Here - Agents Lab - University of Nottingham

Fig. 3. Web Publishingmodel that actors come and go over time, active components do not encouragestatic binding of components but instead rely on dynamic service discovery andusage. In Fig. 2 an overview of the synthesis of SCA and agents to active componentsis shown. It can be seen that the outer structure of SCA components is keptthe same and the main dierence is the newly introduced internal architectureof components. The internal architecture of active components is used to encodethe proactive behavior part of a component that can be specied additionallyto provided service implementations. More details about the integration can befound in [3].3 Web Service Integration ConceptIn this section the overall integration concept as well as more detailed designchoices will be presented.3.1 Making Functionality Accessible as Web ServiceIn order to make functionality of an application available to external systemusers, component services can be dynamically published at the runtime of thesystem. In general, it has to be determined when, where and how a service shouldbe published. A common use case consists in performing the service publicationaccording to the lifecycle state of the underlying component service. For thisreason, per default, service publication and shutdown is automatically checkedfor when a component service is started or ended. The component inspects theprovided service type descriptions for publish information and indirectly usesthis information to publish/shutdown the service via delegation to the infrastructure.The publish information is composed of four aspects: publish type,publish id, mapping and properties. To support arbitrary kinds of service publicationssuch as REST (Representational State Transfer) [4] and WSDL (WebServices Description Language) [15] the component uses the publish type fromthe component specication of a service to dynamically search for a suitableservice publisher (cf. Figure 3). Subsequently, the component asks the servicepublishers one by one if it supports the requested publish type. In case a suitablepublisher could be found it is instructed to publish or retreat the componentservice. Otherwise the publication has failed and an exception is raised within26