Web service

A web service is a broad term referring to various client-server approaches to distributing software across a network. More narrowly, a web service is a program addressable by a URL on a web server, which can be invoked from a client machine elsewhere on the network. Conceptually, a web service may be regarded as an application programming interface (API) distributed across a network and callable by the client without regard for the underlying operating system on which the web service runs.

There are two different situations in which so-called web services (client-server programs) may be invoked across a network. The first case, used widely since scripting languages first appeared to allow web servers to return dynamically-constructed pages, yields up a response web page to be read by or used by human readers. The second case involves a client program calling a server program; the response needs to be in an exact format that the client program expects so that the client program can parse the response. It is this latter situation which is the focus of this article.