WCF - Notes 6 Workflow Service

Creating a Workflow Service

As its name suggests, Windows Workflow Foundation (WF) lets developers create workflow-based applications. WF workflows can be useful in a variety of different situations. For example, WF can make it easier to create scalable applications that pause, perhaps for a long time, while waiting for input. Whatever its purpose, every WF workflow is built from activities, each of which performs some function. An activity might do something simple, such as implement a While statement, or carry out a more complex task, such as executing custom business logic.

While it’s certainly not required, a developer can use WF to create the logic for a WCF service. Known as a workflow service, this technology combination often makes sense. The developer builds scalable business logic as a WF workflow, then lets that workflow’s activities interact with the outside world via WCF services.

To help do this, WF includes several activities specifically intended to allow WCF communication. For example, the Send activity sends a message via WCF, while the Receive activity receives a message. WCF 4 also allows correlation of multiple requests sent to a particular workflow instance. For example, suppose an instance of a workflow sends a request via WCF, then pauses waiting for a message in response. While it’s waiting, this workflow instance will be deactivated, with its state stored on disk. When a response message arrives, WCF can determine from the message’s contents which workflow instance it’s intended for—it can correlate this response message with the request message sent earlier. The target workflow instance can then be reactivated and the message delivered to the appropriate activity in that workflow.

Not every WCF service should be built using WF. In a surprisingly large number of cases, however, using WF and WCF together is the right choice. Workflow services can be useful things.