Is SOAP All Washed Up? Are Web Services In?

In May 2000, SOAP 1.1 was submitted to the W3C as a Note, the Simple Object Access Protocol (SOAP) 1.1. Submitters are

• Ariba, Inc.,
• Commerce One, Inc.,
• Compaq Computer Corporation,
• DevelopMentor, Inc.,
• Hewlett Packard Company,
• International Business Machines Corporation,
• IONA Technologies,
• Lotus Development Corporation,
• Microsoft Corporation,
• SAP AG,
• UserLand Software Inc

SOAP provides a basic mechanism for message delivery and response. In that sense, it is a web service that is flexible and extensible. SOAP allows businesses to access Web Services directly, by using XML messages to invoke method calls on remote objects, and then receiving an XML response. SOAP has four parts:

1. envelope describing what is in the message, who the intended target is, and whether it is optional or mandatory
2. optional data encoding rules useful for failure detection
3. optional RPC convention: rules for forming the Request and Response
4. optional binding to HTTP (or another network protocol like SMTP)

Simeonov, a speaker from Allaire, distinguished between the first generation protocols, such as XML-RPC, WDDX, and LDO which consisted of a single DTD and no namespaces, and second generation protocols built upon XML Schema with namespace support, safe extensions, and better interoperability. He also described the key features of SOAP:

1. mandatory headers
2. header for intermediaries
3. pluggable data encoding
4. error handling model
5. versioning model

Simeonov maintained that we need more than SOAP provides. Web Services is indeed the hot buzzword, with a huge potential for a Network Economy based on a new application interaction model. B2B is rapidly overtaking B2C in its importance. He described a Web Services Interoperability Stack, listed from highest level to lowest level:

1. Service Integration / Workflow - UDDI, The Next Generation, perhaps
2. Service Discovery (and Advertising) - UDDI and DISCO
3. Service Descriptions - WSDL (technical) and UDDI (business)
4. Basic Web Services - SOAP
5. Data Format Specification - XML Schema (and other schema variants)
6. Data Representation - XML
7. Common Protocol - HTTP

Moreover, the W3C started its own XML Protocol Working Group around the beginning of 2000. In fact, W3C is looking at a number of protocols; see the XML Protocol Comparison matrix from March 2000. The speaker stated that emerging XML Protocol will eventually replace SOAP. No further revisions of SOAP are expected. The XML Protocol Working Group has been developing requirements since Sept. 2000.

For more details, see the article The Interoperability Stack by Simeonov in Vol. 2, Issue 1 of XML Journal, Dec. 2000.

Jon Bosak (Sun): Service APIs, UDDI, SOAP, and ebXML
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