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The port mapper is a software component that must be started before any RPC server is invoked. When an RPC server is started, it will tell the port mapper what port number it is listening to, and what RPC program numbers it is prepared to serve. When a client wishes to make an RPC call to a given program number, it will rst contact the port mapper on the server machine to determine the port number where RPC packets should be sent.
one or more network messages. This packaging of arguments is called marshalling. An important aspect of this marshalling is that the byteordering differences among the different platforms are handled automatically using a standard XDR, thus making RPC platform independent. Another important component RPC has introduced is the RPC runtime library. The client stub uses the functions provided in the RPC runtime library to make a systems call into the local kernel in order to send the packaged message over the network to the server machine using a protocol such as TCP. In other words, the RPC runtime encapsulates all the systems calls necessary for the connectivity (that is, to send the packaged arguments over the network). Therefore, the programmer doesn t need to know any systems programming. On the server side, as the network message is received by the network routines in the kernel, it is sent to the server stub via the RPC runtime. The server stub unmarshals the input parameters and invokes the requested local procedure in the server routines. After the local procedure is completed, the server stub marshals the return value into one or more network messages and sends the packaged return value to the server kernel via the RPC runtime. The server kernel sends the message over to the client machines using a network protocol such as TCP. The client stub reads the network messages from the kernel through the use of RPC runtime routines. After possibly converting the return values, the client stub finally returns to the client function. This step appears to be a normal procedure returned to the client.
RPC was an important step in the development of an integration pattern because it outlined (for the rst time) all the steps necessary for applications to share functionality. RPC can be used between any two applications to share functionality. The applications may be running on separate machines connected by a network. Many of the concepts and components introduced by RPC continue to be used in more modern methods of sharing functionality, as discussed in later chapters. Some of these concepts include the marshalling and unmarshalling of arguments, client and server stubs, and the encapsulation of all system and network calls within a library or a separate software component.
Four
RPC Summary
RPC has further extended the concepts introduced with Doors by including applications running on separate machines. The machines are connected by a network. We further consolidated the concept of encapsulating all system-level calls in a library. This library now includes network-associated calls as well. The discussion of RPC also introduced the concept of marshalling of arguments, which packages the arguments in a system-independent manner for transmission over the network. We also introduced client and server stubs. The client stub acts as a proxy for the server-side code and makes it transparent for the client-side programmer to call the exposed server function. In summary, RPC for the first time outlined all the steps necessary for sharing functionality between applications that may be running on different hosts. Conclusion RPC and the associated Doors allowed for the first time real distributed computing by allowing applications to share functionality. A number of new concepts were introduced in this chapter (including service provider (server) and service consumer (client), platform independence, interface definition, the marshalling of input and output parameters, and the encapsulation of systems calls in a library) that are necessary for communication over the network. However, RPC has a number of shortcomings, including the following:
There is little room for code reuse because the code for marshalling and unmarshalling and the code for network communication are buried in the client and server applications. RPC is not language independent, and the client and the server must employ the same programming language. Tight temporal coupling exists between the applications. Because the calls are synchronous, the client application must wait for the server to complete the procedure before it can proceed further. The integration of the client and server is point to point and therefore not suitable when a number of applications need to be integrated. RPC is not suitable if a large number of remote calls are involved. Because of the synchronous nature of the call, the client cannot proceed further before the server completes its work. (Note that this problem can be overcome by using multithreading programming. However, this increases the complexity level of the programming and introduces some risks.)
Remote Procedure Call (RPC)
To improve on RPC, two paths have been taken. The first method involves distributed objects, also known as Object Request Broker (ORB), and the second method involves asynchronous messaging. The distributed objects approach focuses on code reuse and language independence, whereas asynchronous messaging addresses the problem of tight coupling between applications. We discuss the distributed objects (or ORBs) approach in the next chapter because it is more closely aligned with RPC. ORB essentially takes the encapsulation of all system-level calls into a library a step further by making this a separate software component (or executable). Today, most of the application servers such as WebSphere, WebLogic, and JBoss are based on ORB technology.
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