Transition Relationships
A transition is a relationship between two activities that represents workflow from activity to activity. Transitions provide the pathway between activities when modeling a workflow with an activity diagram. When a specific action in an activity is complete, the transition "fires" and the flow continues to the next activity. For example, you could have a source activity named "Take Order" and a destination activity named "Process Order" connected by a transition. When an employee takes an order, the transition moves the workflow along so that another employee can process the order.
Transitions commonly connect activities, send and receive signals, initial states, and final states. Transitions can also connect activities that reside within swimlanes.
Connector
A transition appears as a line with an arrow pointing toward the next activity in the workflow.
Guard Conditions
A guard condition specifies a Boolean expression on an outgoing transition. Guard conditions are used to define the conditional logic that controls the flow of control, and the specific criteria that must be met for a transition to fire. If the specified condition is not met within the activity, the transition does not fire and flow continues to the next activity. For example, you could create a guard condition that evaluates whether someone has entered a correct password. If the user enters the correct password, the transition fires to the next activity. If the user enters an incorrect password, the transition does not fire and the user is prompted to re-enter the password.
Modelers commonly place guard conditions on transitions that depart from activities and decisions. Decisions that have outgoing transitions often have an "else" guard condition on one of the transitions. For example, if an employee processes a "rush" order, the order is shipped overnight. If the order is normal, or "else", the order does not receive overnight delivery.
You can specify guard conditions on transitions in the Properties window. Guard conditions appear within brackets in the diagram window.
Formulating Guard Conditions
You can create guard conditions to describe alternate workflow. You can create a guard condition using the object constraint language that returns a simple "true" or "false" value. For details, see Object Constraint Language. You might place an else value on an outgoing transition or you might use greater than (>) and less than (<) signs when defining numeric expressions. You can also use a simple sentence that explains the condition when specifying guard conditions. For example, "Account balance is sufficient to cover withdrawal.
Related Topics
- Activity Diagrams
- Adding and Modifying Model Elements in Activity Diagrams
- Decisions
- Statechart Diagrams
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