RPC is an easy and popular paradigm for implementing the client-server model of distributed computing. An RPC is initiated by the caller (client) sending a request message to a remote system (the server) to execute a certain procedure using arguments supplied. A result message is returned to the caller. There are many variations and subtleties in various implementations, resulting in a variety of different (incompatible) RPC protocols.
In order to allow servers to be accessed by differing clients, a number of standardized RPC systems have been created. Most of these use an interface description language (IDL) to allow various platforms to call the RPC. Examples of such systems include Microsoft's DCOM (and ActiveX), and CORBA.
Recently a number of vendors have started using XML as the IDL, and HTTP as the network protocol. The advantage of this system, known as web services, is simplicity and standardization, the IDL is a text file that is widely understood, and HTTP is built into almost all modern operating systems. An example of such an RPC system is SOAP, developed in turn from XML-RPC.