An Introduction To Web Services
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Web services are a class of Web application, published, located and accessed via the Web, that communicates via an XML (eXtensible Markup Language) interface [1]. As they are accessed using Internet protocols, they are available for use in a distributed environment, by applications on other computers.
The idea of Internet-accessible programmatic interfaces, services intended to be used by other software rather than as an end product, is not new. Web services are a development of this idea. The name refers to a set of standards and essential specifications that simplify the creation and use of such service interfaces, thus addressing interoperability issues and promoting ease of use.
Well-specified services are simple to integrate into larger applications, and once published, can be used and reused very effectively and quickly in many different scenarios. They may even be aggregated, grouped together to produce sophisticated functionality.
The Google spellchecker service, used by the Google search engine, suggests a replacement for misspelt words. This is a useful standard task; simply hand it a word, and it will respond with a suggested spelling correction if one is available. One might easily imagine using the service in one's own search engine, or in any other scenario in which user input is taken, perhaps in an intelligent "Page not found" error page, that attempts to guess at the correct link. The spellchecker's availability as a Web service simplifies testing and adoption of these ideas.
Furthermore, the use of Web services is not limited to Web-based applications. They may also usefully be integrated into a broad spectrum of other applications, such as desktop software or applets. Effectively transparent to the user, Web service integration permits additional functionality or information to be accessed over the Web. As the user base continues to grow, many development suites focus specifically on enabling the reuse and aggregation of Web services.
'Web services' refers to a potentially huge collection of available standards, so only a brief overview is possible here. The exchange of XML data uses a protocol such as SOAP or XML-RPC. Once published, the functionality of the Web service may be documented using one of a number of emerging standards, such as WSDL, the Web Service Description Language.
WSDL provides a format for description of a Web service interface, including parameters, data types and options, in sufficient detail for a programmer to write a client application for that service. That description may be added to a searchable registry of Web services.
A proposed standard for this purpose is UDDI (Universal Description, Discovery and Integration), described as a large central registry for businesses and services. Web services are often seen as having the potential to 'flatten the playing field', and simplify business-to-business operations between geographically diverse entities.
Due to the popularity of the architecture, many resources exist to support the development and use of Web services in a variety of languages and environments. The plethora of available standards may pose a problem, in that a variety of protocols and competing standards are available and in simultaneous use. Making that choice depends very much on platform, requirements and technical details.
Although Web services promise many advantages, there are still ongoing discussions regarding the best approaches to the underlying technologies and their scope.
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Last Modified 2-December-2005
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