Programmers have made extensive use of XML in SOA to structure data that they wrap in a nearly exhaustive description-container. Analogously, the Web Services Description Language (WSDL) typically describes the services themselves, while the SOAP protocol describes the communications protocols. Whether these description languages are the best possible for the job, and whether they will become/remain the favorites in the future, remain open questions. As of 2008 SOA depends on data and services that are described by metadata that should meet the following two criteria:
- The metadata should come in a form that software systems can use to configure dynamically by discovery and incorporation of defined services, and also to maintain coherence and integrity. For example, metadata could be used by other applications, like a catalogue, to perform autodiscovery of services without modifying the functional contract of a service.
- The metadata should come in a form that system designers can understand and manage with a reasonable expenditure of cost and effort.
SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks. It relies on Extensible Markup Language (XML) for its message format, and usually relies on other Application Layer protocols, most notably Hypertext Transfer Protocol (HTTP) and Simple Mail Transfer Protocol (SMTP), for message negotiation and transmission.
SOAP can form the foundation layer of a web services protocol stack, providing a basic messaging framework upon which web services can be built. This XML based protocol consists of three parts: an envelope, which defines what is in the message and how to process it, a set of encoding rules for expressing instances of application-defined datatypes, and a convention for representing procedure calls and responses. SOAP has three major characteristics: Extensibility (security and WS-routing are among the extensions under development), Neutrality (SOAP can be used over any transport protocol such as HTTP, SMTP, TCP, or JMS) and Independence (SOAP allows for any programming model).
As an example of how SOAP procedures can be used, a SOAP message could be sent to a web site that has web services enabled, such as a real-estate price database, with the parameters needed for a search. The site would then return an XML-formatted document with the resulting data, e.g., prices, location, features. With the data being returned in a standardized machine-parsable format, it can then be integrated directly into a third-party web site or application.
The SOAP architecture consists of several layers of specifications: for message format, Message Exchange Patterns (MEP), underlying transport protocol bindings, message processing models, and protocol extensibility. SOAP is the successor of XML-RPC, though it borrows its transport and interaction neutrality and the envelope/header/body from elsewhere (probably from WDDX).