---+ DatasourceService

A DatasourceService is a webservice with a single AccessPoint giving access to a single source of related data. This is very often equivalent to an access service to a database or a subset of it. But important is that all its data is interrelated. 

Each datasource can be represented with several ConceptualSchemas, thus the relation of the data is required.

The following things apply to a datasource:
   * It is the end point of a message trace and therefore cannot relay any more messages. It therefore needs no destination in its header.
   * It has always only a single response with a single source of data
   * It is the entity where paging takes place