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This type of function call is similar to the remote function call described in the next paragraph. However, this type of function call does not involve any network routines because both applications are running on the same machine and may be considered a type of interprocess communication (IPC). The particular implementation of this type of call we discuss is called Doors. We discuss Doors later in this chapter. The third type of functional call generally involves a client application on one host calling a function in another application running on another host, as long as the two hosts are connected by some form of network. This type of function call is depicted in Figure 4.3. We refer to this type of function call as RPC. We discuss this type of function call in detail later in this chapter. This type of call relies on sockets under the hood. Historically, the RPC type shown in Figure 4.3 was developed before the type shown in Figure 4.2. However, we will discuss the second type of call first because it is simpler to understand because there are no network routines. Normally, with Doors and RPC, the application calling the function is referred to as the client and the application in which the function resides
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Remote Procedure Call (RPC)
Types of Functions
Local function calls involve one piece of code in one application invoking another piece of code in the same application. Restricted remote procedure calls (restricted RPC or Doors) involve a piece of code in one application calling a piece of code in another application. The two applications must be running on the same host. RPC calls are similar to the restricted RPC, with the difference being that the two applications may be running on separate hosts connected by a network.
is called the server. In other words, the application calling the function is the service consumer whereas the application where the function resides is the service provider. Thus, both RPC and Doors involve the concept of services, and we may consider this to be the beginning of the services-based integration pattern.
Doors: Restricted Remote Procedure Calls (Restricted RPC)
In this section we discuss the case where one application (the server) makes a function available to the other applications (clients) running on the same computer, as shown in Figure 4.2. The particular implementation of this type of functionality exchange we discuss here is called Doors. Doors is restricted to Solaris systems and cannot be used on other systems. In addition, there are no other RPC systems of this type that can run on other platforms. When applications are running on a common host, they all share the same operating system, so it is natural to use the kernel of the operating system to provide communication between the applications. Thus in Doors, the kernel of the operating system is used to provide communication between the applications, which make system calls into the kernel. This is shown in Figure 4.4.
Computer Application A Application B
kernel
Figure 4.4 Applications running on the same system using the kernel to communicate. A network is not involved.
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The major contribution of Doors is that all the systems calls are encapsulated in a library with a header file called <door.h>. Therefore, the programmer doesn t need to know any system-level programming. The Doors library consists of a few functions, a couple of data structures, and a simple protocol to implement this restricted type of remote procedure call. It is a very intuitive and the most efficient way to share functionality if the applications are running on the same host. In fact, Doors can be considered a type of interprocess communication. In order to understand how Doors works, we start by looking at a simple example. In this example, one application (the server) provides a function to another application (the client) to invoke. The function provided by the server takes a long integer as an input and returns the cube of the input. The server-side code for this example is discussed next.
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