Activity diagram basics
Activity diagrams are primarily used to describe
behaviour. They are represented as a sequential flow of activities
that describes concepts like workflow.
An activity describes a logical unit of work.
Activities can be broken down in actions. An action is the smallest
unit of work that are not decomposed any further. An activity
diagram has a start and may have multiple end points. The UML 2
also provides for a flow final (a circle with a cross) that
indicates that the process stops.
Activities are joined by process flows or events.
In addition, a decision node can model divergent behaviour based on
a condition. Typically a Start and End node are defined to complete
the full activity diagram representation.
Synchronization points may also be defined to illustrate how
processing may be carried out in parallel, then synchronized at a
point before further activity is undertaken.
Input and output parameters can be shown. This is
done via rectangles that attach to the
activities.
Partitions allow the modeller to create views on
the activity diagram. It may show areas of responsibility,
organizational departments and the like.
The following example shows what happens if a
system becomes unavailable while a user is using it. The user will
receive a message that the system is unavailable. The system will
try to reconnect three times. If not successful it will display an
error message. The display activity makes use of an input
parameter:connect state. This parameter indicates to the activity
what error occurred. The display error message activity is broken
down in the actions that executes.
We have made use of partitions to indicate the
areas of system execution and error management.
