Remote Procedure Calls
The best way to test the RPC application is to run both the client and the server (the caller and callee) on the the same machine. Assuming that you are in the directory where both the client and the server reside, start the server by entering the command:
The rpcinfo utility can be used to verify that the server is running. Typing the command:
$ rpcinfo -p localhostgives the following output:
program vers proto port 100000 2 tcp 111 portmapper 100000 2 udp 111 portmapper 22855 1 udp 1221 22855 1 tcp 1223Note that 22855 is the program number of our application from avg.x and 1 is shown as the version number. Since 22855 is not a registered RPC application, the rightmost column is blank. If we add the following line to the /etc/rpc file:
avg 22855rpcinfo then gives the following output:
program vers proto port 100000 2 tcp 111 portmapper 100000 2 udp 111 portmapper 22855 1 udp 1221 avg 22855 1 tcp 1223 avgTo test the application, use the command:
$ ravg localhost $RANDOM $RANDOM $RANDOMand the following values are returned:
value = 9.196000e+03 value = 2.871200e+04 value = 3.198900e+04 average = 2.329900e+04Since the first argument to the command is the DNS name for the host running the server, localhost is used. If you have access to a remote host that allows RPC connections (ask the system administrator before you try), the server can be uploaded and run on the remote host, and the client can be run as before, replacing localhost with the DNS name or IP address of the host. If your remote host doesn't allow RPC connections, you may be able to run your client from there, replacing localhost with the DNS name or IP address of your local system.
The ONC implementation of RPC is not the only one available. The Open Software Foundation has developed a suite of tools called the Distributed Computing Environment (DCE) which enables programmers to develop distributed applications. One of these tools is DCE RPC which forms the basis for all of the other services that DCE provides. Its operation is quite similar to ONC RPC in that it uses components that closely parallel those of ONC RPC.
Application interfaces are defined through an Interface Definition Language (IDL) which is similar to the language used by ONC RPC to define XDR filters. Network Data Representation (NDR) is used to provide hardware independent data representation. Instead of using programmer-defined integer program numbers to identify servers as does ONC RPC, DCE RPC uses a character string called a universal unique identifier (UUID) generated by a program called uuidgen. A program called rpcd (the RPC daemon) takes the place of portmap. An IDL compiler can be used to generate C headers and client/server stubs in a manner similar to rpcgen.
Although the entire DCE suite is commercially sold and licensed, the RPC component (which is the basis for all the other services) is available as freeware. See the references section for more information on DCE RPC.
The sample application presented here is certainly a naive one, but it serves well in presenting the basic principles of RPCs. A more interesting set of applications can be found in the Network Information System (NIS) package for Linux (see the references section). Also, the Linux kernel sources contain an implementation of Sun's Network File System (NFS), an excellent example of the use of RPC applied to the problem of distributed file access.
In addition to distributed data access, RPC can also be used to harness the unused processing power present on most networks. The book Power Programming with RPC, listed in the references section, presents an image processing application that uses RPC to distribute CPU intensive tasks over multiple processors. With RPC, you have the capability to boost the performance of your applications without spending a dime on additional hardware.
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