Simple Object Access Protocol (SOAP) is a specification from World Wide Web Consortium (W3C) (http://www.w3.org/TR/2007/REC-soap12-part0-20070427/). Although we are referring to SOAP-based web services here, SOAP is one of the specifications used to implement XML-based web services. There are a few other specifications required to implement SOAP web services, which we will see later. One of the premises of SOAP web services is the dynamic discovery and invocation of services. For example, an application can look for a service from the central directory and invoke it dynamically. However, in practice, very few enterprises would be willing to invoke services dynamically without testing them, so this aspect of SOAP web services is less utilized.

W3C has defined many specifications for SOAP web services, for example, specifications for messages, auto discovery, security, and service orchestration. However, at a minimum, we need to understand the following specification before we develop SOAP web services.