Soap, WSDL, UDDI: Digesting The Alphabet Soup



Ask five CIOs to define Web services and you'll probably get five different answers. That's no surprise. Web services are a new and complex set of technologies, even though the phrase has been used generically for years to describe a broad range of Internet technologies.

But one definition is rising to the top: A Web service is a piece of software that can communicate with another application over a network by using a specific set of standard protocols--Simple Object Access Protocol, the Universal Description, Discovery, and Integration framework, the Web Services Description Language.

Soap is the fundamental message-passing protocol that defines how to send data, typically in XML format, among applications across a network. Soap is reminiscent of previous message-passing protocols that didn't quite deliver--the Common Object Request Broker Architecture and Microsoft's Distributed Component Object Model are perhaps the two best-known examples.

Soap can be used to build connections between applications. Those connections can then be described using WSDL. A developer can use the description to design an application to connect to the Web service. When Web services become more sophisticated, an application will be able to use a WSDL description to automatically configure itself to connect to other Web services.

Those Web services have to be found, and that's where UDDI comes in. UDDI is a set of protocols and APIs that define a registry repository where Web services and their associated WSDL descriptions can be catalogued and searched. In the future, businesses or even automated-agent software may first search UDDI registries to find suppliers.

Soap, WSDL, and UDDI are platform-independent technologies that make extensive use of XML, a standard language that's used to define protocols and encode the data stream that applications employ to communicate with each other.

Though Soap, WSDL, and UDDI are frequently referred to as standards, they haven't been blessed by a recognized organization such as the World Wide Web Consortium. Today, Soap is a draft standard within the consortium that will likely be superseded by a specification called XML Protocol, or XMLP. WSDL isn't as far along, but has been submitted to the consortium. UDDI is being guided by the UDDI Project, but it's likely that only parts of that specification will be submitted to the consortium.

Without widely accepted vendor-neutral standards, Web services could end up in the junkyard of also-ran attempts to unify business logic, along with Corba, DCOM, EDI, Java Remote Method Invocation, and Unix Remote Procedure Call. Only Web services have come close to succeeding.

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