# From Service Composition to Mashup Editor: A Multiperspective Taxonomy
## 4 Taxonomy for Services Composition
### Services Composition Using BPEL
#### 4.1.3 Semantic Approach
Unlike OWL-S, WSMO does not provide a notation for building the composite processes in terms of control flow and data flow. Instead, it focuses on the specification of internal and external choreography and orchestration using an approach based on abstract state machines (with guarded transitions). The service basis of WSMO is defined in the same way as the one of OWL-S. This task is achieved by a mediator, which is a key concept in WSMO. In WSMO's approach, mediators perform tasks such as translation between ontologies, or between the messages produced by one Web service and those expected by another. WSMO includes a taxonomy of possible mediators that helps to classify the different tasks that mediators are supposed to solve. The definition of mediators in WSMO calls attention to some important translation tasks associated with Web services. Not surprisingly, these same translation tasks are needed in support of interactions with OWL-S-described Web services. Some OWL-S-based systems [40] also make use of mediator components. However, rather than requiring the existence of a distinguished type of entity in the Web services infrastructure, OWL-S takes the view that mediators are services, and, as such, these mediation services can use the mechanisms provided by OWL-S for discovery, invocation, and composition. Other distinguishing characteristics include WSMO's emphasis on the production of a reference implementation of an execution environment, WSMX, and the specification of mediators (i.e., mapping programs that solve the interoperation problems between Web services). WSMO instances can be created with WSMO Studio [41], which is a real, complete, and open-source semantic Web service and semantic business process modeling environment. It provides support for WSMO editing with an integrated WSML reasoner, WSML text editor and validator, choreography designer, SAWSDL editor for adding semantic annotations to WSDL documents, execution engine, and many other features. Moreover, it also provides semantic business process modeling according to the business process modeling ontology, a semantically extended version of BPEL, called BPEL4 SWS [42]. In conclusion, the semantic approach adds an extra layer on top of the structural approach (Section 4.1.2) by integrating the semantic properties within the operations of description/discovery, and the composition of services [43; 44; 45]. With these properties, it is possible to link services together semantically. For instance, it is possible to propose a schema of the composition from a natural language request (see the natural composer in Section Natural Language Composer). This is a major step forward from the end-user perspective. While this approach is valid for very simple patterns of composition, it is unfortunately not advanced enough to allow for expressing the logic of the composition for complex cases. Another hybrid approach was proposed in [46; 47], combining both semantic capabilities of service description and non-functional aspects of a service (in this case, QoS).