While Scufl defines the data flow, it does not fully describe the service interactions to enable this data flow. It would be impossible to describe the interaction with all the different service interfaces within a language like Scufl. Instead, Scufl is designed to be extensible through the use of processor types. A set of processor plugins extend the core of Taverna 1.7.x and manage service interaction by presenting a common abstraction over different style of service.
The Processor invocation layer (see the Taverna 1.7.x architecture diagram) invokes and interacts with concrete services. It wraps the data sent to the services and unwraps the results from them. It also copes with errors produced by the services.