SOAP is a popular protocol for accessing a web service. My interest in this arose when I needed to schedule a web service using the Oracle Scheduler. This SOAP client had to be written from scratch using PL/SQL so I needed to become familiar with the protocol first.

This is a summarized form of the SOAP Tutorial.

SOAP is an XML dialect so the XML Cheat Sheet might be useful and in particular the XML Namespaces section .

For reference here are the SOAP Specifications .

• The Simple Object Access Protocol (SOAP) is a simple XML based protocol to let apps exchange information over HTTP.

• SOAP is a W3C standard, the latest version is 1.2. SOAP is also a major part of .NET .

• A SOAP message
  • MUST contain an envelope root element
  • MAY contain a header element
  • MUST contain a body element
  • MAY contain a fault element
  • MUST use the SOAP Envelope and a SOAP Encoding namespaces.

• This is a sample SOAP message.

<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">
 
<soap:Header> ... </soap:Header>
<soap:Body>
  ...
  <soap:Fault> ... </soap:Fault>
</soap:Body>
 
</soap:Envelope>

• There are three attributes in the SOAP envelope namespace :
  • actor - URI of endpoint for which the element is intended
  • mustUnderstand - 0 or 1 to specify whether the receiver is obligated to recognize the element or not.
  • encodingStyle

• The SOAP Envelope namespace for elements must always be http://www.w3.org/2001/12/soap-envelope .

• One possible SOAP Encoding namespace for SOAP encoding and data types is http://www.w3.org/2001/12/soap-encoding .

The optional SOAP Fault element is used to hold error and status information for a SOAP message. It can have the following sub-elements :

• faultcode - A code for identifying the fault. The faultcode must be one of the following :
  • VersionMismatch - Found an invalid namespace
  • MustUnderstand - A required element was not understood
  • Client - There was an error in the message
  • Server - There was an error on the server processing the message
• faultactor - The endpoint that caused the fault
• faultstring - A human-readable explanation
• details - Application-specific error info