Web Services Exposed

The uses for application services-type of integration are endless, including creation of composite applications, or applications that aggregate the processes/services and information of many applications. For example, using this paradigm, application developers simply need to create the interface, then add the application services by binding the interface to as many Internet-connected application services as required.

Indeed, Web services promise to deliver additional value to application integration, including a standard application service for publishing and subscribing to software services, local and remote. Applications locate the services using UDDI and determine the interface definition using Web Services Description Language (see the following tidbit.)

Think of Web services as application services exposed by a company or software program that are both discoverable and accessible by other programs or organizations that are in need of a particular service, such as purchasing a product, reserving a flight, or calculating tariffs. These are discrete business services that have value to many organizations.

The downside, as we alluded to above, with serviced-based integration is that this makes it necessary to change the source and target applications or worse, in a number of instances, to create a new application (a composite application). This adds cost to the application integration project and is the reason that many choose to stay at the IOAI level going forward. Indeed, most problem domains only require IOAI.

Still, the upside of this approach is that it is consistent with the "baby step" approach most enterprises find comfortable when implementing solutions to integration problems. Service-based solutions (e.g., Web services) tend to be created in a series of small, lower-risk steps.

WSDL

Web Services Description Language provides a standard approach for Web services providers and those who use the services, or a standard agreement between users and services on interfaces. WSDL provides an automated mechanism to generate proxies for Web services using a standard language. This standard is analogous to Interface Definition Languages (IDLs) found in both COM and CORBA. In other words, it's a simple, standard contract between client and server.

WSDL defines an XML grammar to describe network services as a collection of communication endpoints that can exchange information. The WSDL services definition serves as a recipe to automate the way applications communicate.

Within the world of WSDL, services are a collection of network endpoints, also known as ports, and the abstract definition of endpoint. This mechanism provides for the reuse of abstract definitions, or messages. Messages are abstract descriptions of information flowing from application to application, and messages are separated from the data format bindings. Port types, another WSDL entity, are abstract connections of an operation, which is an abstract description of an action that is supported by the service. Bindings are a concrete protocol and data format specification for an instance of a port type.