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A Web Services Primer by
Venu Vasudevan Introduction
Looking back over the last six years, it is hard to imagine networked computing without the Web. The reason why the Web succeeded where earlier hypertext schemes failed can be traced to a couple of basic factors: simplicity and ubiquity. From a service provider's (e.g. an e-shop) point of view, if they can set up a web site they can join the global community. From a client's point of view, if you can type, you can access services. From a service API point of view, the majority of the web's work is done by 3 methods (GET, POST, and PUT) and a simple markup language. The web services movement is about the fact that the advantages of the Web as a platform apply not only to information but to services. By "services", I don't mean monolithic coarse-grained services like Amazon.com, but, rather, component services that others might use to build bigger services. Microsoft's Passport, for instance, offers an authentication function exported on the Web. So hypothetically, an electronic newspaper like the Washington Post can avoid creating its own user authentication service, delegating it to Passport. Oracle's dynamic services whitepaper provides other examples of component services that are reusable building blocks: currency conversion, language translation, shipping, and claims processing, A more formal definition of a web service may be borrowed from IBM's tutorial on the topic. Web services are a new breed of Web application. They are self-contained, self-describing, modular applications that can be published, located, and invoked across the Web. Web services perform functions, which can be anything from simple requests to complicated business processes...Once a Web service is deployed, other applications (and other Web services) can discover and invoke the deployed service. IBM's web services tutorial goes on to say that the notion of a web service would have been too inefficient to be interesting a few years ago. But the trends like cheaper bandwidth and storage, more dynamic content, the pervasiveness and diversity of computing devices with different access platforms make the need for a glue more important, while at the same time making the costs (bandwidth and storage) less objectionable. Why bother with the Web, you say, when I've got my favorite middleware platform (RMI, Jini, CORBA, DCOM etc.)? While middleware platforms provide great implementation vehicles for services, none of them is a clear winner. The strengths of the Web as an information distributor, namely simplicity of access and ubiquity, are important in resolving the fragmented middleware world where interoperability is hard to come by. The Web complements these platforms by providing a uniform and widely accessible interface and access glue over services that are more efficiently implemented in a traditional middleware platform. Viewed from an n-tier application architecture perspective, the web service is a veneer for programmatic access to a service which is then implemented by other kinds of middleware. Access consists of service-agnostic request handling (a listener) and a facade that exposes the operations supported by the business logic. The logic itself is implemented by a traditional middleware platform.
So what is the web service platform? The basic platform is XML plus HTTP. HTTP is a ubiquitous protocol, running practically everywhere on the Internet. XML provides a metalanguage in which you can write specialized languages to express complex interactions between clients and services or between components of a composite service. Behind the facade of a web server, the XML message gets converted to a middleware request and the results converted back to XML. Wait a minute, you say, access and invocation are only the bare bones (this would be like saying CORBA is only IDL plus remote procedure calls). Life is never quite that simple. What about the platform support services -- discovery, transactions, security, authentication and so on -- the usual raft of services that make a platform a platform? That's where you step up to the next level. The Web needs to be augmented with a few other platform services, which maintain the ubiquity and simplicity of the Web, to constitute a more functional platform. The full-function web services platform can be thought of as XML plus HTTP plus SOAP plus WSDL plus UDDI. At higher levels, one might also add technologies such as XAML, XLANG, XKMS, and XFS -- services that are not universally accepted as mandatory.
Below is a brief description of the platform elements. It should be noted that while vendors try to present the emergent web services platform as coherent, it's really a series of in-development technologies. Often at the higher levels there are, and may remain, multiple approaches to the same problem.
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Generic
Web Service Architecture