Behavior : Activity Diagrams - What is an Activity Diagram?

7 important questions on Behavior : Activity Diagrams - What is an Activity Diagram?

The Activity Diagrams within UML 2.5 concentrate on behavioral steps, process, control flow, and object (data) flow.

  • Behavioral steps
  • process
  • control flow
  • object (data) flow

Many of the diagrams in this chapter are examples of PATTERNS.

What are Patterns?

Patterns are common pieces used in design that we can assemble as needed into structures that are more complex.

They will help us recognize what is going on in an existing diagram, and building, out of these pieces, when configured suitably and tailored to your specific purpose, useful models that will accomplish your goals.

In each of the above figures, there is only one locus of execution in each diagram fragment at any time.

   One way of looking at this is to imagine that...

... there is a token that travels through the diagram conveying permission to execute.

You may read that the UML Fork node spawns a new task or something like that.

We typically do not use the “spawn” terminology, as it usually implies a hierarchical relationship between the associated tasks.

What term do we use?

“Forking” is more appropriate as each token/task is independent.

What happens when a fork occurs?

When a fork occurs, it is not that a new subordinate token is created.

A fork is a clone of a task, while spawn could be considered as a child.

Generally, this distinction is not important, though it may help in understanding what is going on.

There is a special control node used to terminate a token without ending the entire diagram.

This control node is called a ....


If an action has more than one input edge, normally the action requires all the edges to be populated with tokens before the action starts.

This is nearly equivalent...

.....  to having a logical join right before the action

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