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1. Shishir Gundavaram CGI Programming on the World Wide Web O Reilly amp Associates Inc 2000 Nils Klarlund and Anders M ller MONA Version 1 4 User Manual BRICS Notes Series NS 01 1 Department of Computer Science University of Aarhus January 2001 David A Ladd and J Christopher Ramming Programming the Web An application oriented language for hypermedia services In 4th Intl World Wide Web Conference 1995 Open Market Inc FastCGI A high performance Web server interface April 1996 Technical White Paper www fastcgi com Flemming Nielson Hanne Riis Nielson and Chris Hankin Principles of Pro gram Analysis Springer 1999 Dave Raggett Arnaud Le Hors and Ian Jacobs HTML 4 01 specification W3C Recommendation December 1999 http www w3 org TR htm1401 Anders Sandholm and Michael I Schwartzbach Distributed safety con trollers for Web services In Fundamental Approaches to Software Engineering FASE 98 number 1382 in LNCS 1998 Anders Sandholm and Michael I Schwartzbach A type system for dynamic Web documents In Principles of Programming Languages POPL 00 ACM 2000 Sun Microsystems Java Servlet Specification Version 2 3 http java sun com products servlet Sun Microsystems JavaServer Pages Specification Version 1 2 http java sun com products jsp Wolfgang Thomas Automata on infinite objects In Handbook of Theoretical Computer Science volume B pages 133 191 MIT Press Elsevi
2. record score touch HiScore else show lt html gt Sorry no record lt html gt Here the high score document is only regenerated when a player beats the record This code is clearly easier to understand and maintain than the corresponding expanded code The expressive power of syntax macros is extended with a concept of metamor phisms as explained in 10 This declaratively permits tree structures to be trans formed into host language syntax without compromising syntactic safety something not possible with other macro languages Using this mechanism in an extreme way it is possible to define whole new languages We call this concept a very domain specific language or VDSL At the University of Aarhus undergraduate Computer Science students must complete a Bachelor s degree in one of several fields The requirements that must be satisfied are surprisingly complicated To guide students towards this goal they must maintain a so called Bachelor s contract that plans their remaining studies and discovers potential problems This process is supported by a Web service that for each student iteratively accepts past and future course activities checks them against all requirements and diagnoses violations until a legal contract is composed This service was first written as a straight lt bigwig gt application but quickly be came annoying to maintain due to constant changes in the curriculum Thus it was redesig
3. co welcome to who 7 what lt body gt color lt head gt lt title gt Hi lt title gt lt head gt Te SS ee lt body bgcolor 9966f I gt 9966ff who l Stranger j a Oi a BRICS lt body gt Stranger BRICS Figure 3 Building a document by plugging into template gaps The construction starts with the five constants on the left and ends with the complete document on the right languages permit essentially no static guarantees or work separation Furthermore with the script centered solutions the HTML must often be constructed in a linear fashion from top to bottom instead of being composed from components in a more logical manner The lt bigwig gt solution provides the best from the two worlds Higher order HTML templates as first class values are in practice as flexible as print statements and still the MAWL benefits are preserved We define DynDoc as the sub language of lt bigwig gt that deals with document construction that is the control structures HTML template constants variables and assignments plug operations and show receive statements Template con stants are delimited by lt html gt lt htm1 gt Gaps are written with special lt gt tags Special attribute gaps can be used in place of attribute values as shown in the example below Of course o
4. session Hello show lt html gt Hello World lt html gt fancy hello html The compiler retrieves the indicated file and uses its contents in place of the con stant provided it exists and contains well formed HTML In this manner the pro grammer can use plain versions of the templates while a graphical designer simul taneously produces fancy versions The compiler checks that the two versions have the same gaps and fields In order to accommodate the use of HTML authoring tools we permit gaps to be specified in an alternative syntax using special tags The DynDoc sub language was introduced in 26 where it is also shown how this template model can be implemented efficiently with a compact runtime representa tion The plug operation takes only constant time and showing a document takes time linear in the size of the output Also the size of the runtime representation of a document may be only a fraction of its printed size For example a binary tree of height n shown earlier has a representation of size O n rather than O 2 Analysis of Template Construction and Form Input We wish to devise a type checker that allows as liberal a use of dynamic documents as possible while guaranteeing that no errors occur More precisely we would like to verify the following properties at compile time e at every plug operation x lt y z there always exists a y gap in z e the gap types are compatible with the values being plugge
5. even for simple services must be explicitly stored in a name value map called the servlet context instead of using Java s standard variable declara tion scoping mechanism Thirdly the dynamic construction of Web pages is not improved compared to CGI Web pages are built by printing string fragments to an output stream There is no guarantee that the result always becomes valid HTML This situation is slightly improved by using HTML constructor libraries but they preclude the possibility of dividing the work of the programmers and the HTML designers Furthermore since client sessions are split into individual interactions that are only combined implicitly for instance by storing session Ds in cookies it is not possible to statically analyze that a given page sent to a client always contains exactly the input fields that the next servlet in the session expects Both JSP ASP PHP and the countless homegrown variants were designed from a different starting point Instead of aiming for complex services where all parts of the pages are dynamically generated they fit into the niche where pages have mostly static contents and only little fragments are dynamically generated A service written in one of these languages typically consists of a collection of server pages which are HTML pages with program code embedded in special tags When such a page is requested by the client the code is evaluated and replaced by the resulting string Th
6. the server Integrity of shared data is provided by the database An interaction key is generated and embedded in every document shown to the client and serves to prevent submission of old documents Clients and session threads are associated http www w3 org Security faq 25 through a random key which is created by the server upon the first request and car ried across interactions in a hidden input field This mechanism may optionally be combined with other security measures such as SSL to provide the necessary level of security Authenticity and confidentiality is addressed through general declar ative security modifiers that the programmer can attach on a service session or show basis The modifiers ss1 and htaccess enforce that the SSL and HTTP Authentication protocols are used for communication The selective modifier restricts access to a session to those clients whose IP numbers match a given set of prefixes Finally the singular modifier ensures that the client has the same IP address throughout the execution of a session We envision performing some simple static analyses relating to the behavioral security of particular services Values are classified as secret or trusted and in contrast to tainting in Perl the compiler keeps track of the propagation of these properties Furthermore there are restrictions on how each kind of data can be used Form data is always assumed to be untrusted and gaps are never allowed to be pl
7. a collection of loosely related scripts A script is executed upon request from a client receiving form data as input and produc ing HTML as output before terminating Individual requests are tied together by explicitly inserting appropriate links to other scripts in the reply pages A prototypical scripting language is Perl but almost any programming lan guage has been suggested for this role CGI scripting is often supported by a large collection of library functions for decoding form data validating input accessing databases and realizing semaphores Even though such libraries are targeted at the domain of Web services the language itself is not A major problem is that the overall behavior is distributed over numerous individual scripts and depends on the implicit manner in which they pass control to each other This design complicates maintenance and precludes any sort of automated global analysis leaving all errors to be detected in the running service 16 2 HTML documents are created on the fly by the scripts typically using print like statements This again means that no static guarantees can be issued about their correctness Furthermore the control and presentation of a service are mixed together in the script code and it is difficult to factor out the work of programmers and HTML designers 12 The Java Servlets language also fits into this category The overall structure of a service written with servlets is the same as fo
8. a string or template z into the y gaps of the x template Clearly this constitutes a more flexible document construction mechanism but it also calls for new ideas for statically verifying for instance HTML validity This is the topic of Section 3 Other new features include the techniques for improving form field validation and concurrency control together with the syntax macro mechanism all of which are described in the following sections A Session Based Runtime Model The session based model can be implemented on top of the CGI protocol One naive approach is to create session threads as CGI scripts where all local state is stored on disk At every session interaction the thread must be started again and restore its local state including the call stack in order to continue execution A better approach is to implement each session thread as a process that runs for the whole duration of the session For every interaction a tiny transient CGI script called a connector process is executed acting as a pipe between the Web server and the session process This approach resembles FastCGI 22 and is described in detail in 8 Our newest implementation is instead based on a specialized Apache server module Naturally this is much faster than the CGI solutions since it does not create a new process for every single interaction but only for the session processes Two common sources of problems with standard implementations of sessions are the hi
9. by all threads must belong to that set To guide the controller the lt bigwig gt compiler uses the MONA tool 20 to translate the given formula into a minimal deterministic finite state automaton that is used by the controller process to grant permissions to individual threads When a thread asks to pass a given event label it is placed in a corresponding queue The controller continually looks for non empty queues whose event labels correspond to enabled transitions from the current DFA state When a match is found the corresponding transition is performed and the chosen thread is resumed Of course the controller must be implemented to satisfy some fairness requirements All regular trace languages can be expressed in the logic Applying temporal logics is a very abstract approach that can be harsh on the av erage programmer However using syntax macros which are described in Section 6 it is possible to capture common concurrency primitives such as semaphores mutex regions the readers writers protocol monitors and so on and provide high level language constructs hiding the actual formulas The advantage is that lt bigwig gt can be extended with any such constructs even some that are highly customized to particular applications while maintaining a simple core language for concurrency control The following example illustrates a simple service that implements a critical region using the event labels enter and leave service sha
10. checked that the input fields match what the program ex pects One practical limitation of the MAWL approach is that the HTML template mechanism is quite restrictive as one cannot insert markup into the template gaps We describe more details of these existing languages in the following sections By studying services written in any of these languages some other common problems show up First of all often surprisingly large portions of the service code tend to deal with form input validation Client server interaction takes place mainly through input forms and usually some fields must be filled with a certain kind of data perhaps depending on what has been entered in other fields If invalid data is submitted an appropriate error message must be returned so that the client can try again All this can be handled either on the client side typically with JavaScript 17 in the server code or with a combination In any case it is tedious to encode Secondly one drawback of dynamically generated Web pages compared to static ones is that traditional caching techniques do not work well Browser caches and proxy servers can cause major improvements in saving network bandwidth load time and clock cycles but when moving towards interactive Web services these benefits disappear Thirdly most Web services act as interfaces to underlying databases that for instance contain information about customers products and orders Accessing databases from g
11. corresponding URLs The following example shows a cluster of two documents that are cyclically connected Notice that the cluster can be browsed freely without cluttering the control flow service session Cluster html greeting lt html gt Hi Click lt a href where gt here lt a gt for a kind word lt html gt html kind lt html gt How nice to see you lt a href there gt Back lt a gt lt html gt kind kind lt there amp Greeting show greeting lt where amp kind The compiler checks that all cluster documents with submit buttons contain the same form fields It is also necessary to perform an escape analysis to ensure that document variables are not exported out of their scope 4 Form Field Validation A considerable effort in Web programming is expended on form field validation that is checking whether the data supplied by the client in form fields are valid and when it is not producing error messages and requesting the fields to be filled in again Apart from details about regular expression matching the main problem is to program a solution that is robust efficient and user friendly One approach is server side validation where the form fields are validated on the server when the page has been submitted None of the languages mentioned in Section 1 provide any help for this except the regular expression matching in Perl Therefore the main logic of the service often becomes cluttered with vali
12. expressions and boolean logic e temporal logic is a useful formalisms for expressing concurrency constraints and synthesizing safety controllers and e syntax macros can be used to create very domain specific high level languages for extremely narrow application domains We focus on these key contributions in the remainder of this paper but also describe less central contributions such as a technique for performing client side caching of dynamically generated pages a built in relational database and simple security mechanisms The individual results have been published in previous more special ized papers 25 8 26 7 9 6 10 Together these results show that there is a need for high level programming languages that are tailor made to the domain of Web service development Overview We begin in Section 2 by classifying the existing Web service languages as either script page or session centered arguing for the latter as the best choice for com plex services In Section 3 we show how the HTML template mechanism from MAWL can be extended to become more flexible using a notion of higher order tem plates Using novel type systems and static analyses the safety benefits of MAWL templates remain in spite of the increased expressibility Also we show how our solution can be used to cache considerable parts of the dynamically generated pages in the browser In Section 4 we address the problem of validating form input more easily S
13. increasingly sophisticated abstractions Macros are also used to tie together different sub languages making them col laborate to provide tailor made extensions of the language For instance the sub languages dealing with sessions dynamic documents and concurrency control can be combined into a publish macro This macro is useful when a service wishes to publish a page that is mostly static yet once in a while needs to be recomputed when the underlying data changes The following macros efficiently implements such an abstraction macro lt toplevels gt publish lt id D gt lt exp E gt shared html lt D gt cache shared bool lt D gt cached session lt D gt exclusive if lt D gt cached lt D gt cache lt E gt lt D gt cached true show lt D gt cache 22 macro lt stm gt touch lt id d gt lt d gt cached false The publish macro recomputes the document if the cache has expired and then shows the document while the touch macro causes the cache to expire The operator is used to create new identifiers by concatenation of others Using this extended syntax a service maintaining for example a high score list can look like require publish wigmac service shared int record shared string holder publish HiScore computeWinnerDoc record holder session Play int score play if score gt record show EnterName receive holder name
14. specialized proxy servers and careful programming to ensure consistency between the proxies and the main server HPP tries to separate the constant parts from the dynamic parts of the gener ated documents We apply a similar technique In contrast to HPP our solution is entirely automatic while HPP requires extra programming The idea is to ex ploit the clear division between the service code and the HTML templates present in lt bigwig gt In our normal implementation of DynDoc the internal template representation is converted to an HTML document on the server when the show statement is executed Instead we now store each template constant in a fixed file on the server and defer the conversion to the client using a JavaScript representa tion of the dynamic parts The template files can now be cached by the ordinary browser caches More details of the technique can be found in 6 We summarize our evaluation results in Section 8 Code Gaps and Document Clusters In the following we describe two extensions to the DynDoc language Occasion ally the page centered approach is admittedly more appropriate than the session centered Consider the following example which gives the current time of day service session Time html h lt html gt Right now the time is lt t gt lt html gt show h lt t now An equivalent but less clumsy version can be written using code gaps which implic itly represent expressions whose
15. still precludes general tree like structures The following lt bigwig gt example uses a recursive function to construct an HTML document representing a binary tree service html list lt html gt lt ul gt lt li gt lt gap gt lt li gt lt gap gt lt ul gt lt html gt html tree int i if i 0 return lt html gt foo lt html gt return list lt gap tree i 1 session ShowTree show tree 10 Something similar could not be done with MAWL s first order templates In a script centered or a page centered language it is of course possible but not with such a simple program structure reflecting the logical composition of the document because it must be generated linearly by printing to the output stream An alter native is to use an HTML tree constructor library however that forces documents to be built bottom up which is often inconvenient The use of higher order templates generally leads to programs with a large num ber of relatively small template constants For that reason it is convenient to be able to inline the constants in the program code as in these examples rather than always placing them in separate files However we do offer explicit support for factoring out the work of graphical designers using a include construct like in C Alternatively each HTML constant appearing in a lt bigwig gt program may have associated a URL pointing to an alternate presumably more elaborate version service
16. the active threads A simple case is to control access to the shared variables using mutex regions or the readers writers protocol Another issue is enforcement of priorities between different session kinds such that a management session may block other sessions from running Another example is event handling where a session thread may wait for certain events to be caused by other threads We deal with all of these scenarios in a uniform manner based on a central controller process in the runtime system which is general enough to enforce a wide range of safety properties 25 The support for concurrency control in the previously mentioned Web languages is limited to more traditional solutions such as file locking monitor regions or synchronized methods A lt bigwig gt service has an associated set of event labels During execution a session thread may request permission from the controller to pass a specific event checkpoint Until such permission is granted the session thread is suspended The policy of the controller must be programmed to maintain the appropriate global in variants for the entire service Clearly this calls for a domain specific sub language 19 We have chosen a well known and very general formalism temporal logic In partic ular we use a variation of monadic second order logic 29 A formula describes a set of strings of event labels and the associated semantics is that the trace of all event labels being passed
17. Fortunately the macros again permit high level abstractions to be introduced with more palatable syntax service shared int i event Increment itt session Application cause Increment The runtime model with a centralized controller process ensuring satisfaction of safety constraints is described in 8 The use of monadic second order logic for controller synthesis was introduced in 25 where additionally the notions of triggers and counters are introduced to gain expressive power beyond regular sets of traces and conditions for distributing the controller for better performance are defined The session model provides an opportunity to get a global view of the concurrent behavior of a service Our current approach does not exploit this knowledge of the control flow However we plan to investigate how it can be used in specialized program analyses that check whether liveness and other concurrency requirements are complied with 6 Syntax Macros As previously mentioned lt bigwig gt contains a notion of macros Although not specific to Web services this abstraction mechanism is an essential part of lt bigwig gt that serves to keep the sub languages minimal and to tie them together A macro language can be characterized by its level of operation which is either lexical or syntactic Lexical macro languages operate on sequences of tokens and conceptually precede parsing Because of this independence of syntax macros oft
18. INPUT NAME handle gt lt BODY gt lt HTML gt and greeting mhtml lt HIML gt lt HEAD gt lt TITLE gt MAWL Demo lt TITLE gt lt HEAD gt lt BODY gt Hello lt MVAR NAME who gt you are user number lt MVAR NAME count gt lt BODY gt lt HTML gt The template for goodbye is similar A form tag and a continue button are im plicitly inserted Variables declared static contain persistent data while those declared auto contain per session data also called local data The form variables are declared with two record types The former defines the set of gaps that occur in the template and the latter defines the input fields In the templates gaps are written with MVAR tags Template variables all have a put method When this is executed the arguments are inserted in the gaps the resulting page is sent to the client who fills in the fields and submits the reply which is turned into a record value in the program Note how the notion of sessions is explicit in the program that private and shared state is simply a matter of variable declaration modifiers and that the templates are cleanly separated from the service logic Obviously the session flow is more clear both to the programmer and to the compiler than with the non session based approaches One concrete benefit is that it is easy to statically check both validity and correct use of input fields The main force of the session centered approach is for services where the control flow
19. The lt bigwig gt Project Claus Brabrand Anders M ller and Michael I Schwartzbach BRICS Department of Computer Science University of Aarhus Denmark brabrand amoeller mis brics dk Abstract We present the results of the lt bigwig gt project which aims to design and implement a high level domain specific language for programming interactive Web services A fundamental aspect of the development of the World Wide Web during the last decade is the gradual change from static to dynamic generation of Web pages Generating Web pages dynamically in dialogue with the client has the advantage of providing up to date and tailor made information The develop ment of systems for constructing such dynamic Web services has emerged as a whole new research area The lt bigwig gt language is designed by analyzing its application domain and identifying fundamental aspects of Web services inspired by problems and solutions in existing Web service development languages The core of the design consists of a session centered service model together with a flex ible template based mechanism for dynamic Web page construction Using specialized program analyses certain Web specific properties are verified at compile time for instance that only valid HTML 4 01 is ever shown to the clients In addition the design provides high level solutions to form field validation caching of dynamic pages and temporal logic based concurrency control and it propose
20. action and the invocation syntax including the typed arguments As expected the type of the repeat until macro is lt stm gt representing statements This causes the body of the macro to be parsed as a statement and announces that invocations are only allowed in places where an ordinary statement would be We allow the programmer to design the invocation syntax of the macro This is used to guide parsing and adds to the transparency of the macro abstractions This partic ular macro is designed to parse two appropriately delimited arguments a statement s and an expression E The body of the macro implements the abstraction using a boolean variable and a while loop When the macro is invoked the identifiers occurring in the body are a converted to avoid name clashes with the invocation context With a concept of packages macros can be bundled up in collections Our experience with lt bigwig gt programming has led us to develop a standard macro package std wigmac that extends the sub languages of lt bigwig gt in various ways and has helped keep the language minimal For instance the form field validation language is extended with an optional regular expression construct and database language macros transform SQL like queries into our own iterative factor con struction Also various composite security modifiers are defined and concurrency control macros such as the region from Section 5 gradually build on top of each other to implement
21. as the session model dynamic documents form field validation and syntax macros Since the design of lt bigwig gt has focused on the Web specific areas we hope that the many standard programming issues of Web services become easier to develop with JWIG However a number of new challenges arise For instance the program analyses described in Section 3 all assume that we have access to precise control flow graphs of the programs This is trivial for lt bigwig gt but certainly not for Java Other future plans include type safe support for XML document transformation WML and VoiceXML support and broadening the view towards development and management of whole Web sites comprising many services Acknowledgments Tom Ball provided us with extensive and very helpful information about experiences with the MAWL language Anders Sandholm has been a key participant during his PhD studies at BRICS Mikkel Ricky Christensen and Steffan Olesen have worked tirelessly as student programmers during the entire project Niels Damgaard Uffe Engberg Mads Johan Jurik Lone Haudrum Olesen Christian Stenz and Tommy Thorn have provided valuable feedback and suggestions We also appreciate the efforts made by the participants of the WIG Projects course in Spring 1998 Finally we are grateful for the insightful comments we have received from the anonymous reviewers 28 References 1 Ken Arnold James Gosling and David Holmes The Java Programming Lan g
22. ather than all Web service program mers Also the programmer needs not anymore write essentially the same code in a server side version and a client side version PowerForms is a declarative language Informally this means that the pro grammer specifies what the input requirements are not how to check them In its simplest form PowerForms allows regular expression formats to be associated to form fields service format Digit range 0 9 format Alpha union range a z range A Z format Word concat Alpha star union Digit Alpha format Email concat Word Word star concat Word session Validate html form lt html gt Please enter your email address lt input name email type text size 20 gt lt format name Email field email gt lt html gt string s show form receive s email This example shows how to constrain input in the email field to a certain regular expression The lt bigwig gt compiler generates the JavaScript code that checks the user input on the client side and provides help and error messages and also the code performing the server side double check Using traffic light icons next to the input fields the user is provided with continuous feedback about the string entered so far Green means valid yellow means invalid but prefix of something valid and red means not prefix of something valid Other alternatives ca
23. d in in particular HTML with markup tags is never inserted into attribute gaps 13 e for every show receive statement the fields in the receive part always exist in the document being shown e the field types are compatible with the receive parts for instance a select menu allowing multiple items to be selected yields a vector value and e only valid HTML 4 01 24 is ever sent to the clients The first four properties are addressed in 26 as summarized in the following The last property is covered in the next section It is infeasible to explicitly declare the exact types of higher order templates for two reasons Firstly all gaps and all fields and their individual capabilities would have to be described which may become rather voluminous Secondly this would also imply that an HTML variable has the same type at every program point which is too restrictive to allow templates to be composed in an intuitive manner Consequently we rely instead on a flow analysis to infer the types of template variables and expressions at every program point In our experience this results in a liberal and useful mechanism We employ a monovariant interprocedural flow analysis which guarantees that the form fields in a shown document correspond to those that are received and that gaps are always present when they are being plugged This analysis fits into standard data flow frameworks 23 however it applies a highly specialized lattice structu
24. dation code In a sense every program part that sends a page to a client must be wrapped into a while loop that repeats until the input is valid Other disadvantages include wasting bandwidth and causing delays to the users The alternative is client side validation which usually requires the programmer to use JavaScript in the pages being generated This permits more sophisticated user interactions and reduces the communication overhead However client side validation should not be used alone The reason is that the client is perfectly capable of bypassing the JavaScript code so an additional server side validation must always be performed Thus the same code must essentially be written both in JavaScript and in the server scripting language In practice writing JavaScript 17 input validators that at the same time capture all validity requirements and also are user friendly can be very difficult since most browsers unfortunately differ in their JavaScript support Whole Web sites are dedicated to explaining how the various subsets of JavaScript work in different browsers In lt bigwig gt we have introduced a domain specific sub language called Power Forms for form field validation 7 It handles complex interdependencies between form fields and the compiler generates the required code for both client and server By compiling into JavaScript only the PowerForms implementors need to know the details of how browsers support JavaScript r
25. e server Eventually the client submits the form which causes the session thread to be resumed and any form data entered by the client to be received into program variables A simple lt bigwig gt service that communicates with a client as in the Servlet and MAWL examples is the following service html hello lt html gt Enter your name lt input name handle gt lt html gt html greeting lt html gt Hello lt who gt you are user number lt count gt lt html gt html goodbye lt html gt Goodbye lt who gt lt html gt shared int users 0 session Hello string name show hello receive name handle userstt show greeting lt who name count users show goodbye lt who name The program structure is obviously as in MAWL except that the session code and the templates are wrapped into a service block For instance the show receive statements produce the client interactions similarly to the put methods in MAWL However lt bigwig gt provides a number of new features Most importantly HTML templates are now first class values That is html is a built in data type and its values can be passed around and stored in variables as for any other data type Also the HTML templates are higher order This means that instead of only allowing text strings to be inserted into the template gaps we also allow insertion of other templates This is done with the the special plug operator x lt y z which inserts
26. ection 5 describes a technique for generating concurrency controllers from temporal logic specifications Section 6 gives an introduction to the syntax macro mechanism that ties together the sub languages of lt bigwig gt In Section 7 we men tion various less central aspects of the lt bigwig gt language Finally in Section 8 we describe our implementation and a number of applications and evaluate various practical aspects of lt bigwig gt 2 Session Centered Web Services Web programming covers a wide spectrum of activities from composing static HTML documents to implementing autonomous agents that roam the Web We focus in our work on interactive Web services which are Web servers on which clients can initiate sessions that involve several exchanges of information mediated by HTML forms This definition includes large classes of well known services such as news services search engines software repositories and bulletin boards but also covers services with more complex and specialized behavior There are a variety of techniques for implementing interactive Web services These can be divided into three main paradigms the script centered the page centered and the session centered Each is supported by various tools and suggests a particular set of concepts inherent to Web services The Script Centered Approach The script centered approach builds directly on top of the plain stateless HTTP CGI protocol A Web service is defined by
27. en have unintended effects and parse errors are only discovered at invocation time Consequently programmers are required to consider how individual macro invoca tions are being expanded and parsed Syntactic macros amend this by operating on parse trees instead of token sequences 31 Types are added to the macro argu ments and bodies in the form of nonterminals of the host language grammar Macro definitions can now be syntax checked at definition time guaranteeing that parse errors no longer occur as a consequence of macro expansion Using syntax macros the syntax of the programming language simply appears to be extended with new productions 21 Our macros are syntactic and based entirely on simple declarative concepts such as grammars and substitution making them easy and safe to use by ordinary Web service programmers Other macro languages such as MS Scheme macros and Maya instead apply full Turing complete programming languages for manipulating parse trees at compile time making them more difficult to use As an initial example we will extend the core language of lt bigwig gt with a repeat until control structure that is easily defined in terms of a while loop macro lt stm gt repeat lt stm S gt until lt exp E gt bool first true while first lt E gt lt S gt first false The first line is the header of the macro definition It specifies the nonterminal type of the macro abstr
28. eneral purpose programming languages where database queries are not integrated requires the queries to be built as text strings that are sent to a database engine This means that there is no static type checking of the queries As known from modern programming languages type systems allow many pro gramming bugs to be caught at compile time rather than at run time and thereby improve reliability and reduce development cost Fourthly since running Web services contain many concurrently executing threads and they access shared information for instance in databases on the server there is a fundamental need for concurrency control Threads may require exclusive access to critical regions be blocked until certain events occur or be required to satisfy more high level behavioral constraints All this while the service should run smoothly without deadlocks and other abrupt obstacles Existing solutions typically provide no or only little support for this for instance via low level semaphores as Perl or synchronized methods in Servlets This can make it difficult to guarantee correct concurrent execution of entire services Finally since Web services usually operate on the Internet rather than on secure local networks it is important to protect sensitive information both from hostile attacks and from programming leaks A big step forward is the Secure Sockets Layer SSL protocol 18 combined with HTTP Authentication 5 These techniques can ensure c
29. entire program is the argument to a single macro studies that expands into the complete code for a corresponding Web service The file bach elor wigmac is only 400 lines and yet defines a complete implementation of the new language Thus the lt bigwig gt macro mechanism offers a rapid and inexpen sive realization of new ad hoc languages with almost any syntax desired Similar features do not occur in any of the Web service languages mentioned in the previous sections 7 Other Web Service Aspects There are of course other features in lt bigwig gt that are necessary to support Web service development but for which we have no major innovations These are briefly presented in this section HTML Deconstruction The template mechanism is used to construct HTML documents but when run in reverse it also allows for deconstruction This is realized by using the templates as patterns in which the gaps play the role of variables as illustrated in this example service html Template lt html gt lt gt lt img src source alt today s Dilbert comic gt lt gt lt html gt session Dilbert string data get http www dilbert com string s match data Template s source exit Template lt source http www dilbert com s 24 which grabs the daily strip from the Dilbert home page Gaps without names serve as wildcards Seslets For some interaction patterns a strict session model can be inappropriate
30. er 1990 Arie van Deursen Paul Klint and Joost Visser Domain specific languages An annotated bibliography SIGPLAN Notices 35 6 26 36 2000 Daniel Weise and Roger F Crew Programmable syntax macros In Program ming Language Design and Implementation PLDI 93 1993 30
31. guage support The Session Centered Approach The pure session centered approach was pioneered by the MAWL project A service is here viewed as a collection of distinct sessions that access some shared data A client may initiate a session thread which is conceptually a process running on the server Interaction with the client is viewed as remote procedure calls from the server as known from classical construction of distributed systems but with the roles reversed The flow of an entire session is programmed as a single sequential program which is closer to ordinary programming practice and offers the compiler a chance to obtain a global view of the service Figure 1 illustrates the flow of control in this approach Important issues such as concurrency control become simpler to understand in this context and standard programming solutions are more likely to be applicable The following MAWL program is equivalent to the previous Servlet example static int users 0 session GreetingSession auto form gt handle hello auto string name hello put handle auto form string who int count gt greeting userst greeting put name users auto form string who gt goodbye goodbye put name The HTML templates hello greeting and goodbye are placed in separate files Here is hello mhtml lt HIML gt lt HEAD gt lt TITLE gt MAWL Demo lt TITLE gt lt HEAD gt lt BODY gt Enter your name lt
32. h as ASP PHP and JSP where the dynamic code is embedded in the HTML pages In a sense this is the inverse of the script centered languages where HTML fragments are embedded in the program code When a client requests a page a specialized Web server interprets the embedded code which typically produces additional HTML snippets while accessing a shared database In the case of JSP implementations work by compiling each JSP page into a servlet using a simple transformation This approach is often beautifully motivated by simple examples where pages are mainly static and only sporadically contain computed contents For example a page that displays the time of day or the number of accesses clearly fits this mold The following JSP page dynamically inserts the current time together with a title and a user name based on the CGI input parameters lt HTML gt lt HEAD gt lt TITLE gt JSP Demo lt TITLE gt lt HEAD gt lt BODY gt Hello lt String name request getParameter who if name null name stranger out print name 5 gt lt P gt This page was last updated lt new Date gt lt BODY gt lt HTML gt The special lt gt tags contain Java code that is evaluated at the time of the request As long as the code parts only generate strings without markup it is easy to statically guarantee that all shown pages are valid HTML and other relevant properties But as the services become more complex the page cente
33. he script is written in the general purpose scripting language Perl but any language supported by the server can be used Being based on general purpose programming languages there is no special support for Web specific tasks such as generation of HTML pages and knowledge of the low level details of the HTTP protocol are required Also HTTP CGI is a stateless protocol that by itself provides no help for tracking and guiding users through series of individual interactions This can to some degree be alleviated by libraries In any case there are no compile time guarantees of correct run time behavior when it comes to Web specific properties for instance ensuring that invalid HTML is never sent to the clients Servlets are a popular higher level Java specific approach Servlets which are special Java programs offers the common Java advantages of network support strong security guarantees and concurrency control However some significant problems still exist Services programmed with servlets consist of collections of re quest handlers for individual interactions Sessions consisting of several interactions with the same client must be carefully encoded with cookies URL rewriting or hid den input fields which is tedious and error prone even with library support and it becomes hard to maintain an overview of large services with complex interaction flows A second although smaller problem is that state shared between multiple client sessions
34. ht It has many active users and has been integrated into a proprietary Web deployment system Performance When evaluating the performance of the lt bigwig gt implementation we want to focus on the areas where we have attempted to provide improvements We are not aiming for simple high load services but are focusing on services with intricate flow of control Still informal tests show that the throughput of our services is certainly comparable with that of straight CGI based services or Servlet applications running on J2SE The automatic caching scheme based on our HTML templates is designed to exploit their intricate structure to cache static fragments on the client side We have obtained real benefits from this approach The experiments reported in 6 show that the size of the transmitted data may shrink by a factor of 3 to 30 which on a dial up connections translates into a reduction in download times by a factor of 2 to 10 27 It is also relevant to evaluate the performance of the lt bigwig gt compiler since we employ a series of theoretically quite expensive static analyses However in practice they perform very well as documented in 26 9 The EATCS service is analyzed for HTML validity in 6 7 seconds and the JAOO service in 2 4 seconds 9 Conclusion The lt bigwig gt project has identified central aspects of interactive Web services and has provided solutions in a coherent framework based on programming language theor
35. ific language 30 meaning that it employs special syntax and constructs that are tailored to fit its particular application domain and allow spe cialized program analyses in contrast to library based solutions Its core is a C or Java like skeleton which is surrounded by domain specific sub languages covering the above key aspects A notion of syntax macros tie the sub languages together and provide additional layers of abstraction This macro language which operates on the parse tree level rather that the token sequence level as conventional macro languages has proved successful in providing extensions of the core language This has helped each of the sub languages remain minimal since desired syntactic sugar is given by the macros Syntax macros can be taken to the extreme where they with little effort can define a completely new syntax for very domain specific languages tailored to highly specialized application domains It is important that lt bigwig gt is based on compilation rather than on interpre tation of a scripting language Unlike many other approaches we can then apply type systems and static analysis to catch many classes of errors before the service is actually installed The lt bigwig gt compiler uses common Web technologies as target languages This includes HTML 24 HTTP 5 JavaScript 17 and Java Applets 1 Our current implementation additionally relies on the Apache Web server It is impor tant to apply only sta
36. is complex Many simple Web services are in actuality more loosely structured If all sessions are tiny and simply do the work of a server module from the page centered approach then the overhead associated with sessions may seem to large Script centered services can be seen as a subset of the session centered where every session contains only one client interaction Clearly the restriction in the script centered and the page centered languages allow significant performance improve ments For instance J2EE Servlet JSP servers employ pools of short lived threads that store only little local state For more involved services however the session centered approach makes programming easier since session management comes for free Structure of lt bigwig gt Services The overall structure of lt bigwig gt programs is directly inspired by MAWL A lt bigwig gt program contains a complete specification of a Web service A service contains a collection of named sessions each of which essentially is an ordinary sequential program A client has the initiative to invoke a thread of a given session which is a process on the server that executes the corresponding sequential code and exclusively communicates with the originating client Communication is performed by showing the client an HTML page which implicitly is made into a form with an appropriate URL return address While the client views the given document the session thread is suspended on th
37. is gives better control over the HTML construction but it only gives an advantage for simple services where most of every page is static The MAWL language was designed especially for the domain of interactive Web services One innovation of MAWL is to make client sessions explicit in the program logic Another is the idea of building HTML pages from templates A MAWL service contains a number of sessions shared data and HTML templates Sessions serve as entry points of client initiated session threads Rather than producing a single HTML page and then terminating as CGI scripts or Servlets each session thread may involve multiple client interactions while maintaining data that is local to that thread An HTML template in MAWL is an HTML document containing named gaps where either text strings or special lists may be inserted Each client interaction is performed by inserting appropriate data into the gaps in an HTML template and then sending it to the client who fills in form fields and submits the reply back to the server The notions of sessions and document templates are inherent in the language and being compilation based it allows important properties to be verified statically without actually running the service Since HTML documents are always con structed from the templates HTML validity can be verified statically Also since it is clear from the service code where execution resumes when a client submits form input it can be statically
38. is the following where the client chooses a letter group and the select menu is then dynamically restricted to those letters service format Vowel charset aeiouy format Consonant charset bcdfghjklmnpqrstvwxz html form lt html gt Favorite letter group lt input type radio name group value vowel checked gt vowels lt input type radio name group value consonant gt consonants lt br gt Favorite letter lt select name letter gt lt option value a gt a lt option value b gt b lt option value c gt c lt option value Z gt z lt select gt lt format field letter gt lt if gt lt equal field group value vowel gt lt then gt lt format name Vowel gt lt then gt lt else gt lt format name Consonant gt lt else gt lt if gt lt format gt lt html gt session Letter string s show form receive s letter ColdFusion 13 provides a mechanism reminiscent of PowerForms However it does not support field interdependencies or validation of non text fields PowerForms have shown to be a simple language with a clean semantics that appears to handle most realistic situations We have implemented it both as part of the lt bigwig gt compiler and as a stand alone tool that can be used to add input validation to general HTML pages 5 Concurrency Control As services have several session threads there is a need for synchronization and other concurrency control to discipline the concurrent behavior of
39. may prevent the client from becoming impatient and abandoning the session The lt bigwig gt runtime system additionally contains a garbage collector pro cess that monitors the service and shuts down session processes which have been abandoned by the clients By default this occurs if the client has not responded within 24 hours The sessions are allowed to execute some clean up actions before terminating 3 Dynamic Construction of HTML Pages In MAWL all HTML templates are placed in separate files and viewed as a kind of procedures with the arguments being strings that are plugged into gaps in the tem plate and the results being the values of the form fields that the template contains This allows a complete separation of the service code and the HTML code Two benefits are that static guarantees are possible and that the work of programmers and HTML designers can be separated as previously mentioned A disadvantage is that the template mechanism becomes too rigid compared to the flexibility of the print like statements available in the script centered languages However those 11 brics lt head gt lt title gt Hi lt title gt lt head gt lt body bgcolor color gt contents lt body gt greeting Hello who welcome to h lt head gt lt title gt Hi lt title gt lt head gt lt body bgcolor color gt
40. munication An important use of seslets is to allow client side code to synchronize with other active threads on the server For example the chat room solution could employ a seslet that uses the concurrency control mechanisms of lt bigwig gt to wait until the next message is available which then is returned to the applet In this way no client pulling or busy waiting is required Databases Most Web services are centered around a database In the general case this is an existing external database which the service must connect to The lt bigwig gt system supports the ODBC interface for this purpose However queries are not built as strings but are written in a query language that is part of the lt bigwig gt syntax This allows for compile time checking of the syntax and types of queries eliminating another source of errors Since many smaller services use only simple data we also offer an internal database that is implemented on top of the file system Security There are many aspects of Web service security The security in lt bigwig gt can be divided into two categories depending on whether it is generically applicable to all services or specific to the behavior of a particular service The former category mostly relates to the runtime environment and commu nication dealing with concepts such as integrity authenticity and confidentiality Integrity of a session thread s local state is achieved by keeping it exclusively on
41. n be chosen such as checkmark symbols arrows etc We also allow the usual Perl style syntax for regular expressions in the subset of our notation that excludes the intersection and complement operators Formats can be associated to all kinds of form fields not just those of type text For select fields the format is used to filter the available options For radio and checkbox fields only the permitted buttons can be depressed As noted in 14 many forms contain fields whose values may be constrained by those entered in other fields A typical example is a field that is not applicable if some other field has a certain value Such interdependencies are almost always han dled on the server even if the rest of the validation is performed on the client The reason is presumably that interdependencies require even more delicate JavaScript programming The lt bigwig gt solution is to allow such field interdependencies to be specified using an extension of the regular expressions the format tags are extended to describe boolean decision trees whose conditions probe the values of other form fields and whose leaves are simple formats The interdependence is resolved by a fixed point process that is computed on the client by JavaScript code automatically 3See e g http www webdevelopersjournal com articles javascript limitations html or http www xs4all nl ppk js version5 html 18 generated by the lt bigwig gt compiler A simple example
42. ndard technologies on the client side in order not to place restrictions on the clients In particular we do not use browser plug ins and we only use the subset of JavaScript that works on all common browsers As new technologies become standard the compiler will merely obtain corresponding op portunities for generating better code To the degree it is possible we attempt to hide the low level technical details of the underlying technologies We have made no effort to contribute to the graphical design of Web services Rather we provide a clean separation between the physical layout of the HTML pages and the logical structure of the service semantics Thus we expect that standard HTML authoring tools are used conceivably by others than the Web programmer Also we do not focus on efficiency but on providing higher levels of abstraction for the developers For now we regard it as less important to generate solutions that seamlessly scale to thousands of interactions per second although scalability of course is an issue for the design The main contributions of the lt bigwig gt project are the following results e The notion of client sessions can and should be made explicit in Web service programming languages e dynamic construction of Web pages can be made at the same time flexible and fast while still permitting powerful compile time analyses e form field validation can be made easier with a domain specific language based on regular
43. ned in the form of a VDSL where study fields and requirements are conceptualized and defined directly in a more natural language style This makes it possible for non programmers to maintain and update the service An small example input is require bachelor wigmac studies course Math101 title Mathematics 101 2 points fall term course Phys202 title Physics 202 2 points spring term course Lab304 23 title Lab Work 304 1 point fall term exclusions Math101 lt gt MathA Math102 lt gt MathB prerequisites Math101 Math102 lt Math201 Math202 Math203 Math204 CS101 CS102 lt CS201 CS203 Math101 CS101 lt CS202 Math101 lt Stat101 CS202 CS203 lt CS301 CS302 CS303 CS304 Phys101 Phys102 lt Phys201 Phys202 Phys203 Phys301 Phys203 lt Phys302 Phys303 Lab301 Lab302 Lab303 Lab101 Lab102 lt Lab201 Lab202 Lab201 Lab202 lt Lab301 Lab302 Lab303 Lab304 field CS Mathematics field courses Math101 Math102 Math201 Math202 Stat101 CS101 CS102 CS201 CS202 CS203 CS204 CS301 CS302 CS303 CS304 Project other courses MathA MathB Math203 Math204 Phys101 Phys102 Phys201 Phys202 constraints has passed CS101 CS102 at least 2 courses among CS201 CS202 CS203 at least one of Math201 Math202 at least 2 courses among Stat101 Math202 Math203 has 4 points among Project CS303 CS304 in total between 36 and 40 points None of the syntax displayed is plain lt bigwig gt except the macro package require instruction The
44. neering PASTE 2001 ACM 2001 10 Claus Brabrand and Michael I Schwartzbach Growing languages with meta morphic syntax macros In Proceedings of Workshop on Partial Evaluation and Semantics Based Program Manipulation PEPM 2002 ACM 2002 11 Aske Simon Christensen Anders Meller and Michael I Schwartzbach Ex tending Java for high level Web service construction In preparation 12 K Cox T Ball and J C Ramming Lunchbot A tale of two ways to pro gram web services Technical Report BL0112650 960216 06TM AT amp T Bell Laboratories 1996 13 John Desborough Cold Fusion 3 0 Intranet Application International Thom son Publishing 1997 14 Micah Dubinko Sebastian Schnitzenbaumer Malte Wedel and Dave Raggett XHTML extended forms requirements W3C Working Draft April 2001 http www w3 org TR xhtml forms req html 15 Alex Fedorov et al Professional Active Server Pages 2 0 Wrox Press 1998 16 Mary Fernandez Dan Suciu and Igor Tatarinov Declarative specification of data intensive Web site In USENIX Conference on Domain Specific Lan guages October 1999 29 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 David Flanagan JavaScript The Definitive Guide O Reilly June 1998 Alan O Freier Philip Karlton and Paul C Kocher The SSL protocol version 3 0 Internet Draft November 1996 http home netscape com eng ss13 draft302 txt
45. nly strings should be plugged into such gaps not templates with markup The plug operation x lt y z creates a new template by inserting a copy of z in the y gaps of a copy of x When used in a show receive statement a template is converted to a complete document by implicitly plugging empty strings into all remaining gaps Also it is automatically wrapped into a form element whose action is to continue the session unless the session terminates immediately after And finally it is inserted into an outermost template like lt html gt lt head gt lt title gt service lt title gt lt head gt lt body gt lt body gt lt html1 gt unless already inside a body element The following example illustrates that docu ments can be built gradually using higher order templates service html brics lt html gt lt head gt lt title gt Hi lt title gt lt head gt lt body bgcolor color gt lt contents gt lt body gt lt html gt html greeting lt html gt Hello lt who gt welcome to lt what gt lt html gt session Welcome html h brics lt contents greeting show h lt color 9966ff who Stranger what BRICS The construction process is shown in Figure 3 Note that gaps may be plugged in any order not necessarily bottom up MAWL does provide little functionality 12 beyond plugging text strings into gaps The special MITER tag allows list struc tures to be built iteratively but that
46. nteger users session putValue name name out println Hello name you are user number users session putValue state 2 else state equals 2 String name String session getValue name out printin Goodbye name session invalidate out printin lt BODY gt lt HTML gt lhttp www apl jhu edu hall java Servlet Tutorial and http java sun com docs books tutorial servlets Clients running this service are guided through a series of interactions first the service prompts for the client s name then the name and the total number of invocations is shown and finally a goodbye page is shown The ServletContext object contains information shared to all sessions while the HttpSession object is local to each session The code is essentially a switch statement that branches according to the current interaction An alternative approach is to make a servlet for each kind of interaction In spite of the API one still needs to explicitly maintain both the state and the identity of the session The model of sessions that is supported by Servlets and other script centered approaches tends to fit better with shopping basket applications where the client browses freely among dynamically generated pages than with complex services that need to impose more strict control of the interactions The Page Centered Approach The page centered approach is covered by language suc
47. oid to reject this program since the branching of the if statement depends on a function applied to information derived from a secret value These analyses are not particularly original but are not seen in other Web service programming languages There is still much work to be done in this area So far we have not considered using cryptological techniques to ensure service integrity the role of certificates or more sophisticated static analyses 26 8 Evaluation The lt bigwig gt language should be evaluated according to two different criteria First the quality of our language design as seen from concrete programming expe riences This is necessarily a somewhat intangible and subjective criterion Second the performance of our language implementation as seen from observed benchmarks Experience with lt bigwig gt lt bigwig gt is still mainly an experimental research tool but we have gained experi ences from numerous minor services that we have written for our own edification a good number of services that are used for administrative purposes at the University of Aarhus and a couple of production services on which we have collaborated Apart from these applications we estimate that lt bigwig gt has been downloaded roughly 2500 times from our Web site and we have mainly received positive feedback from the users One production service is the Web site of the European Association for Theo retical Computer Science www eatcs o
48. ommunication authenticity and confidentiality but using them properly requires insight of technical protocol and implementation details Furthermore they do not protect against programming bugs that unintentionally leak secret information The taint mode in Perl offers some solution to this However it is run time based so no compile time guarantees are given Also it only checks for certain predefined properties and more specialized properties cannot be added The lt bigwig gt Language Motivated by the languages and problems described above we have identified the following areas as key aspects of Web service development e sessions the underlying paradigm of interactive Web services e dynamic documents HTML pages must be constructed in a flexible efficient and safe fashion e concurrency control Web services consist of collections of processes running concurrently and sharing resources e form field validation validating user input requires too much attention of Web programmers so a higher level solution is desirable e database integration the core of a Web service is often a database with a number of sessions providing Web access and e security to ensure authenticity and confidentiality both regarding malicious clients and programming bugs To attack the problems we have from scratch designed a new language called lt bigwig gt as a descendant of the MAWL language This language is a high level domain spec
49. periments show that they work well in practice even for large intricate programs These experiments are mentioned in Section 8 15 Caching of Dynamically Generated HTML Traditional Web caching based on HTTP works by associating an expiration time to all documents sent by the servers to the clients This has helped in decreasing both network and server load and response times By default no expiration is set and by using now caching is effectively disabled This technique was designed primarily for documents whose contents rarely or never changes not for documents dynamically generated by interactive Web services The gradual change from stat ically to dynamically generated documents has therefore caused the impact of Web caching to degrade Existing proposals addressing this include Active Cache HPP and various server based techniques as explained in the survey in 6 Server based techniques aim for relieving the server of redundant computations not for decreasing net work load They typically work by simplifying assumptions for instance that many interactions can be handled without side effects on the global service state that interactions are often identical for many clients or that the dynamics of the pages is limited to e g banner ad rotation None of this applies to complex interactive services Active Cache is a proxy based solution that employs programmable cache applets This can be very effective but it requires both
50. r Perl Every possible interaction is essentially defined by a separate script and one must use cookies hidden input fields or similar techniques to connect sequences of interactions with the clients Servlets provide a session tracking API that hides many of the details of cookies hidden input fields and URL rewriting Many servlet servers use cookies if the browser supports them but automatically revert to URL rewriting when cookies are unsupported or explicitly disabled This API is exemplified by the following code inspired by two Servlet tutorials public class SessionServlet extends HttpServlet public void doGet HttpServletRequest request HttpServletResponse response throws ServletException IOException ServletContext context getServletContext HttpSession session request getSession true response setContentType text htm1L PrintWriter out response getWriter out printin lt HTML gt lt HEAD gt lt TITLE gt Servlet Demo lt TITLE gt lt HEAD gt lt BODY gt if session isNew out println lt FORM ACTION SessionServlet gt Enter your name lt INPUT NAME handle gt lt INPUT TYPE SUBMIT gt lt FORM gt session putValue state 1 else String state String session getValue state if state equals 1 String name String request getParameter handle int users Integer context getAttribute users intValue 1 context setAttribute users new I
51. re representing the template types For every template variable and expres sion that occurs in the given program we associate a lattice element that abstractly captures the relevant template properties The lattice consists of two components a gap map and a field map The gap map records for every occurring gap name whether or not the gap occurs at that point and in case it does occur whether it is an HTML gap or an attribute gap Similarly the field map records for every occurring input field name information about the input fields which can be of type text radio select or checkbox representing the different interaction methods Given a lt bigwig gt program we construct a flow graph This is quite easy since there are no higher order functions or virtual methods All language constructs that are not included in DynDoc are abstracted away It is now possible to define transfer functions which abstractly describe the effect of the program statements This produces a constraint system which we solve using a classical fixed point iteration technique From this solution we inspect that the first three properties mentioned above are satisfied and if not generate error messages indicating the cause With this approach the programmer is only restricted by the requirement that at every program point the template type of an expression must be fixed In practice this does not limit the expressibility rather it tends to enforce a more comprehen
52. red approach tends to converge towards the script centered one Instead of a mainly static HTML page with some code inserted the typical picture is a single large code tag that dynamically computes the entire contents Thus the two approaches are closely related and the page centered technologies are only superior to the degree in which their scripting languages are better designed The ASP and PHP languages are very reminiscent of JSP ASP is closely tied to Microsoft s Internet Information Server although other implementations exist Instead of being based on Java it defines a language independent connection be tween HTML pages and scripting languages typically either Visual Basic Script or Microsoft s version of JavaScript PHP is a popular Open Source variant whose scripting language is a mixture of C Java and Perl oo N ee SESSION THREAD eee Figure 1 Client server sessions in Web services On the left is the client s browser on the right is a session thread running on the server The tread is initiated by a client request and controls the sequence of interactions Se oo These languages generally provide only low level support for tracking client ses sions and maintaining session state Cookies hidden input fields and some li brary support is the common solution Also for other Web service aspects such as databases and security there is often a wide range of libraries available but no direct lan
53. red int i session Critical constraint label leave enter all t1 t3 t1 lt t3 amp amp enter tl amp amp enter t3 gt is t2 t1 lt t2 amp amp t2 lt t3 amp amp leave t2 wait enter i itl wait leave The formula states that for any two enter events there is a leave event in between which implies that at any time at most one thread is allowed in the critical region Using syntax macros programmers are allowed to build higher level abstractions such that the following can be written instead service shared int i session Critical region i itl We omit the macro definitions here In its full generality the wait statement is more like a switch statement that allows a thread to simultaneously attempt to pass several event labels and request a timeout after waiting a specified time A different example implements an asynchronous event handler Without the macros this could be programmed as service shared int i 20 constraint label handle cause all t1 handle t1l gt is t2 t2 lt tl amp amp cause t2 amp amp all t3 t2 lt t3 amp amp t3 lt t1 gt handle t3 session Handler while true wait handle itt session Application wait cause This non trivial formula allows the handler to proceed without blocking the ap plication whenever the associated event has been caused at least once since the last invocation of the handler
54. rg handling newsletters webboards and several membership services It is written in 5 345 lines of lt bigwig gt using 133 HTML templates and 114 show statements Another is the Web site of the JAOO 2001 conference www jaoo dk handling all aspects of advertisement schedules registration and attendant services It is written is 7 943 lines of lt bigwig gt using 248 HTML templates and 39 show statements These experiences have shown that lt bigwig gt has two very strong points First the session concept greatly simplifies the programming of complicated control flow with multiple client interactions Second the HTML templates are very easy and intuitive to use and the static guarantees catch numerous errors many of which are difficult to find by other means It is particularly helpful that the HTML analyzers provide precise and intuitive error messages The JAOO application has been particularly interesting since it involved col laboration with an external HTML designer This experience confirmed that our templates are successful in defining an interface between programmers and designers and that gaps and fields define a useful contract between the two The main weak point that we have identified is the core language which is often found to be lacking minor features We plan to address this in future work as mentioned in Section 9 The stand alone version of the PowerForms sub language has been surprisingly popular in its own rig
55. s syntax macros for making highly domain specific lan guages The language is implemented via widely available Web technologies such as Apache on the server side and JavaScript and Java Applets on the client side We conclude with experience and evaluation of the project 1 Introduction The lt bigwig gt project was founded in 1998 at the BRICS Research Center at the University of Aarhus to design and implement a high level domain specific language for programming interactive Web services In the following we will argue that existing Web service programming languages in various ways provide only low level solutions to problems specific to the domain of Web services Our overall ambitions of the project are to identify the key areas of the Web service domain analyze the problems with the existing approaches and provide high level solutions that will support development of complex services Motivation Specifically we will look at the following Web service technologies the HTTP CGI Web protocol 19 Sun s Java Servlets 27 and their JavaServer Pages JSP 28 Microsoft s Active Server Pages ASP 15 the related Open Source language PHP 4 and the research language MAWL 8 2 21 CGI was the first platform for development of Web services It is based on the simple idea of letting a script generate the reply to incoming HTTP requests dynamically on the server rather than returning a static HTML page from a file Typically t
56. sible structure of the programs Also the compiler silently resolves conflicts at flow join points by implicitly plugging superfluous gaps with empty contents HTML Validity Analysis The fifth property HTML validity is addressed with a similar but more complicated approach as described in 9 The main idea is the following We define a finite structure called a summary graph that approximates the set of templates that a given HTML expression may evaluate to This structure contains the plug operations and the constant templates and strings that are involved As an example consider the summary graph in Figure 4 The nodes correspond to program constants and the edges correspond to plug operations For instance the li template may here be plugged into the items gaps in the ul template The e node represents arbitrary text strings and e is the empty string The root of the 14 large items e kind Pa lt li gt lt text gt text lt litems gt FP ys Baana lt ul gt lt items gt lt ul class kind gt items Figure 4 A summary graph representing a set of HTML fragments graph corresponds to the outermost template By unfolding this graph according to the plug edges this summary graph defines a possibly infinite set of HTML fragments without gaps in this case the set of all ul lists of class large with one or more character data items This struc
57. since the client and server must alternate between being active and suspended Furthermore information cannot be pushed on the server s initiative while the client is viewing a page A simple example is a chat room where new messages should appear au tomatically without the client having to reload the page being viewed and where only the new message and not the entire new page is transmitted The essence of this concept is client side computations that are able to contact the server on their own accord The lt bigwig gt solution is a notion of seslets A seslet is a kind of light weight session which is allowed to do anything an ordinary session can do except perform show operations It is invoked by the client with some arguments and eventually returns a reply of any lt bigwig gt type Typically it performs database operations or waits for certain events to occur and reports back to the client Since we are limited by the existing technologies on the client side our current implementation is restricted to using Java applets or JavaScript To facilitate the writing of applets the lt bigwig gt compiler generates the Java code for an abstract class extending Applet which must be inherited from in order to access the available seslets Alternatively we have experimented with a JavaScript interface However this approach is limited by the lack of client server communication support from JavaScript so we currently apply cookies for the com
58. splaying its results again lt bigwig gt provides a simple but unique solution to these problems Each session thread is associated a URL which points to a file on the server containing the latest HTML page shown to the client Instead of sending the contents directly to the client at every show statement we redirect the browser to this URL as illustrated in Figure 2 Since the URL serves as the identification of the session thread this solves the problems mentioned above The history list of the browser now only contains a single entry for the duration of the session the sessions can now be bookmarked for later use and in addition the session identity URL can be passed around manually to another browser for instance without problems When using URLs instead of cookies to represent the session identity it also becomes possible for a single user to simultaneously run multiple sessions in different windows but with the same browser With this simple solution we can furthermore automatically provide the client with feedback while the server is processing a request This is done by after a few seconds writing a temporary response to the HTML file which informs the client about the status of the request This temporary file reloads itself frequently allowing for updated status reports When the final response is ready it simply overwrites the temporary reply file causing the reloading to stop and the response to be shown This simple technique
59. story buffers and the bookmarking features found in most browsers With the history buffers and the back button the users can step back to a previous interaction and either intentionally or unintentionally resubmit an old input form Sometimes this can be a useful feature but more often this causes confusion and annoyance to the users who may for instance order something twice It is a general problem that the information shown to the user in this way can be obsolete since it was tailor made only for the exact time of the initial request Since the information was generated from a shared database that may have changed entirely it does generally not make sense to step back in time using the history buffer This is no different from ordinary programs Even if the programmer has been aware of this and has added serial number checks the history buffer will be full of URLs to obsolete requests If the service really needs a back feature it can be programmed explicitly into the flow of the sessions It also becomes hazardous to try to use 2See http httpd apache org 10 SESSION PROCESS D HTML FILE Figure 2 Session based runtime model with reply indirection Each session thread is implemented as a separate process that writes its HTML reply to a designated file bookmarks to temporarily suspend a session Invoking the bookmark will then typically cause a CGI script to be executed a second time instead of just di
60. t of names of attributes and subelements that may occur and a boolean expression constraining the element type instances with respect to their attribute values and contents The official DTD for HTML is easily rewritten into our abstract DTD notation In fact the abstract DTD version captures more va lidity requirements than those expressible by standard DTDs and merely appear as comments in the HTML DTD As a technicality we actually work with XHTML 1 0 which is an XML reformulation of HTML 4 01 There are no conceptual differ ences except that the XML version provides a cleaner tree view of documents for the analysis Given a summary graph and an abstract DTD description of HTML validity can be checked by a recursive traversal of the summary graph starting at the roots We memoize intermediate results to ensure termination since the summary graphs may contain loops If no violations are encountered the summary graph is valid Since all validity properties are local to single elements and their contents we are able to produce precise error messages in case of violations Analysis soundness is ensured by the following property if all summary graphs corresponding to show expressions are verified to be valid with respect to the abstract DTD then all concrete documents are guaranteed to be valid HTML The program analyses described here all have high worst case complexities be cause of the complex lattices Nevertheless our implementations and ex
61. ture turns out to provide an ideal abstraction level for verifying HTML validity Again we apply a data flow analysis to approximate the flow of template values in the program This time we use a lattice consisting of summary graphs It is pos sible to model plug operations with good precision using transfer functions however two preliminary analyses are required One for tracking string constants and one called a gap track analysis for tracking the origins of gaps The latter tells us for each template variable and gap name which constant templates containing such a gap can flow into that variable at any given program point Clearly these anal yses are highly specialized for the domain of dynamic document construction and for lt bigwig gt s higher order template mechanism but they all fit into the standard data flow analysis frameworks For more details we refer to 9 Once we have the summary graphs for all the show statements we need to ver ify that the sets of document fragments they define all are valid HTML according to W8C s official definition To simplify the process we reformulate the notion of Document Type Definition DTD as a simpler and more convenient formalism that we call abstract DTD An abstract DTD consists of a number of element declara tions whereof one is designated as the root An element declaration defines the requirements for a particular type of elements Each declaration consists of an el ement name a se
62. uage Addison Wesley 3rd edition 2000 2 David Atkins Thomas Ball Michael Benedikt Glenn Bruns Kenneth Cox Peter Mataga and Kenneth Rehor Experience with a domain specific language for form based services In Useniz Conference on Domain Specific Languages October 1997 3 David Atkins Thomas Ball Glenn Bruns and Kenneth Cox Mawl a domain specific language for form based services In EEE Transactions on Software Engineering June 1999 4 Leon Atkinson Core PHP Programming Prentice Hall 1999 5 T Berners Lee R Fielding and H Frystyk Hyper text transfer protocol HTTP 1 0 RFC1945 May 1996 http www w3 o0org Protocols rfc1945 rfc1945 6 Claus Brabrand Anders M ller Steffan Olesen and Michael I Schwartzbach Language based caching of dynamically generated HTML May 2001 Submit ted for publication 7 Claus Brabrand Anders Meller Mikkel Ricky and Michael I Schwartzbach Powerforms Declarative client side form field validation World Wide Web Journal 3 4 2000 8 Claus Brabrand Anders Meller Anders Sandholm and Michael I Schwartzbach A runtime system for interactive Web services Computer Net works 31 1391 1401 1999 Also in Proceedings of the Eighth International World Wide Web Conference 9 Claus Brabrand Anders M ller and Michael I Schwartzbach Static validation of dynamically generated HTML In Proceedings of Workshop on Program Analysis for Software Tools and Engi
63. ugged with secret values Variables can be declared with the modifiers secret or trusted and may then only contain the corresponding values The system function can only be called with a trusted string value To change the classification of a value there are two functions trust and disclose The programmer must make the explicit choice of using these coercions An example involving trust is the following service service session Lookup html Error lt html gt Invalid URL lt html gt html EnterURL lt html gt Enter a URL lt input type text name URL gt lt html gt string u domain show EnterURL receive u URL if jul lt 7 u 0 7 http show Error for i 7 i lt lul amp amp u i i domain u 7 i if system usr sbin nslookup domain stderr show Error This code performs an nslookup on the URL supplied by the user to check whether its domain exists Since the value of domain is derived from the form field URL it should not be trusted and its use in the call of system will be flagged by the com piler And indeed it would be unfortunate if the client enters http foo rm rf in the form A similar analysis is performed for secret Consider the example service shared secret string password bool odd int n return n 2 1 session Reveal if odd password show lt html gt foo lt html gt The compiler is sufficiently paran
64. values are computed and plugged into gaps when the document is being shown service session Time html h lt html gt Right now the time is lt now gt lt html gt show h 16 Documents with code gaps remain first class values since the code can only access the global scope Note that code gaps in lt bigwig gt are more powerful than the usual page centered approach since the code exists in the full context of sessions shared variables and concurrency control In fact with the idea of published documents described in Section 6 the page centered approach is now included as a special case of lt bigwig gt Some services may want to offer the client more than a single document to browse for example the response could be a tiny customized Web site In lt bigwig gt we have experimented with support for showing such document clusters The diffi culty is to provide a simple notation for specifying an arbitrary graph of documents connected by links We introduce for an HTML variable x the document reference notation amp x which can be used as the right hand side of a plug operation It will eventually expand into a URL but not until the document is finally shown Until then the flow analysis just records the connection between the gap and the variable When a document is shown the transitive closure of document references is com puted and the resulting cluster of documents is produced with references replaced by
65. y At the same time the lt bigwig gt project is a case study in applications of the domain specific language design paradigm We have argued that the notion of sessions is essential to Web services and should constitute the basic structure of a Web service programming language Together with higher order document templates such as in the DynDoc sub language the dynamic construction of Web pages becomes at the same time flexible making it is easy to use and safe through compile time guarantees regarding document validity and the use of input forms We have shown that form field validation compared to traditional approaches can be made easier with a domain specific sub language such as PowerForms which automatically translates high level specifications into a combination of more low level server side and client side code We have examined how temporal logics can be use to synthesize concurrency controllers Finally we have demonstrated how macro mechanisms can be made effective for extending and combining languages in the context of the sub languages of lt bigwig gt Version 2 0 of the lt bigwig gt compiler and runtime system is freely available from the project home page at www brics dk bigwig where documentation and examples also can be found Regarding the future development of lt bigwig gt we now move towards Java We are developing JWIG 11 as an extension of Java where we add the most successful features of lt bigwig gt such

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