Stub (distributed computing)

A stub in distributed computing is a piece of code used for converting parameters passed during a Remote Procedure Call (RPC).

The main idea of an RPC is to allow a local computer (client) to remotely call procedures on a remote computer (server). The client and server use different address spaces, so conversion of parameters used in a function call have to be performed, otherwise the values of those parameters could not be used, because of pointers to the computer's memory pointing to different data on each machine. The client and server may also use different data representations even for simple parameters (e.g., big-endian versus little-endian for integers.) Stubs are used to perform the conversion of the parameters, so a Remote Function Call looks like a local function call for the remote computer.

Stub libraries must be installed on client and server side. A client stub is responsible for conversion of parameters used in a function call and deconversion of results passed from the server after execution of the function. A server skeleton, the stub on server side, is responsible for deconversion of parameters passed by the client and conversion of the results after the execution of the function.

Stub can be generated in one of the two ways:

1. Manually: In this method, the RPC implementer provides a set of translation functions from which a user can construct his or her own stubs. This method is simple to implement and can handle very complex parameter types.
2. Automatically: This is more commonly used method for stub generation. It uses an interface description language (IDL), that is used for defining the interface between Client and Server. For example, an interface definition has information to indicate whether, each argument is input, output or both — only input arguments need to be copied from client to server and only output elements need to be copied from server to client.

function.