not themselves derivative works of the Document If the Cover Text requirement of section 3 is applicable to these copies of the Document then if the Document is less than one quarter of the entire aggregate the Document s Cover Texts may be placed on covers that surround only the Document within the aggregate Otherwise they must appear on covers around the whole aggregate 8 TRANSLATION Translation is considered a kind of modification so you may distribute translations of the Document under the terms of section 4 Replacing Invariant Sections with translations requires special permission from their copyright holders but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections You may include a translation of this License provided that you also include the original English version of this License In case of a disagreement between the translation and the original English version of this License the original English version will prevail 72 GLADE User s Guide 9 TERMINATION You may not copy modify sublicense or distribute the Document except as expressly provided for under this License Any other attempt to copy modify sublicense or distribute the Document is void and
83. on License Version 1 1 or any later version published by the Free Software Foundation with the Invariant Sections being LIST THEIR TITLES with the Front Cover Texts being LIST and with the Back Cover Texts being LIST A copy of the license is included in the section entitled GNU Free Documentation License If you have no Invariant Sections write with no Invariant Sections instead of saying which ones are invariant If you have no Front Cover Texts write no Front Cover Texts instead of Front Cover Texts being LIST likewise for Back Cover Texts If your document contains nontrivial examples of program code we recommend releasing these examples in parallel under your choice of free software license such as the GNU General Public License to permit their use in free software Table of Contents About This HIE ios Mts NEE EE What This Guide Contains 0 0 ccc cece cnn eens 1 Introduction to Distributed Systems 1 1 Using OS Network Services 0 00 cece eee e eee 1 2 Using a Middleware Emironment 1 8 Using a Distributed Language 2 The Distributed Systems Annex 2 1 Architecture of a Distributed Ada95 Application 2 2 Categorization Pragmas 0 00 c cece eese 2 3 Pragma Declared Pure 2 4 Pragma Remote Call Interface 2 4 1 Overview of Pragma Remote_Call_Interface 2 4 2 Regular Remote Subprograms RCI 2 4 3 Remote Access to Subprograms R
84. oon as the partition is restarted remote calls to this partition are executed normally e When this partition is configured with the Block_Until_Restart reconnection policy the dead partition partition can be restarted Any remote call to a subprogram located on this partition is suspended until the partition is restarted As soon as the partition is restarted remote calls to this partition are executed normally The suspended remote procedure calls to this partition are resumed 3 5 22 Channel Declaration The configuration language not only describes partitions but also the connections be tween them Such a connection is called a Channel and represents a bi directional link between two partitions CHANNEL_DECLARATION CHANNEL_IDENTIFIER Channel PARTITION_PEER PARTITION_PEER PARTITION_IDENTIFIER PARTITION_IDENTIFIER A partition peer is a pair of distinct partition names The list order is not important Of course the designated partitions have to be declared prior to the channel itself A Channel Channel Partition 1 Partition 2 This gives the link between partitions Partition_1 and Partition 2 the name A Channel It is not possible to declare more than one channel between the same two partitions 3 5 23 Partition and Channel Attribute Filter GLADE contains a transparent extensible filtering mechanism that allows the user to define various data transformations to be performed on the arguments and re
85. own or else a unique number Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work In the combination you must combine any sections entitled History in the various original documents forming one section entitled History likewise combine any sections entitled Acknowledgements and any sections entitled Dedications You must delete all sections entitled Endorsements Heading 6 COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License and replace the individual copies of this License in the various documents with a single copy that is included in the collection provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects You may extract a single document from such a collection and distribute it individually under this License provided you insert a copy of this License into the extracted document and follow this License in all other respects regarding verbatim copying of that document 7 AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works in or on a volume of a storage or distribution medium does not as a whole count as a Modified Version of the Document provided no compilation copyright is claimed for the compilation Such a c
86. packages are on the same partition Partition_1 and that RemPkgl1 is on partition Partition_2 with RemPkgi Ada Exceptions use Ada Exceptions package body RemPkg2 is procedure Subprogram is begin RemPkg1 Subprogram exception when E others gt Raise_Exception Exception_Identity E Exception_Message E end Subprogram end RemPkg2 20 GLADE User s Guide with Internal Ada Exceptions use Ada Exceptions package body RemPkg1 is procedure Subprogram is begin Raise_Exception Internal Exc Identity Message end Subprogram end RemPkg 1 with Ada Text IO Ada Exceptions use Ada Text IO Ada Exceptions with RemPkg2 Internal procedure RemExcMain is begin RemPkg2 Subprogram exception when E Internal Exc gt Put_Line Exception_Message E Output Message end RemExcMain When RemPkg1 Subprogram on Partition_1 raises Internal Exc this exception is prop agated back to Partition_2 As Internal Exc is not defined on Partition_2 it is not possible to catch this exception without an exception handler when others When this exception is reraised in RemPkg1 Subprogram it is propagated to Partition 1 But this time Inter nal Exc is visible and can be handled as we would in a single partition Ada program Of course the exception message is also preserved 2 7 4 Pragma Asynchronous By default a remote call is blocking the caller waits until the remote call is complete and the output stream is received
87. partition_2 gt lt partition_3 gt 3 4 Gnatdist Behind the Scenes Here is what goes on behind the scenes in gnatdist when building a distributed appli cation e Each compilation unit in the program is compiled into an object module as for non distributed applications This is achieved by calling gnatmake on the sources of the various partitions e Stubs and skeletons are compiled into object modules these are pieces of code that allow a partition running on machine A to communicate with a partition running on machine B Several timestamp checks are performed to avoid useless code recompila tion and stub generation e gnatdist performs a number of consistency checks For instance it checks that all packages marked as remote call interface RCI and shared passive SP are mapped onto partitions It also checks that a RCI or SP package is mapped onto only one partition e Finally the executables for each partition in the program are created The code to launch partitions is embedded in the main partition except if another option has been specified see undefined Pragma Starter page undefined In this case a shell script or nothing is generated to start the partitions on the appropriate machines This is specially useful when one wants to write client server applications where the number of instances of the partition is unknown 3 5 The Configuration Language The configuration language is Ada like As the capabilities of
88. partitions which can be ex ecuted concurrently on the same machine or and this is the interesting part can be dis tributed on a network of machines The way in which partitions communicate is described in Annex E of the Ada 95 reference manual A partition is a set of compilation units that are linked together to produce an executable binary A distributed program comprises two or more communicating partitions The Distributed Systems Annex DSA does not describe how a distributed application should be configured It is up to the user to define what are the partitions in his program and on which machines they should be executed The tool gnatdist and its configuration language allows the user to partition his program and to specify the machines on which the individual partitions are to execute gnatdist reads a configuration file whose syntax is described in section undefined The Configuration Language page undefined and builds several executables one for each partition It also takes care of launching the different partitions default with parameters that can be specific to each partition 3 2 How to Configure a Distributed Application e Write a non distributed Ada application to get familiar with the GLADE environment Use the categorization pragmas to specify the packages that can be called remotely e When this non distributed application is working write a configuration file that maps the user categorized packages onto sp
89. ramming language with the addition of distributed features The distributed object paradigm provides a more object oriented approach to programming distributed systems The notion of a distributed object is an extension to the abstract data type that allows the services provided in the type interface to be called independently of where the actual service is executed When combined with object oriented features such as inheritance and polymorphism distributed objects offer a more dynamic and structured computational environment for distributed applications The Distributed Systems Annex DSA of Ada95 defines several extensions that allow the user to write a distributed system entirely in Ada The types of distributed objects the services they provide and the bodies of the remote methods to be executed are all defined in conventional Ada packages The Ada95 model is analogous the Java RMI model In both languages the IDL is replaced by well defined language constructs Therefore the language 1 ESTEREL is an imperative synchronous language designed for the specification and the development of reactive systems Chapter 1 Introduction to Distributed Systems 5 supports both remote procedure calls and remote object method invocations transparently and the semantics of distribution are consistent with the rest of the language A program written in such a language is intended to communicate with a program written in the same language but this appar
90. rtition_3 Any attempt to reconnect to Partition 3 when this partition is dead will be blocked until Partition_3 restarts By default any restart is rejected Reject On Restart Another policy is 50 10 11 12 13 14 15 16 1T 18 19 20 21 22 GLADE User s Guide to raise Communication Error on any reconnection attempt until Partition 3 has been restarted Line 23 Specify additional arguments to pass on the command line when a given par tition is launched Line 24 Specify a termination mechanism for partition 4 The default is to compute a global distributed termination When Local Termination is specified a partition terminates as soon as local termination is detected standard Ada termination Line 26 Specify the kind of startup method the user wants There are 3 possibilities Shell Ada and None Specifying Shell builds a shell script All the partitions will be launched from a shell script If Ada is chosen then the main Ada procedure itself is used to launch the various partitions If method None is chosen then no launch method is used and the user must start each partition manually If no starter is given then an Ada starter will be used In this example Partition 2 Partitions 3 and Partition 4 will be started from Parti tion 1 ie from the Ada procedure Master Procedure Line 30 Specify the use of a particular boot server It is especially useful when the de fault port use
91. s that a Remote_Types package can make visible in its specification only Remote_Types Shared_Passive and Pure units 2 8 Partition Communication Subsystem 2 8 1 Marshaling and Unmarshaling Operations The Partition Communication Subsystem PCS is the runtime library for distributed features It marshals and unmarshals client and server requests into a data stream suitable for network transmission type Params_Stream_Type Initial_Size Ada Streams Stream_Element_Count is new Ada Streams Root_Stream_Type with private This type is a container for the data to be transmitted between partitions Its root is Root_Stream_Type which defines the basic stream type and two abstract operations Write and Read Its purpose is to insert remove objects of type Stream_Element_Array which are array of bytes representing a particular data Streams are read and written using four attributes e Write write an element into a stream valid only for constrained types e Read read a constrained element from a stream e Output same as Write but write bounds and discriminants as well if needed 26 GLADE User s Guide e Input same as Read but read bounds and discriminants from the stream the Input attribute denotes a function An Ada compiler provides default Read and Write operations But it is up to the implementation of the PCS to provide default Read and Write to ensure proper operation between heterogeneous architectures see undefin
92. t to the end of the list of Cover Texts in the Modified Version Only one passage of Front Cover Text and one of Back Cover Text may be added by or through arrangements made by any one entity If the Document already includes a cover text for the same cover previously added by you or by arrangement made by the same entity you are acting on behalf of you may not add another but you may replace the old one on explicit permission from the previous publisher that added the old one The author s and publisher s of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version 5 COMBINING DOCUMENTS You may combine the Document with other documents released under this License under the terms defined in section 4 above for modified versions provided that you include Appendix B GNU Free Documentation License 71 in the combination all of the Invariant Sections of all of the original documents unmodified and list them all as Invariant Sections of your combined work in its license notice The combined work need only contain one copy of this License and multiple identical Invariant Sections may be replaced with a single copy If there are multiple Invariant Sections with the same name but different contents make the title of each such section unique by adding at the end of it in parentheses the name of the original author or publisher of that section if kn
93. t us take the following example package Node2 is pragma Remote Call Interface procedure Send X Integer pragma Asynchronous Send end Node2 package body Node2 is V Integer 0 procedure Send X Integer is begin V X end Send end Node2 package Node1 is pragma Remote_Call_Interface procedure Send X Integer pragma Asynchronous Send end Node1 with Node2 package body Node1 is procedure Send X Integer is begin Node2 Send X end Send end Node1 with Node1 Node2 procedure NonDeterministic is begin Chapter 2 The Distributed Systems Annex 23 Nodei Send 1 Node2 Send 2 end NonDeterministic Let us say that Main is configured on Partition 0 Nodel on Partition 1 and Node on Partition 2 If Nodel Send and Node2 Send procedures were synchronous or if no latency was introduced during network communication we would have the following RPC order Main remotely calls Nodel Send which remotely calls Node2 Send which sets V to 1 Then Main remotely calls Node2 Send and sets V to 2 Now let us assume that both Send procedures are asynchronous and that the connection between Partition 1 and Partition 2 is very slow The following scenario can very well occur Main remotely calls Nodel Send and is unblocked Immediately after this call Main remotely calls Node2 Send and sets V to 2 Once this is done the remote call to Nodel Send completes on Partition_1 and it remotely calls Node Se
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95. the user may also require a number of more complex services to develop his distributed appli cation Some of these services are indispensable for example a location service that allows clients to reference remote services via higher level names instead of a traditional scheme for addressing remote services that use Internet host addresses and communication port numbers Other services provide domain independent interfaces that are frequently used by distributed applications If we return to the multi thread programming comparison the middleware solution is close to what a POSIX library or a language like Esterel would provide for developing concurrent applications A middleware framework like DCE is close to a POSIX library in terms of abstraction levels Functionalities are very low level and very complex CORBA is closer to Esterel in terms of development process The control part of the application can be specified in a description language The developer then has to fill in automatically generated source code templates stub and skeletons to build the computational part of the application The distribution is a pre compilation process and the distributed boundaries are always explicit Using CORBA the distributed part is written in IDL and the core of the application is written in a host language such as C 1 3 Using a Distributed Language Rather than defining a new language like the CORBA IDL an alternative is to extend an existing prog
96. the other partitions The remote shell used as a default is determined during GLADE configuration and installa tion It is either rsh remsh or the argument passed to with rshcmd ARG The pragma Remote_Shell allows the user to override the default Chapter 3 Getting Started With GLADE 39 PRAGMA pragma Remote_Shell Command gt STRING LITERAL Options gt STRING LITERAL The Command parameter indicates the name of the remote shell command name and the Options parameter corresponds to the additional flags to pass to the remote shell command 3 5 9 Pragma Boot Location When a partition starts executing one of the first steps consists in a connection to the boot partition where the boot server is located see undefined The GARLIC PCS page undefined This pragma provides one or several locations in order to get a connection with the boot partition PRAGMA PRAGMA WITH NAME AND DATA PRAGMA WITH LOCATION PRAGMA WITH LOCATION LIST PRAGMA WITH NAME AND DATA pragma Boot Location Protocol Name gt STRING LITERAL Protocol Data STRING LITERAL PRAGMA WITH LOCATION pragma Boot Location Location gt LOCATION PRAGMA WITH LOCATION LIST pragma Boot Location Locations gt LOCATION LIST This boot server location can be concatenated into a single string to be used as a com mand line option or an environment variable see undefined Partition Comm
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98. to develop such an application using GLADE we shall describe how to do this in the following sections 8 GLADE User s Guide 2 1 Architecture of a Distributed Ada95 Application A distributed system is an interconnection of one or more processing nodes and zero or more storage nodes A distributed program comprises one or more partitions A partition is an aggregate of library units Partitions communicate through shared data or RPCs A passive partition has no thread of control Only a passive partition can be configured on a storage node An active partition has zero or more threads of control and has to be configured on a processing node The library unit is the core component of an Ada95 distributed application The user can explicitly assign library units to a partition Partitioning is a post compilation process The user identifies interface packages at compile time These packages are categorized using pragmas Each of these pragmas supports the use of one of the following classical paradigms e Remote subprograms For the programmer a remote subprogram call is similar to a regular subprogram call Run time binding using access to subprogram types can also be used with remote subprograms These remote subprograms are mostly declared in library units categorized as remote call interface RCI e Distributed objects Special purpose access types can be defined which designate re mote objects When a primitive dispatching operation is invok
99. turn values of remote calls One possible application is to compress all data before sending it and to decompress it on the receiving partition With GLADE it is no longer necessary for the application to take care of such trans formations Instead users can write their own data transformations and hook them into GLADE so that they are automatically and transparently applied depending on the config uration of the distributed application By default no filtering is performed by GLADE even though the compression filter is always available The user can choose to configure his distributed application to use this filter In order to define filtering one must first declare the channels between the partitions of an application Once a channel is defined the data transformation that is to be applied on all data sent through it can be specified 46 GLADE User s Guide A Channel Channel Partition 1 Partition 2 for A_Channel Filter use ZIP This specifies that all data sent over this channel should be transformed by the filter named ZIP There must be a filter with this name implemented in the package Sys tem Garlic Filters Zip It may also be useful to specify that a partition use a certain filter for all remote calls regardless of the channel i e regardless of the partition that will receive the remote call This can be specified using the attribute Filter on a partition for Partition 1 Filter use ZIP
100. type Balance Type is access function Customer in Customer Type Password in Password Type return Integer procedure Register Balance in Balance Type function Get Balance return Balance Type Other services end RASBank In the code below a mirroring bank registers its services to the central bank with Types use Types package MirrorBank is 12 GLADE User s Guide pragma Remote_Call_Interface function Balance Customer in Customer_Type Password in Password_Type return Integer Other services end MirrorBank with RASBank Types use RASBank Types package body MirrorBank is function Balance Customer in Customer_Type Password in Password_Type return Integer is begin return Something end Balance begin Register a dynamically bound remote subprogram Balance through a statically bound remote subprogram Register Register Balance Access Register other services end MirrorBank In the code below a central bank client asks for a mirroring bank and calls the Balance service of this bank by dereferencing a remote access type with Types use Types with RASBank use RASBank procedure BankClient is B Integer C Customer Type rich P Password Type xxxx begin Through a statically bound remote subprogram Get Balance get a dynamically bound remote subprogram Dereference it to perform a dynamic invocation B
101. unication Subsystem The communication subsystem is one of the key points of a distributed system it offers basic services such as the capability to transmit a message from one part of the distributed program to another Those elementary services are then used by higher level services to build a fully functional distributed system The limit between what belongs to the communication subsystem and what belongs to an external service may sometimes be difficult to draw Moreover something considered as a service in CORBA may be viewed as purely internal in DSA A 3 1 DSA PCS In the DSA world everything that is not done by the compiler in regard to the distribu tion belongs to the partition communication subsystem PCS For example figuring out on which partition a package that will be called remotely is located is part of the PCS s responsibility The PCS entry points are well defined in DSA and described in the System RPC package declaration By looking at this package one can notice that there is nothing related to abortion of remote subprogram calls although the Annex states that if such a call is aborted an abortion message must be sent to the remote partition to cancel remote processing That means that the PCS is in charge of detecting that a call to one of its entry points has been aborted and must send such an abortion message without any help from the compiler Another interesting characteristic of the PCS is its behavior regar
102. ure PROCEDURE_IDENTIFIER REPRESENTATION_CLAUSE for PARTITION_IDENTIFIER Main use PROCEDURE_IDENTIFIER 3 5 7 Pragma Starter As a default the main executable is a full Ada starter procedure That means that it launches all the other partitions from an Ada program The pragma Starter allows the user to ask for one starter or another When the partition host is not statically defined see undefined Partition Attribute Host page undefined the starter subprogram will ask for it interactively when it is executed CONVENTION_LITERAL Ada Shell None PRAGMA pragma Starter Convention gt CONVENTION LITERAL e The default method consists in launching partitions from the main partition Ada sub program using a remote shell see below e The user may ask for a Shell script that starts the different partitions one at a time on the appropriate remote machines using a remote shell As the Ada starter the Shell script starter ask for partition hosts interactively when a partition host is not already defined Having a textual shell script allows the user to edit it and to modify it easily e The user may ask for a None starter In this case it is up to the user to launch the different partitions The user may have to provide on the command line the boot server location see undefined The GARLIC PCS page undefined 3 5 8 Pragma Remote Shell When pragma Starter is Ada or Shell the main partition launches
103. uted program is not to be confused with a distributed application composed of only one program The later is built with the help of the configuration tool and includes the communication library Once the non distributed version of the program is complete it has to be configured into separate partitions This step is surprisingly simple compared to that of developing the application itself The configuration step consists in mapping sets of compilation units into individual partitions and specifying the mapping between partitions and nodes in the computer network This mapping is specified and managed by means of GLADE The distributed version of the user application should work as is but even when a pro gram can be built both as a non distributed or a distributed program using the same source code there may still be differences in program execution between the distributed and non distributed versions These differences are discussed in subsequent sections see undefined Pragma Asynchronous page undefined and undefined Pragma All_Calls_Remote page undefined Developing a non distributed application in order to distribute it later is the natural approach for a novice Of course it is not always possible to write a distributed application as a non distributed application For instance a client server application does not belong to this category because several instances of the client can be active at the same time It is very easy
104. ved from the first interface and extends the list of its primitive operations to achieve inheritance from other interfaces Another unnatural feature of CORBA for an Ada programmer comes from forward declarations In Ada two package specifications cannot with each others but this can occur between two IDL interfaces To solve this the mapping can create forward packages This can result in a very non intuitive situation where the client stub does not with its usual interface packages but withs forward packages instead When developing a distributed application with CORBA two situations can arise On the server side the programmer is responsible for the IDL file He has to understand the Ada95 language mapping in order to avoid structures with a non trivial implementation whenever possible such as forward declaration and multiple inheritance On both the server and the client side the programmer has to deal with the generated code A good understanding of the mapping is useful to get back and forth from the IDL file to the generated code in order to keep an overview of the distributed application Understanding this mapping can be a tedious task depending of the host language IDL interface information can be stored on line in a database called Interface Repository IR A CORBA specification describes how the interface repository is organized and how 62 GLADE User s Guide to retrieve information from it The reader w
105. vers and clients to interact through asyn chronous events between anonymous objects A supplier produces events when a consumer receives notification and data An event channel is the mediator between consumers and suppliers consumer admins and supplier admins are in charge of providing prozies to allow consumers and suppliers to get access to the event channel Suppliers and consumers pro duce and receive events through their associated proxies From the event channel point of view a proxy supplier or prozy consumer is seen as a consumer or a supplier Therefore a proxy supplier or proxy consumer is an extended interface of consumer or supplier The Events service defines push and pull methods to exchange events This allows to define four models to exchange events and data A 5 Some Elements of Comparison CORBA provides an outstanding and very popular framework The IDL syntax is close to C The object model is close to Java CORBA defines only distributed objects Fur thermore when using the Ada mapping the stub and skeleton generated code is close to Java with two root classes Ref for clients and Object for servers DSA provides a more general model This includes distributed objects but also regular remote subprograms and references to remote subprograms Shared passive packages can be defined as an abstraction for a distributed shared memory a persistency support or a database Basically the IDL is a subset of Ada95 and th
106. wWorker Q Integer N Notify private type NewNewWorker is new NewWorker with record NewField Field Type Other fields end record end NewNewWorkers In the following code we define a unique place where workers wait for jobs Work erCity is a Remote Call Interface package with services to hire and free workers Unlike Remote Types packages Remote Call Interface packages cannot be duplicated and are as signed to one specific partition with Common use Common Chapter 2 The Distributed Systems Annex 31 package WorkerCity is pragma Remote Call Interface procedure Insert W in Any Worker procedure Remove W out Any Worker end WorkerCity In order to use even more DSA features Factory is defined as a generic RCI package see sample above Any instantiation defines a new factory see sample above To be RCI this instantiation has to be categorized once again with Storage use Storage generic package Factory is pragma Remote Call Interface procedure Notify Q Integer pragma Asynchronous Notify end Factory with Factory package NewFactory is new Factory pragma Remote Call Interface NewFactory 32 GLADE User s Guide Chapter 3 Getting Started With GLADE 33 3 Getting Started With GLADE This chapter describes the usual ways of using GLADE to compile Ada distributed programs 3 1 Introduction to GLADE An Ada 95 distributed application comprises a number of
107. will automatically terminate your rights under this License However parties who have received copies or rights from you under this License will not have their licenses terminated so long as such parties remain in full compliance 10 FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new revised versions of the GNU Free Doc umentation License from time to time Such new versions will be similar in spirit to the present version but may differ in detail to address new problems or concerns See http www gnu org copyleft Each version of the License is given a distinguishing version number If the Document specifies that a particular numbered version of this License or any later version applies to it you have the option of following the terms and conditions either of that specified version or of any later version that has been published not as a draft by the Free Software Foundation If the Document does not specify a version number of this License you may choose any version ever published not as a draft by the Free Software Foundation ADDENDUM How to use this License for your documents To use this License in a document you have written include a copy of the License in the document and put the following copyright and license notices just after the title page Copyright c YEAR YOUR NAME Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentati
108. xtra information is not needed The feature is activated as soon as the environment variable exists or as soon as the option is passed on the command line The precedence order for specifying a run time parameter is as follows first the the command line option then the environment variable and finally the configuration attribute or pragma A location can be concatenated into a single string to be used as a command line option or an environment variable The formatted string must conform to the nota tion support name support data Most commonly a network location string is tcp machine port that means that the protocol name is tcp the protocol data which is specific to the protocol name is machine port A list of locations can be concatenated into a single string as well Location strings are separated by spaces To be used as a command line option it is possible to quote this string Most commonly a network locations string is tcp lt machine gt lt port1 gt tcp machine port2 3 6 1 Partition Option boot location This option sets the boot server location see undefined Pragma Boot Location page undefined for details Environment Variable Command Line Option Type BOOT LOCATION boot location Formatted String 52 GLADE User s Guide The formatted string must conform to the location notation see undefined Loca tion Declaration page undefined and undefined Partition C
109. yorb requires a working PolyORB setup Please refer to the PolyORB User s Guide for complete installation instructions 3 9 3 Heterogeneous System The GNAT environment provides default stream attributes except for non remote access types see undefined Transmitting Dynamic Structure page undefined and undefined Marshaling and Unmarshaling Operations page undefined The implementation of the default attributes of predefined types can be found in System Stream_Attributes s stratt adb The GLADE implementation overloads the GNAT default marshaling and unmarshal ing subprograms with its own subprograms which format data according to a XDR like protocol Therefore any GLADE application will work in an heterogeneous environment If the user wants to keep using the GNAT default attributes for performance purposes or to use another protocol to marshal and unmarshal predefined types he can replace s stratt adb by a more appropriate implementation 3 9 4 Allocating Partition Ids The Partition_ID is allocated dynamically at run time Each partition connects to a Partition ID Server which is located on the boot server and asks for a free Partition_ID The advantage of this approach is that it supports easily client server solution client partitions may be duplicated they will obtain different Partition Ids There is no need to recompile or relink all the partitions when a new partition is added to the system The Partition_

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