Understanding XML-Based Web Services

For many years, developers have sought ways to reuse the products of their work and to bring together disparate systems that their organizations have already invested in to form a cohesive whole. This quest has been fraught with difficulties, ranging from dissimilar platforms and systems with no common standard for communication to security measures that prevented potential clients outside a corporate network from being able to use existing functionality.

Some of the solutions for these difficulties have included creating custom Web front ends for existing functionality, or using proprietary communications and/ or packaging technologies to bridge the gap between systems. The former solution worked well in some situations, but the organization hosting the application had to create and maintain a UI for the functionality that they made available, as well as creating a front end for any additional functionality added later. Moreover, this UI might or might not fully meet the needs of their existing and/or future clients. The use of proprietary communications or bridging software typically meant greater expense, reliance on a single vendor, and dependence on that vendor’s maintenance and upgrades. XML-based Web services address both of these issues squarely, allowing programmatic functionality to be exposed without the need for a custom Web-based UI, and without the need for proprietary bridging software.

Web services are discrete units of programmatic functionality exposed to clients via standardized communication protocols and data formats, namely HTTP and XML. These protocols and formats are both well understood and widely accepted. (While XML is still a relatively immature technology, it has rapidly gained acceptance in the industry because of its promise as a bridging technology.) By communicating over HTTP, Web services overcome the problem of communicating across the Internet and across corporate firewalls without resorting to proprietary solutions that require additional communications ports to be opened to external access. Because Web services use XML for the formatting of requests and responses, they can be exposed from and consumed by any platform that can format and parse an XML message. This allows XML-based Web services to bring together disparate pieces of functionality—existing or new, internal or external to an organization—into a coherent whole. The following illustration shows how XML-based Web services can bring together a variety of applications and platforms. Note that because XML-based Web services can communicate over standard protocols such as HTTP, they can work over firewalls. In addition to the interactions shown here, server-to-server Web services calls are also possible.

A client accesses Web services by calling into a listener that is able to provide a contract to describe the Web service and also processes incoming requests or passes them on to other application logic and passes any responses back to the client. The following illustration shows how a Web service application can be layered. You can also use Web services to access both legacy logic and new application logic, or to wrap legacy logic for access by a Web service.