The World Wide Web (WWW) is rapidly evolving from a platform for serving up static documents to a platform for deploying distributed, cross-platform, and language and location independent applications. There is thus a need for appropriate frameworks that would allow not only the deployment but also the development of such applications. In this paper, we describe the Web Request Broker (WRB), a system developed here at Oracle that provides a distributed, scalable, and daemon-independent framework for developing and deploying distributed web-based applications. The framework description is along three dimensions. The first dimension is the overall architecture of the WRB. The next dimension is the application programming interface (API) offered by the WRB to applications, and the final dimension is the kinds of programming paradigms that can be used with the above API to develop applications.
The WRB is part of Oracle's Web Application Server. The WRB provides a distributed, scalable, daemon-independent framework for developing and deploying distributed web-based applications. Bernstein defines a framework in the context of application development environments as follows [2]:
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A framework is a software environment that is defined
to simplify application development and system management for a
specialized application domain.
The application domain in our case is quite general and encompasses web-based distributed applications. The WRB provides a framework to support the development and deployment of such applications. We describe this framework here. The framework description is along three dimensions. The first dimension is the overall architecture of the WRB. The next dimension is the application programming interface (API) offered by the WRB to applications, and the final dimension is the kinds of programming paradigms that can be used with the above API to develop applications.
The rest of the paper is organized as follows: Section 2 describes the overall architecture of the WRB. Section 3 describes cartridges, an application structuring mechanism provided by the WRB. Section 4 discusses the API offered by the WRB to application developers. Section 5 describes programming paradigms that can be used with the WRB API. Section 6 discusses related work and Section 7 concludes the paper.
These include the dispatcher, application and system cartridges , and a Corba [4] compliant Object Request Broker (ORB) . The dispatcher and cartridges are built over the ORB and utilize the ORB for inter-component communication. This allows the components to be distributed on remote machines, for performance and scalability reasons.
The dispatcher is responsible for routing requests received from the HTTP daemon to the appropriate cartridge. It is designed to be extensible and daemon independent.
Cartridges are modules of code performing specific application or system functions. They form the centerpiece of the WRB framework. We discuss cartridges in detail in the following section. For now it suffices to think of cartridges as specialized code modules.
The typical stages involved in processing a client request are as follows. Upon receiving the client request from the underlying daemon, the dispatcher determines which cartridge needs to handle the request by using the naming service system cartridge. It then authenticates the request using the authentication service system cartridge . Once authenticated, the dispatcher obtains an instance of the cartridge that will handle the request from the resource manager system cartridge. Finally, it dispatches the request for execution to that cartridge by calling the execution manager system cartridge.
interface Cartridge {
boolean init();
boolean authenticate(in Principal user_passwd);
boolean exec(in Request req_obj, out Response resp_obj);
boolean shutdown();
boolean quiesce();
boolean unquiesce();
}
The init() routine is responsible for intializing the cartridge instance. This may include invoking the constructors of several subobjects, preforking threads and acquiring all other required shared resources. The shutdown() routine is responsible for cleaning up all of the resources and shutting down the cartridge instance. Once the shutdown() routine is invoked on a cartridge instance, it immediately becomes unavailable for servicing subsequent requests. The authenticate() routine validates whether the client requesting the services of the cartridge is authorized to use those services. The exec() routine is the generic way to dispatch all service requests to the cartridge. The quiesce() and unquiesce() routine are called to temporarily suspend/resume the cartridge instance without bringing it down. Once the cartridge instance has been quiesced, it will ignore all routines except for unquiesce(). Normal processing resumes after unquiescing.
The typical states that the cartridge goes through when implementing the request/response paradigm are shown in Fig. 3. When the cartridge receives a request, it transitions from the initial state (state 1) to the next state (state 2). In state 2 it does various things related to the request received. When the cartridge sends the response to the requesting client, it transitions to state 3, where it is done executing. Note that there are other intermediate states possible, but the three states shown capture the essential nature of the request/response paradigm.
The typical states that the cartridge goes through when implementing the session paradigm are shown in Fig. 4. When the cartridge receives a request, it transitions from the initial state (state 1) to the next state (state 2). In state 2 it does various things related to the request received. When the cartridge sends the response to the requesting client, it transitions back to state 1, where it can receive additional requests from clients. When the session has been in existence for the specified duration, the cartridge transitions to state 3, where it is done executing.
In the next example, as shown in Fig. 7, a client makes a request to a cartridge that involves the cartridge initiating a transaction on the database. After the cartridge responds to the client's original request, the cartridge maintains state regarding the client's original request, in particular information about the active transaction. The client then sends another request, possibly with information to update the database. The cartridge includes this information within the active transaction. Finally, the client sends a third request to commit the transaction to the cartridge. Thus the cartridge retains state about the client's request across multiple requests for the duration of the session. The transactional semantics ensure that the operations executed by the cartridge during the session are atomic, i.e., either all are applied to the database, or none are.
The systems offered by Apache [1], Purveyor/Microsoft [5], Netscape [7], and Spyglass [8] are all structured such that applications run in the same address space as the web server. In the WRB, the application runs in a separate process. However, unlike CGI [3], we do not create a new process for each request. The WRB maintains a pool of processes, and processes are re-used from this pool in order to satisfy incoming requests. Java servlets from Sun [6] uses a similar approach by maintaining a pool of threads.
[1] Apache API Notes, Apache, 1996.
[2] P. A. Bernstein, "Middleware: A Model for Distributed System Services," Communications of the ACM, Vol. 39, No. 2, Feb. 1996, pp. 86-98.
[3] CGI version 1.1 Specification, 1996.
[4] The Common Object Request Broker: Architecture and Specification, revision 2.0, Object Management Group, Cambridge, MA, 1995.
[5] ISAPI Specification, Purveyor Corp., Framingham, MA, 1996.
[6] Java Servlet API White Paper, Sun Microsystems Inc., Mountain View, CA, 1996.
[7] Netscape API Functions, Netscape Communications, Mountain View, CA, 1996.
[8] Spyglass Web Server SDK 2.1, Spyglass Inc., Naperville, IL, 1996.
[9]
Web Request Broker API,
Oracle Corp., Redwood Shores, CA, 1996.