Web services is one of those concepts made all the more difficult to understand because of the myriad acronyms and abbreviations that pepper any conversation. The diagram below puts the most important concepts in their place: Soap/XML, UDDI and WSDL.
UDDI says where services are, WSDL describes what they do and Soap talks to them.
As Figure 1
shows, the process flow of a Web service is:
- Search UDDI site(s) for the proper Web service.
Latest Standards:http://www.UDDI.org 2. Description
- A description of the selected Web service is returned to the client application as a Web Services Description Language (WSDL) file.
Latest Standards:http://www.w3.org/2002/ws/desc/ 3. Proxy creation
- A local proxy to the remote service is created.
There are no standards associated with a proxy, apart from the Soap/XML message that it creates (see below). The proxy converts an object's means of method invocation into an XML message, and vice versa.
4. Soap Message Creation
- a Soap/XML message is created and sent to the URL specified in the WSDL file.
Latest Standards: http://www.w3.org/http://www.ws-i.org 5. Listener
- A Soap listener at the host site receives the call and interprets it for the Web Service.
6. The Web service
performs its function, and returns the result back to the client, via the listener and the proxy.
Of course to achieve this utopia of Web services talking to each other, the technologies have to be there and work together properly. On the next page, we explain just how far they have each come -- and much farther they have to go. UDDI:
UDDI (Universal Description, Discovery and Integration) has yet to live up to its original promise. UDDI acts a registry and locator service for Web services; applications access UDDI directories automatically, in the background, to find the information they need to complete transactions.
Although UDDI was introduced nearly three years ago, adoption has been slow compared with that of other Web services standards, such as the Simple Object Access Protocol (Soap) and Web Services Description Language (WSDL).
Originally, UDDI registries were envisioned as a locator service on the Internet, allowing companies to tap into a broad network of third-party Web services applications. In practice, the directories have been used within corporate networks as a way to store information on available Web services, according to analysts.
Even here, progress has at best been slow. Gary Barnett of analyst firm Ovum believes that most companies have no immediate need for UDDI, or at least UDDI in its existing form. Perhaps more surprisingly, this view appears to be shared by HP. "UDDI has a precursor problem," says George Bathurst, HP UK software marketing manager. "Before Web services become reality, companies need to integrate their own applications. Most companies are just not at that stage yet."
Bathurst says that UDDI has been overhyped and is still a year away from being commonplace, even though it could enable savings in the supply chain. Part of the problem, say analysts, is that UDDI currently lacks useful tools, which means many firms have little reason to use it, but this could change if it can gain the interest of developers.
Things looked up for UDDI in May 2003 when the Organisation for the Advancement of Structured Information Standards (Oasis) said that an update to UDDI had been ratified as a standard. With the ratification of UDDI version 2.0, software companies and individual developers should now be able to build software based on the standard and know that their applications will work together.
Version 2 of UDDI introduces the ability to attach a classification to a Web services directory. This allows a company to identify the contents of UDDI directory to include information such as the company's industry, product categories or geographical information, according to Oasis.
Members of group say that the UDDI technical committee is well on its way to completing the standardisation of a much-anticipated version 3 of the specification. Version 3 introduces the ability to have several interconnected registries and improves the security of UDDI directories with digital signatures.
Soap (Simple Object Access Protocol) acts as a transport mechanism to send data between applications or from applications to people. Soap, along with Extensible Markup Language (XML) and the Web Services Description Language (WSDL), is considered to be the foundation of Web services.
The World Wide Web Consortium (W3C) in spring 2003 proposed a recommendation for Soap 1.2 that puts this version in prime position to become an official standard. Version 1.2 of Soap includes enhancements designed to simplify Web services development with Soap toolkits. Soap 1.2 resolves a number of lingering problems with Soap 1.1 and introduces a "processing model" that allows a developer to establish rules on how Soap messages are handled. It also has a number of XML-oriented enhancements that will make it easier to manipulate data formatted as XML documents, the W3C said.
If you think of the Web services example on the previous page as an ordinary phone call, then in Web services parlance, XML represents the conversation, Soap describes the rules for how to call someone, and UDDI is the phone book. WSDL, finally, describes what the phone call is about and how one can participate. It enables one to separate the description of the abstract functionality offered by a service from concrete details of a service description such as "how" and "where" that functionality is offered
The W3C published its latest working draft of the latest version of the specification, WSDL 1.2, in June 2003. According to W3C, WSDL 1.2 will be easier and more flexible for developers than the previous version. The latest iteration of WSDL includes better component definition, language clarifications, a conceptual framework that defines description components, and support of the XML Schemas and XML Information Set standards. Furthermore, 1.2 removes non-interoperable features from WSDL 1.1 and more effectively works with HTTP and Soap. The W3C has given no indication as to when WSDL 1.2 will be ratified.
Find out more about Web services in this IT Priorities Special Report
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