The WSDL document describes the names of the operations, the names and types of each of the parameters, and the name and type of the return value. After you add reliable messaging support (to ensure delivery) and a business process management engine (to route based on high-level business logic), your integration problems are over.
Of course it's not so simple. Even when using the loose coupling afforded by Web services, you must marshal data into the correct format. Organizations that do not carefully plan their integration strategy usually end up with a multiplicity of point-to-point integrations, because the simplest way to expose a Web service is through a thin veneer on application data structures. A single point-to-point integration is manageable; dozens become a maintenance nightmare. Semantic inconsistency arises between disparate systems. Exposing application data as a Web service without considering the consumers of the data defeats the loose-coupling tenet of an SOA. Fortunately, a set of best practices is emerging to address these concerns. The essence of these practices is to separate data logic from business logic, capturing the data logic in a model with accompanying rules and transformations that are preserved as metadata. This model-based approach to integration promotes reuse, error management, and controlled change.
