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Distributed Objects and Application Servers
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directly access the CORBA ORB, if required for communication with other CORBA applications that may be written in a different language. Figure 5.10 summarizes some of the interactions an EJB component can have with other J2EE components and other CORBA applications that may be written in a language other than Java. However, this kind of interaction is assured only by some implementation vendors. Notable among these vendors is IBM, who ensures interoperability between their various ORB products. In the context of J2EE-specific application servers, it is interesting to consider the parallels between the CORBA and J2EE programming models. Java has another independent method of distributed objects, called Remote Method Invocation (RMI). The RMI architecture is very similar to the CORBA architecture but is limited to the Java language. In other words, RMI provides functionality and services very similar to CORBA for applications written in Java only. To use services provided by RMI, the client first must obtain a reference to the remote object. To do that, the client must know where to find the remote object, what it is called, and what method call it provides. These location services are provided by an RMI registry, to which the remote object must register first. Java provides an interface called the Java Naming and Discovery Interface (JNDI), which is used to locate and bind to the server object. Table 5.1 summarizes some of the parallels between CORBA and RMI.
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Figure 5.10 Interoperation of the Java/J2EE application server
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Five Parallels Between the CORBA and RMI Programming Models CORBA Programming Model EJB Programming Model Java EJB home and remote interface or IDL RMI code or IDL code IIOP and RMI-to-IDL mapping (if required) CORBA security interface JNDI or CosNaming Service Java Transaction Service or Object Transaction Service
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Table 5.1
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Interface de nition language Object invocation Communication protocols Security protocol Naming service Transaction service
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The second most common type of application servers are those that cater to C++ applications and components. Notable among this type of application server is IBM s WebSphere Application Server Enterprise Edition and VisiBroker for C++. In general, various products from a given vendor interoperate quite well. For example, IBM s WebSphere Application Server works together with IBM s WebSphere Application Server Enterprise Edition quite well. In theory, the various ORB products from different vendors should also interoperate quite well. However, in practice, the following factors restrict or limit the full interaction between CORBA applications from different vendors:
Proprietary extensions Some ORB implementations have added proprietary extensions to the CORBA speci cations. Speci cation levels Some ORB implementations conform to different levels of the CORBA speci cations, and these different levels are not always compatible with each other. Ambiguities Some ORB implementations differ in the way they implement parts of the CORBA speci cations because of ambiguities in the speci cations. Bugs Some CORBA ORBs simply have bugs. Interpretability problems Problems with interpretability are more serious if the products are designed to be used with different languages.
Conclusion In this chapter, we introduced the concept of distributed objects by moving away from procedural languages such as C and into the realm of object-oriented programming (OOP) and object-oriented design (OOD). Examples of object-oriented languages include Java, C++,
Distributed Objects and Application Servers
and Visual Basic (VB). We extended the concept of objects to include distributed objects, where the objects can be distributed over a network. Furthermore, we described the standard CORBA, which allows remote objects to interact with one another. We took a big step forward in application integration by encapsulating the code for parameter marshalling and unmarshalling and the code for networking into a separate software component (or application). We called this component Object Request Broker (ORB). This remediates the problem of the lack of code reuse in the case of RPC, which was described in the last chapter. Various implementations of ORB form the backbone of all the modern commercial application servers, which are needed to support distributed objects. In addition, ORB has allowed us to move away from point-to-point integration, which is important if a large number of applications need to be integrated. Also, this move away from point-to-point integration leads to the concept of the Enterprise Service Bus (ESB), as you will see later. In addition, we introduced the concept of language independence via an interface definition language (IDL). The interfaces declared through an IDL can be mapped to any programming language and can allow, in principle, the client and server to be implemented in two different languages. Another important concept introduced in this chapter is the registry, which is used by the server objects to register themselves so they can be located by the client. The major issue we did not address in this chapter that was mentioned in 4 as a shortcoming of RPC, is the issue of scalability. This lack of scalability in cases of RPC and distributed objects results from the synchronous nature of the interaction between the server and the client. For synchronous method calls, the client is blocked from performing further work until the server completes its work and returns control to the client. In the next chapter, we discuss asynchronous messaging and address this issue in detail.
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