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Sequential and State Machine Workflows

The WF API provides support for modeling two flavors of business process workflows: sequential

workflows and state machine workflows. Ultimately, both categories are constructed by piecing

together any number of related activities; however, exactly how they execute is what sets them

apart.

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The most straightforward workflow type is sequential. As its name implies, a sequential work-

flow allows you to model a business process where each activity executes in sequence until the final

activity completes. This is not to say that a sequential workflow is necessarily linear or predictable

in nature—it is entirely possible to build a sequential workflow that contains various branching and

looping activities, as well as a set of activities that execute in parallel on separate threads.

The key aspect of a sequential workflow is that it has a crystal-clear beginning and ending

point. Within the Visual Studio 2008 workflow designer, the path of execution begins at the top of

the WF diagram and proceeds downward to the end point. Figure 26-3 shows a simple sequential

workflow that models a partial business process for verifying a given automobile is in stock.

Figure 26-3.

Sequential workflows have a clear starting point and ending point.

Sequential workflows work well when the workflow models interactions with various system-

level entities, and when there is no requirement for backtracking in the process. For example, the

business process modeled in Figure 26-3 has two possible outcomes: the car is in stock or it isn’t. If

the car is indeed in stock, the order is processed using some block of custom code (whatever that

may be). If the car isn’t in stock, we send a notification e-mail, provided that we have the client’s 

e-mail address at our disposal.

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