Web APIs

Machine-based interaction, a common feature of Web 2.0 sites, uses two main approaches to Web APIs, which allow web-based access to data and functions: REST and SOAP.

1. REST (Representational State Transfer) Web APIs use HTTP alone to interact, with XML (eXtensible Markup Language) or JSON payloads;

2. SOAP involves POSTing more elaborate XML messages and requests to a server that may contain quite complex, but pre-defined, instructions for the server to follow.

Often servers use proprietary APIs, but standard APIs (for example, for posting to a blog or notifying a blog update) have also come into wide use. Most communications through APIs involve XML or JSON payloads.

See also Web Services Description Language (WSDL) (the standard way of publishing a SOAP API) and this list of Web Service specifications.