Sequential workflows can best be represented graphically as a flowchart of actions, with a beginning, an end, and a sequential flow direction from start to finish. Sequential workflows can incorporate flow structures such as repetition, looping, and parallel branches, but ultimately progress from the initial action to the final action.
For example, suppose you were to chart the simple workflow that routes a document in Windows SharePoint Services for approval. When the workflow starts, the system notifies the specified reviewer, by e-mail message, that he or she has a document to review. The reviewer then reviews the document and notifies the system that the task is completed and whether the reviewer approves or rejects the document. Based on the reviewer response, the workflow executes one of two parallel branches. If the reviewer approved the document, the system moves the approved document to a specific SharePoint document library and then sends an e-mail message to the entire team notifying them of the approved document. If the reviewer rejects the document, the system notifies the document author. In either case, the workflow then reaches its end and terminates.
State machine workflows Represents a set of states, transitions, and actions. One state is denoted as the start state, and then, based on an event, a transition can be made to another state. The state machine can have a final state that determines the end of the workflow.
Unlike sequential workflows, state machine workflows do not have a prescribed execution flow, and need not have an end. Instead, state machine workflows define any number of states which an item may inhabit, and the events that transition the item from one state to another.
Figure 3 represents a simple document publishing process, modeled as a state machine workflow. The workflow is initiated when a document is created, and ends when the document state is set as completed. In between, however, the document does not travel in a predetermined path, but instead transitions from state to state as events occur.
The state machine workflow is composed of state activities. Each state activity represents a state for the item. Each state activity can contain optional state initialization, state finalization, and one or more event handlers. Each event handler activity can handle one event. In response to the event handled, some processing can be done, and a transition can be made to another state.