Workflow Concepts Reference

OpenACS Documentation : Workflow : Workflow Conceptual Reference

And-split

A case of parallel routing, where several tasks are performed in parallel or in no particular order. It is modeled by a transition with one input place and two or more output places.

And-join

A transition with two or more input places and one output place. This will only be enabled, once there is a token in all of the input places, which would be after each parallel thread of execution has finished.

Arc

Arcs are the arrows that connect transitions and places. They always go either from a place to a transition (thus defining an input place for that transition), or from a transition to a place (thus defining an output place for the transition).

Case Attribute

Correspond to "for-internal-use-only" fields on the forms. They're filled out as part of a transition, and can be used to branch on in or-splits.

Enabled Transition

An enabled transition is a transition for which there is at least one token in each of its input places. In the office metaphor this is the same as saying that all the forms necessary to complete a certain task are present.

Explicit Or-split

An example of conditional routing, where the decision is made as early as possible. It is modeled by attaching conditions or guards the arcs going out of a transition.

Firing a Transition

When a transition fires, it consumes one token from each of its input places and produces one token in each of its output places, for which the guard evaluates to true. Thus that a transition is enabled is a necessary condition for the transition to fire. In the office metaphor, this corresponds to taking each of the forms in the in-boxes, perform the task, and put a form in each of the in-boxes of the next task to be performed. We'll just photocopy or discard forms as necessary to make the numbers fit.

Guard

An expression attached to an arc, shown in brackets, that evaluates to true or false. Tokens can only travel over arcs when their guard evaluates to true. The expression will typically involve the case attributes.

Implicit Or-Split

An example of conditional routing, where the decision is made as late as possible. Implicit or-splits are modeled as two arcs going from the same place but to different tasks. That way, the transition that happens to fire first will get the token. Once the token is gone, the others are no longer enabled and thus cannot fire.

Or-join (Explicit and Implicit)

Is simply a place, that serves as the output place of two different transitions. That way, the next transition after the or-join place will be enabled when either of the two conditional threads are done.

Place

Places are the in-boxes of the office. They're represented as circles on the diagram. Places hold Tokens.

Place (Start and End Places)

There are two special places. When the workflow starts, a token is put in the start place, which should enable at least transition. The end place is a place which does not serve as the input place for any transitions. When the token lands in this place, the case is closed.

Task

Used as a synonym for an enabled transition. In practice, though, people often use the word task as a synonym for transition as well.

Token

A token is the form representing a case. Tokens stay in Places.

Transition

A transition represents a task (or a desk) in the office metaphor. They correspond to activity. They move tokens from one or more places to one or more other places governed by certain rules.

Transition Trigger

A transition need not fire the instant it is enabled. If the task is something that has to be done by a human being, she might not be able, available or willing to perform the task. In general there are four types of triggers:

1. User: The transition must be executed by some person. Instances of these transitions (the transition for a specific case) is also referred to as tasks.
2. Time: The transition is fired automatically at some specific time, typically after the transition has been enabled for a certain number of days. An example is when we mail a form to a customer and we wait for a response. We could then add a transition that automatically closes the case if we haven't heard back from the customer in three weeks.
3. Message: Some tasks need to be executed by something outside the control of the workflow application. An example of that is when we mail a form the some person and asks him to fill it out. The way the workflow package learns that the task has been executed is through a message of some form.
4. Automatic: The workflow engine automatically fires the transition as soon as it becomes enabled. These can be used to build control structures like and-splits, and-joins, or-splits and or-joins. Or they can be used for their side-effects, such as executing some code.

Workflow

A workflow is the formal definition of the process used to manage a certain case. The example of an insurance claim was given above. More examples of workflows are: