SOAP (protocol)

SOAP is a client-server protocol for RPC messaging between a Web Service and its clients in computer networks. The SOAP request and response format is Extensible Markup Language (XML), and the messages are transmitted using HTTP). The SOAP specification grew out of an industry consortium and relies on compilers to automatically generate SOAP messages based on a web service contract (WSDL) file.

SOAP also refers to a dialect (encoding) of XML; SOAP-encoded XML is designed to be more concise that pure XML by providing pointer attributes for any duplicated data values. However, SOAP-encoded XML was not popular for use in SOAP protocol messaging, and after early experiments, pure XML became widely preferred as the SOAP protocol message format.

The name SOAP was originally defined as Simple Object Access Protocol, but this acronym fell out of favor because it is confusing to people.

As a layman's example of how SOAP procedures can be used, a SOAP message could be sent to a web service enabled web site (for example, a house price database) with the parameters needed for a search. The site would then return an XML-formatted document with the resulting data (prices, location, features, etc). Because the data is returned in a standardized machine-parsable format, it could then be integrated directly into a third-party site.

Sample SOAP message

Request

<soapenv:Envelope
    xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://schemas.xmlsoap.org/soap/envelope/
                        http://schemas.xmlsoap.org/soap/envelope/">
  <soapenv:Body>
    <req:echo xmlns:req="http://localhost:8080/axis2/services/MyService/">
      <req:category>classifieds</req:category>
    </req:echo>
  </soapenv:Body>
</soapenv:Envelope>

Response

<soapenv:Envelope
    xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
    xmlns:wsa="http://schemas.xmlsoap.org/ws/2004/08/addressing"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://schemas.xmlsoap.org/soap/envelope/
                        http://schemas.xmlsoap.org/soap/envelope/">
  <soapenv:Header>
    <wsa:ReplyTo>
      <wsa:Address>http://schemas.xmlsoap.org/ws/2004/08/addressing/role/anonymous</wsa:Address>
    </wsa:ReplyTo>
    <wsa:From>
      <wsa:Address>http://localhost:8080/axis2/services/MyService</wsa:Address>
    </wsa:From>
    <wsa:MessageID>ECE5B3F187F29D28BC11433905662036</wsa:MessageID>
  </soapenv:Header>
  <soapenv:Body>
    <req:echo xmlns:req="http://localhost:8080/axis2/services/MyService/">
      <req:category>classifieds</req:category>
    </req:echo>
  </soapenv:Body>
</soapenv:Envelope>