Distributed systems
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1. Because CORBA is an industry standard CORBA compliant ORBs from different software vendors can inter operate with each other just as well as ORBs from the same vendor Thus a client and a server do not have to be running the same ORB software 2 CORBA is cross platform Any CORBA compliant ORB on one machine can communicate with any CORBA compliant ORB on another machine regardless of the machine or operating system 3 CORBA is cross language A CORBA compliant ORB allows a program written in say Java to access objects in a program written in say Smalltalk on another machine a This is accomplished by specifying the interface to an object using a language neutral Interface Definition Language IDL b A CORBA compliant ORB includes IDL compilers which translate an IDL specification into appropriate language specific interfaces and implementation skeletons c Note that the Java library includes a package org omg that provides the Java side of the link to a CORBA compliant ORB on the same computer which can then be used to link 7 to ORB s on other computers The org omg package is not an ORB it provides facilities for using an ORB c A major disadvantage of CORBA is that it is complex and CORBA compliant ORBs are fairly expensive d We will not discuss CORBA further Using Microsoft s NET framework a Microsoft s NET framework includes support for distributed applications that replaces an ol2. in the order in which you say it The other person can reply to you over the same connection The network improves on the phone system in that if there are problems with the network the message will still get through in the proper sequence but maybe after some delay c At the implementation level connection based networking is actually implemented by using the datagram mechanism to transmit information and acknowledgments that ensure that message delivery is done reliably D A fundamental notion in IP networking is the notion of a socket A socket is an endpoint for communication between two systems 1 Datagram sockets are used for UDP communication a Rather than go into a lot of detail regarding datagram sockets we Il look at a simple example 10 b To illustrate the language independent nature of IP networking we will use a program written in C on one machine to send a message to another program written in Java on another machine which will simply display the message along with information about where it came from 1 DEMO datagramdemo cc client on moses DatagramDemo java server on laptop Use port 4242 e First demo with no one listening note message is ignored e Then demo with server listening 2 HANDOUT Code excerpts 2 Stream sockets are used for TCP communication a In this protocol a connection is established involving a socket on one system and a socket on another system A stream of byt
3. sent so we close the socket after reading the single line but we could leave it open and receive additional lines if that were the protocol 13 V Using RMI A A significant limitation of low level networking using the facilities of java net is that the communicating programs must assume full responsibility for managing the details of marshaling and unmarshaling parameters to requests RMI provides a much higher level approach It is based on an earlier technology developed by Sun called remote procedure call RPC which has been used with many programming languages In effect RMI is the Java version of RPC B When using RMI the basic idea is that a program is allowed to work with two kinds of objects local objects which exist inside of the memory of the machine that is running the program and remote objects which exist in the memory of some other machine However this distinction is made transparent to the program as follows 1 If the program executes a method of a local object that method is performed by the system running the program 2 If the program executes a method of a remote object the rmi system creates a message which is sent over the network to the system where the object really lives That system then executes the method and sends a message back with any results return value The messages are sent received automatically transparently to the program which references remote objects in the same way it
4. the Socket constructor used in this code attempts to establish a connection at the specified address on the specified port b If a connection is established the Socket thus created becomes one end of the pair of connecting sockets that can be used for communication If a connection cannot be established an exception is thrown c We now create a PrintWriter object from this socket using a wrapper What is printed into the print writer will be transmitted over the network to the corresponding socket on the other end d Note that in this case we send a single line of data before closing the socket and terminating the connection shortly after establishing it We could just as easily send many lines of data over an extended period of time if we wished Back on the Responding System we have seen that when a connection request arrives a new ordinary socket is created a ReceiveDialog box is created to display the message and a new thread is created This thread actually receives the message and displays it in the dialog A separate thread is created for this purpose so the listener thread can go back to listening for other connections HANDOUT Code for receiving message a connectionSocket is the socket that was created when the connection request was accepted b Note how this is wrapped in a BufferedReader that allows us to read line s of input sent by the sender c In this case we know only one line will be
5. the server where it resides 17 E DEMO The RMI version of the registration system 1 The Server will run on a workstation a login to micah b cd cs211 RMIExample RMIExampLeServer build cLasses c rmiregistry amp d cd e java cp build classes controller Controller 2 The client will run on laptop DEMO a cd PROJECTION cs211 RMIExample RMIExampleClient b java cp build classes gui RegistrationLabsGUI micah cs gordon edu c Repeat the above to run a second copy hence a second window d Observe how changes made in one window are visible in the other two copies of the client both accessing the same server e Observe how reports are printed on the server not the client F One thing that is missing from the our discussion thus far is and is not present in the version of the code we are using is synchronization 1 Each time the rmi registry is contacted by a client it creates a separate thread 2 Thus if our GUI is running on two or more different systems accessing the same server two different threads could perform conflicting operations at the same time Oo How should this be done and where should it go ASK Each of the methods that alters the database doEnrol1Q doDrop doGradeC needs to be synchronized Since they all This should be handled by explicit synchronization in the server code share the same controller object this will ensure that two threads don t perfor
6. CS211 Lecture Distributed Systems Remote Method Invocation Deployment Diagrams in UML Last revised November 19 2007 Objectives 1 To introduce the notion of distributed systems including the use of deployment diagrams 2 To introduce the client server and three layer architecture models 3 To introduce the facilities in java net 4 To introduce the facilities in java rmi Materials 1 Projectables from UML User Manual pp 412 414 415 2 Executable code and handout of datagram demo implemented with C sender on jonah datagramdemo cc and datagramdemo and Java receiver DatagramDemo java 3 Handout of excerpts from code for CS112 Encryption lab 4 Projectable of before and after deployment diagrams for converting registration system to RMI plus apparent and actual situation 5 Demonstration of RegistrationLabs reimplemented as a client server system using java rmi facilities Run server on micah two copies of client on laptop project all three on laptop L Introduction A In our discussion of threads we introduced the idea of breaking up a computation being done on a single system into multiple concurrent threads that execute in parallel on the same computer We now pursue that idea further to consider distributed systems systems in which computation is spread over multiple physical sites 1 The term distributed system is actually used in both a broad and a narrow sense a In the bro
7. ad sense it refers to any system in which computation is spread out over two or more physically distinct processors The processors may be located in the same room or may be on opposite sides of the world b In the narrow sense it refers to what are sometimes called fully distributed systems in which information and computation is physically distributed in such a way that even if one physical site fails the computation can continue though at reduced speed 1 2 In this lecture we will deal with distributed systems in the broad sense of the term In particular we will discuss systems built using the Client Server model B In the bulk of this lecture we will focus on mechanisms available in Java and other languages for allowing client software running on one machine to interact with server software running on another machine C First however we want to spend a little bit of time looking at a type of UML diagram the Deployment diagram that can be used to show how components of any system are physically deployed If UML Deployment Diagrams A The major components of a deployment diagram are nodes symbols representing either a processor or some significant piece of hardware that the system utilizes 1 The standard symbol for a node is a cube 2 Nodes representing specialized pieces of hardware can be represented by specialized symbols as we shall see shortly 3 Nodes are connected by associations
8. der now deprecated Microsoft technology known as DCOM Distributed Component Object Model b In some respects NET was developed to compete with Sun s Enterprise Edition java though it can be used with Microsoft s clone of Java c Although the open source community is developing versions of the NET framework for non Microsoft platforms at this point it is largely a proprietary technology d We will not discuss NET further Using Java RMI Remote method invocation a RMI provides capabilities similar to those provided by an ORB but only to Java programs b A major advantage of RMI for us is that it is integrated into the Java language and libraries c A major disadvantage of RMI is that it is integrated into the Java language and libraries d We will discuss RMI in some detail later in this lecture because it is readily available to us If you understand RMI you will find it easier to understand approaches like CORBA because there are significant similarities though obviously there are important differences as well IV Low Level Networking Using the java net Package A The java net package contains classes that allow a Java programs on one machine to communicate with another program on another machine using industry standard TCP IP protocols Note that the other program does not have to be written in Java B Our goal is not to cover this package in depth but rather to give something of the flavor
9. ementation class 1 A stub name implementation classname_Stub 2 A skeleton name implementation classname_Skel EXAMPLE In the above case we would run the rmic compiler on Controller class to create two new classes ControLLer_Stub class and Controller_Skel class Note that rmic compiles from class file to class file there is no java source code involved b The skeleton file is used by the server and the stub file is used by the client The stub is downloaded to the client automatically the first time it uses a remote object of the class c The roles of the stub and skeleton can be pictured as follows 16 CLIENT SYSTEM SERVER SYSTEM Local client i Local server object s j object s Local client Local server object s object s result operation Stub object Skeleton object ACTUAL SITUATION PROJECT D What needs to happen when the program is run 1 Before running any rmi server program it is necessary to run a program on the server called rmiregistry This program manages the task of establishing initial access to a remote object 2 When the server program starts up it must include a call to a method called Naming bindCname object or Naming rebindCname object which places a reference to a remote object in the registry 3 When a client program starts up it must include a call to a method called Naming LookupCurlL which accesses the remote object by invoking the registry on
10. ent diagram is not required for the VideoStore example system as specified 1 The system we have been designing thus far has been designed presumably but perhaps unrealistically to run on a single computer 2 Suppose however that we were to design it as a distributed system perhaps with several clerk consoles computers and perhaps some kiosks that customers can use to make inquiries What might such a system look like ASK DEVELOP A DEPLOYMENT DIAGRAM AS A CLASS Some options to consider a Server perhaps a separate system b Manager console perhaps a separate system or server system could double in this capacity 3 Note for purposes of the project do not attempt a distributed implementation MI Inter System Communication A We now move from a general consideration of distributed systems to a specific consideration of how participating systems communicate with each other Since we are using Java in this course we will deal with mechanisms available to programs written in Java however most will be usable with other languages as well B The specific question we want to explore then is how does a program running on one computer go about communicating with a program running on another computer 1 We assume that the computers are connected via some sort of network either a local area network or the Internet 2 We assume that information is transmitted over the network using TCP IP the basic pr
11. es written into one socket can be read from the other socket and vice versa A pair of sockets supports two way communication 1 A tin can telephone of the type you may have used as a kid is a good model for a pair of connected sockets 2 Note that sockets support communication at the level of streams of bytes More complex data numbers objects etc must be reduced down to a stream of bytes in order to be sent and must be converted back to the appropriate type of entity at the other end This process is known as marshaling and unmarshaling b TCP IP incorporates the notion of a server socket which is a special kind of socket that waits for connection requests from another system 1 When an appropriate connection request is received a socket is created on each system and the two sockets are connected to each other 2 The only role of the server socket is to initiate the connection the same server socket can be used to initiate any number of connections 11 3 Typically server programs create a single server socket which is used to receive incoming connection requests When a connection request is received the resulting socket is given to a separate thread to carry out the requested service while the server socket and its thread remain available to handle new requests c Again rather than discussing this protocol in detail we will look at an example in this case a lab you did i
12. ine a concrete class that implements the interface a On the server side objects that can be accessed remotely will be instances of this class b Typically the implementation class is made a subclass of java rmi server UnicastRemoteObject This class provides some facilities needed by remote objects which they inherit by being a subclass of it EXAMPLE Suppose we wanted to use rmi to convert the registration system you used in earlier labs into a client server version a The following deployment diagrams show the structure of the original version and the new distributed version Registration System Deploys b Original controller Controller gui RegistrationGUI gui RegController model RegistrationModel model Course model Student model EnrollediIn PROJECT 15 Distributed PROJECT Registration Client LAN Registration Server Deploys Deploys controller Controller gui RegistrationGUI gui RegController gui RegController model RegistrationModel model Course model Student model EnrollediIn c What class shows up on both the client and the server sides ASK gui RegController the interface that defines the behavior of the controller 5 After compiling the client and server software in the normal way we need an additional step of running a program called rmic rmi compiler on each of the implementation class files a This creates two additional class files for each impl
13. m an operation that alters the database at the same time 18
14. n CS112 E Example The CS112 Encryption lab 1 Deployment diagram Z LAN L Initiating System Responding System Deploys Lab8 amp Note that in this case both systems run identical code any system can initiate communication with any other system as you recall from lab Deploys Lab8 amp 2 When the program starts up it starts a connection listener thread that listens for connection requests from another system This happens on both systems but our interest is in the one created on the Responding System HANDOUT Encryption lab code excerpts discuss connection listener section a A ServerSocket is a socket that can accept connection requests from another system b Its acceptC method blocks until another system attempts to connect This is why we need a separate thread for this the thread spends most of its life waiting for a connection request c When a connection request is received accept returns an ordinary socket that represents one end of the connection that was established Communication can take place with the other system through this socket until one end or the other or both closes the connection 12 Meanwhile on the Initiating System clicking the Send button causes a SendDialog box to be created When the dialog is all filled in and the user requests that a message be sent the following code is executed HANDOUT message send section a The form of
15. of what s involved when programs communicate at this level if nothing else to motivate the more abstract approaches C TCP IP networking defines several different models of communication of which two are particularly important 1 Connectionless communication using datagrams This is technically called UDP the user datagram protocol a In this model a machine can send a message to another system called a datagram without first establishing a connection with that machine b The network makes a good faith effort to deliver the message correctly but there is no guarantee that the message will not be lost in transit corrupted or even delivered twice c Further if one machine sends several datagrams to another machine there is no guarantee that the datagrams will arrive in the same order in which they were sent d If the receiving machine wishes to reply it may send a message back to the first machine But this is treated as a separate message in its own right 2 Connection oriented communication using streams This is technically called TCP transmission control protocol Note that in the acronym TCP IP IP stands for the underlying network protocol and TCP for the commonly used stream protocol built on top of it UDP is actually an alternative to TCP also built on IP a In this model a client first establishes a connection to a server b While the connection remains open i e until either machine closes i
16. otocol used by the Internet and many LAN s Certainly other networking protocols can be used but the mechanisms built into the Java libraries assume TCP IP 3 In the Client Server model the communication is initiated by a client program wishing to communicate with a server program However much of what we say here is applicable to other models as well C There is more than one possible answer to this question Each possible answer has advantages and disadvantages 5 1 A low level approach using TCP IP services directly through facilities in the package java net a Advantages most flexible b Disadvantages makes programmer responsible for managing a lot of details c We will discuss this approach in some detail later so that you will be familiar with it 2 Using a technology connected to web pages a For example a technology called CGI is supported by most web servers 1 In brief the web server recognizes a URL having a certain format as being a CGI request rather than an ordinary page request Each type of web server has its own rules for the format of CGI requests 2 When the web server receives such a request it runs a program whose name is typically specified as part of the request The request may also specify parameters to be sent to the program 3 The program creates a web page on the fly which the server then sends to the client as the response to its request b There are numer
17. ous similar systems such as servlets little Java programs that can execute inside a server application Microsoft ASP php etc c Advantage supported by existing web servers and browsers d Disadvantage really designed for this specific purpose Not a general purpose mechanism for letting two programs communicate with each other A Java program could pretend to be a web browser and could communicate with another program through a web server on the server system but this is a rather roundabout approach e We will not discuss CGI or similar systems further 3 Using an object request broker ORB a An object request broker is a piece of software that needs to be running on both machines It allows a client program running on one machine to invoke methods of an object in a program running on a server machine In effect once access to a remote object has been established a client program can interact with the remote object the same way it interacts with a local object the location is not a factor in how it uses an object The remote object looks just like a local object to the program using it 1 There is an industry standard for ORBs called CORBA Common Object Request Broker Architecture 2 Thus when we talk about using an ORB we usually mean using an ORB that complies with the CORBA standards b The major advantage of CORBA is that it is a cross language cross platform industry standard 1
18. previous lectures Note that in this case we are depicting hardware not the software that controls the hardware E g in previous discussions we have used a class CardReader to represent the software that controls the physical device that reads ATM cards In this diagram that class is part of the software that is deployed on the processor that controls the system and the card reader symbol stands for the physical hardware device it controls ASK FOR A VOLUNTEER TO PRODUCE SUCH A DRAWING ON THE BOARD D Client Server Systems 1 The following example appears in the UML User s Manual depicting a human resource Client Server system which can be accessed from PC s and dedicated information kiosks PROJECT UML User Manual p 414 2 Example Gordon s WebAdvisor system Lax S computer webera webera Browser College LAN HOPE Deploys Registration system L S computer KTA KTA Browser Note how the network connecting the advisor computers to the registration system is depicted 3 Example Our ATM system viewed now as a node connected to the bank s computer ASK FOR A VOLUNTEER TO PRODUCE SUCH A DRAWING ON THE BOARD E Fully Distributed Systems Example PROJECT UML User Manual page 415 Note how the Internet is depicted potentially connecting any console to any regional server while direct hard wired connections between specific servers are shown as associations F A Deploym
19. references local objects 3 The initial setup to use RMI in a program is a bit complex but once it is set up the program author can proceed without further consciousness of it We will first consider what needs to be done as the software is being developed and then what needs to happen when it is run C What needs to be done during software development 1 Identify remote classes i e classes whose objects will live on the server side but whose methods may be invoked on the client side 2 For each such class it is necessary to define an interface that spells out what methods the class makes available remotely The class may define some methods that are only available locally these are omitted from the interface a The interface must be made a sub interface of java rmi Remote 14 b Each method in the interface must be declared as being capable of throwing java rmi RemoteException This class of exception represents exceptions that can arise as a direct consequence of using an object remotely e g various sorts of communication problems that can happen on a network c On the client side remote objects will be instances of this interface d When the client calls methods of a remote interface it must include appropriate code for dealing with the possibility of a RemoteException the one place where the client code needs to differ for a remote object as opposed to a local object It is also necessary to def
20. representing the flow of information back and forth between them 4 We may also explicitly show the network which allows the nodes to communicate if they communicate via a general purpose network rather than via dedicated hard wired connections 5 Note that in contrast to other diagrams we have drawn in a Deployment diagram the symbols represent hardware not software The software is deployed on one or more processors depicted by node symbols B Deployment diagrams can be used to depict several different types of systems 1 Embedded Systems 2 Client Server Systems 3 Fully Distributed Systems C Embedded Systems 1 An embedded system is one in which a computer is embedded in some piece of physical equipment to control it e g a home appliance a car or a chemical plant 2 The following example appears in the UML User Manual depicting an embedded system controlling an autonomous robot PROJECT UML User Manual page 412 Note that there is only one processor the remaining nodes are specialized hardware controlled by software in the processor Note also the used of specialized symbols for these nodes chosen ad hoc to help the reader understand the diagram These are not standard UML symbols Oo We could depict our ATM example system using such a diagram to depict the various specialized pieces of hardware in relationship to the controlling processor that runs the software we have discussed in
21. t either machine may send messages to the other machine c The network guarantees correct accurate delivery of the messages 9 d Further if one machine sends several messages to another machine they are guaranteed to appear to arrive in the same order as they were sent A later message that arrives before an earlier one is held by the TCP software at the receiving end until the earlier one arrives e The receiving machine can reply to a message over the same connection on which it was sent 3 There are significant analogies between these two models and more familiar communication mechanisms a Connectionless networking using datagrams is similar to postal mail When you mail a letter to another person the Postal Service makes a good faith effort to deliver it intact However there is no guarantee that the letter will not be lost or damaged in transit and if you send several letters to the same destination they may arrive in a different order than the order in which you sent them One thing that the US Postal Service cannot do that the network can is to duplicate messages b Connection based networking streams is similar to the use of a telephone Before you can communicate with someone else via telephone you must first establish a connection by placing a call and having it answered Absent serious problems with the phone system what you say is heard accurately and completely by the other person and
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