In computer science, a remote procedure call (RPC) is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space (commonly on another computer on a shared network) without the programmer explicitly coding the details for this remote interaction. That is, the programmer writes essentially the same code whether the subroutine is local to the executing program, or remote. When the software in question uses object-oriented principles, RPC is called remote invocation or remote method invocation.
Many different (often incompatible) technologies have been used to implement the concept.
The idea of treating network operations as remote procedure calls goes back at least to the 1980s in early ARPANET documents.Bruce Jay Nelson is generally credited with coining the term. One of the first business uses of RPC was by Xerox under the name "Courier" in 1981. The first popular implementation of RPC on Unix was Sun's RPC (now called ONC RPC), used as the basis for Network File System.
An RPC is initiated by the client, which sends a request message to a known remote server to execute a specified procedure with supplied parameters. The remote server sends a response to the client, and the application continues its process. While the server is processing the call, the client is blocked (it waits until the server has finished processing before resuming execution), unless the client sends an asynchronous request to the server, such as an XHTTP call. There are many variations and subtleties in various implementations, resulting in a variety of different (incompatible) RPC protocols.
An important difference between remote procedure calls and local calls is that remote calls can fail because of unpredictable network problems. Also, callers generally must deal with such failures without knowing whether the remote procedure was actually invoked. Idempotent procedures (those that have no additional effects if called more than once) are easily handled, but enough difficulties remain that code to call remote procedures is often confined to carefully written low-level subsystems.
Sequence of events during an RPC
The client calls the client stub. The call is a local procedure call, with parameters pushed on to the stack in the normal way.
The client stub packs the parameters into a message and makes a system call to send the message. Packing the parameters is called marshalling.
The client's local operating system sends the message from the client machine to the server machine.
The server stub unpacks the parameters from the message. Unpacking the parameters is called unmarshalling.
Finally, the server stub calls the server procedure. The reply traces the same steps in the reverse direction.
Standard contact mechanisms
To let different clients access servers, a number of standardized RPC systems have been created. Most of these use an interface description language (IDL) to let various platforms call the RPC. The IDL files can then be used to generate code to interface between the client and server.