The SOAP standard by W3C defines a communication protocol based on a textual form of message encoding in XML. Each message consists of a series of optional headers and a body, with the headers carrying information intended for systems that route the message and the body intended for the final recipient of the message. The messages are extensible and easy to transport via HTTP.

<SOAP:Envelope xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/"
  SOAP:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <!-- Header with additional information -->
  <SOAP:Header>
    <wscoor:CoordinationContext
      xmlns:wscoor="http://schemas.xmlsoap.org/ws/2003/09/wscoor"
      SOAP:mustUnderstand="true">
      <wscoor:Identifier>
        http://example.com/context/1234
      </wscoor:Identifier>
    </wscoor:CoordinationContext>
  </SOAP:Header>

  <!-- Body with message content -->
  <SOAP:Body>
    <m:aMethodRequest xmlns:m="http://example.com/soap.wsdl">
      <aNumber xsi:type="xsd:int">42</aNumber>
    </m:aMethodRequest>
  </SOAP:Body>

</SOAP:Envelope>

The SOAP standard also introduces a data model, which allows describing simple and compound types, as well as encoding rules, which allow encoding graphs of typed data. Unfortunately, the data model is not explicitly related to XML Schema, which is used to describe simple and compound types in WSDL. Encoding of types described using XML Schema therefore does not necessarily pass validation using the same XML Schema. This discrepancy makes it difficult to validate a SOAP encoded message given the WSDL description of the service for which the message is intended.

Many technologies prefer literal to encoded messages, with the language binding defined directly between the XML Schema in WSDL and the implementation language, rather than between the SOAP data model and the implementation language. This is the case of JAX-RPC and JAX-WS with JAXB.