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SOAP

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SOAP(Simple Object Access Protocol) is a protocol for exchanging XML-based messages over a computer network, normally using HTTP. SOAP forms the foundation layer of the Web services stack, providing a basic messaging framework that more abstract layers can build on. SOAP can be used to facilitate a Service-Oriented architectural pattern.

There are several different types of messaging patterns in SOAP, but by far the most common is the Remote Procedure Call (RPC) pattern, where one network node (the client) sends a request message to another node (the server), and the server immediately sends a response message to the client. Indeed, SOAP is the successor of XML RPC.

Overview

Originally designed by Dave Winer, Don Box, Bob Atkinson, and Mohsen Al-Ghosein in 1998 with backing from Microsoft (where Atkinson and Al-Ghosein worked at the time) as an object access protocol, the SOAP specification is currently maintained by the XML Protocol Working Group of the World Wide Web Consortium.

At the time of the creation of the standard, SOAP was an acronym for "Simple Object Access Protocol". Version 1.2 of the standard, which became a W3C Recommendation on June 24 2003, dropped this acronym as it was considered to be misleading.

Transport methods

Both SMTP and HTTP are valid application layer protocols for SOAP, but HTTP has gained wider acceptance as it works well with today's Internet infrastructure; specifically, SOAP works well with network firewalls. This is a major advantage over other distributed protocols like GIOP/IIOP or DCOM which are normally filtered by firewalls. A key issue under discussion is whether or not HTTP is the right transport given its inherent asynchronous nature.

XML was chosen as the standard message format because of its widespread acceptance by major corporations and open source development efforts. Additionally, a wide variety of freely available tools significantly ease the transition to a SOAP-based implementation.

The somewhat lengthy syntax of XML can be both a benefit and a drawback. Its format is easy for humans to read, but can be complex and can have slow processing times. For example, CORBA, GIOP and DCOM use much shorter, binary message formats. On the other hand, hardware appliances are available to accelerate processing of XML messages. Binary XML (the use of the word "XML" is controversial here) is also being explored as a means for streamlining the throughput requirements of XML.


Weaknesses

  • Because of the lengthy XML format, SOAP is considerably slower than competing middleware technologies such as CORBA. Typically, SOAP is about 10 times slower than binary network protocols such as RMI or IIOP. This may not be an issue when only small messages are sent.
  • Because of its main reliance on HTTP as a transfer protocol, the roles of the interacting parties are fixed. Only one party (the client) can use the services of the other. So developers must use polling instead of notification in most cases.
  • Relies on Web Services Description Language, an external standard. There's no standard way to dynamically discover the services (methods, parameters) offered, nor to get a WSDL for a particular endpoint.

Example SOAP messages

Here is an example of how a client might format a SOAP message requesting product information from a fictional warehouse web service. The client needs to know which product corresponds with the ID 827635: 

 <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
   <soap:Body>
     <getProductDetails xmlns="http://warehouse.example.com/ws">
       <productID>827635</productID>
     </getProductDetails>
   </soap:Body>
 </soap:Envelope>

Here is how the warehouse web service might format its reply message with the requested product information: 

 <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
   <soap:Body>
     <getProductDetailsResponse xmlns="http://warehouse.example.com/ws">
       <getProductDetailsResult>
         <productName>Toptimate 3-Piece Set</productName>
         <productID>827635</productID>
         <description>3-Piece luggage set. Black Polyester.</description>
         <price>96.50</price>
         <inStock>true</inStock>
       </getProductDetailsResult>
     </getProductDetailsResponse>
   </soap:Body>
 </soap:Envelope>

See also

Alternatives to SOAP

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