Introducing SOAP (Simple Object Access Protocol)

In Lesson 15, "Web Services and the REST Protocol," we discussed web services and in particular saw how the REST (Representational State Transfer) protocol can be used to provide a consistent application programming interface (API) to such services.

REST is a good example of a protocol designed to operate with resource-oriented services, those that provide a simple mechanism to locate a resource and a set of basic methods that can manipulate that resource. In a resource-oriented service, those methods normally revolve around creating, retrieving, modifying, and deleting pieces of information.

In the case of REST, the methods are those specified in the HTTP specificationsGET, POST, PUT, and DELETE.

In certain cases, however, we are more interested in the actions a web service can carry out than in the resources it can control. We might perhaps call such services action-oriented. In these situations the resources themselves may have some importance, but the key issues concern the details of the activities undertaken by the service.

Perhaps the most popular and widely used protocol for designing action-oriented web services is SOAP, the Simple Object Access Protocol.

This lesson looks at SOAP, comparing and contrasting it where appropriate with the REST protocol discussed in Lesson 15.

Note

The full name Simple Object Access Protocol has been dropped in the later versions of the SOAP specifications, as it was felt that the direction of the project had shifted and the name was no longer appropriate. The protocol continues to be referred to as SOAP.

The Background of the SOAP Protocol

SOAP began in the late 1990s when XML was itself a fledgling web technology and was offered to the W3C in 2000. SOAP and another XML-based web service protocol, called XML-RPC, had a joint upbringing.

SOAP was designed essentially as a means of packaging remote procedure calls (requests to invoke programs on remote machines) into XML wrappers in a standardized way.

Numerous enterprises contributed to the early development of SOAP, including IBM, Microsoft, and Userland. The development of SOAP later passed to the XML Protocols Working Group of the W3C.

Tip

You can get the latest information on the SOAP specification from the W3C website at http://www.w3.org/2000/xp/Group/.