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1. Automatic Web service discovery. Automatic Web service discovery is an automated process for location of Web services that can provide a particular class of service capabilities, while adhering to some client-specified constraints. For example, the user may want to find a service that sells airline tickets between two given cities and accepts a particular credit card. Currently, this task must be performed by a human who might use a search engine to find a service, read the Web page, and execute the service manually, to determine if it satisfies the constraints. With OWL-S markup of services, the information necessary for Web service discovery could be specified as computer-interpretable semantic markup at the service Web sites, and a service registry or ontology-enhanced search engine could be used to locate the services automatically. Alternatively, a server could proactively advertise itself in OWL-S with a service registry, also called middle agent [4,25,15], so that requesters can find it when they query the registry. Thus, OWL-S enables declarative advertisements of service properties and capabilities that can be used for automatic service discovery.
2. Automatic Web service invocation. Automatic Web service invocation is the automatic invocation of an Web service by a computer program or agent, given only a declarative description of that service, as opposed to when the agent has been pre-programmed to be able to call that particular service. This is required,for example, so that a user can request the purchase, from a site found by searching and then selected by that user, of an airline ticket on a particular flight. Execution of a Web service can be thought of as a collection of remote procedure calls. OWL-S markup of Web services provides a declarative, computer-interpretable API that includes the semantics of the arguments to be specified when executing these calls, and the semantics of that is returned in messages when the services succeed or fail. A software agent should be able to interpret this markup to understand what input is necessary to invoke the service, and what information will be returned. OWL-S, in conjunction with domain ontologies specified in OWL, provides standard means of specifying declaratively APIs for Web services that enable this kind of automated Web service execution.
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