Among the different styles of service-oriented architectures, the YAWL System adopts the Representational State Transfer (REST) style. In line with the principles of REST (or RESTful) architectures, YAWL services provide access to resources, each with a Unique Resource Identifier (URI). For example, each workflow specification is exposed by the Engine as a resource, and when an instance of a workflow specification (i.e., acase) is created, this case is also represented as a resource. Also, in line with the principles of RESTful architectures, YAWL services inter-act with one another using the basic operations defined by the HyperText TransferProtocol (HTTP). Specifically, YAWL services interact by means of HTTP’s GET and POST operations: GET being used to obtain information about a given resource, and POST being used to create resources and to update existing resources.Messages exchanged through these operations are encoded using the eXtensible Markup Language (XML) and (with some exceptions) conform to predefined XML schemas. Messages may contain references to resources, which are concretely represented as Uniform Resource Locators (URLs)