Home > Service-Oriented Computing in the Real World > About Web Services (Part II)
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A typical Web service is comprised of the following:
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A physically decoupled technical service contract consisting of a WSDL definition, an XML schema definition, and possibly a WS-Policy definition. This service contract exposes public functions (called operations) and is therefore comparable to a traditional application programming interface (API).
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A body of programming logic. This logic may be custom-developed for the Web service, or it may exist as legacy logic that is being wrapped by a Web service in order for its functionality to be made available via Web services communication standards. In the case that logic is custom-developed, it generally is created as components and is referred to as the core service logic (or business logic).
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Message processing logic that exists as a combination of parsers, processors, and service agents. Much of this logic is provided by the runtime environment, but it can also be customized. The programs that carry out message-related processing are primarily event-driven and therefore can intercept a message subsequent to transmission or prior to receipt. It is common for multiple message processing programs to be invoked with every message exchange.
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Figure: Three variations of a single Web service showing the different physical parts of its architecture that come into play, depending on the role it assumes at runtime.
A Web service can be associated with temporary roles, depending on its utilization at runtime. For example, it acts as a service provider when it receives and responds to request messages, but can also assume the role of service consumer when it is required to issue request messages to other Web services.
When Web services are positioned within service compositions, it is common for them to transition through service provider and service consumer roles. Note also that regular programs, components, and legacy systems can also act as service consumers as long as they are able to communicate using Web services standards.
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