UML 1.x Activity diagram for a guided brainstorming process

Activity diagrams are graphical representations of workflows of stepwise activities and actions with support for choice, iteration and concurrency. In the Unified Modeling Language, activity diagrams can be used to describe the business and operational step-by-step workflows of components in a system. An activity diagram shows the overall flow of control.

Construction

Activity diagrams are constructed from a limited number of shapes, connected with arrows. The most important shape types:

• rounded rectangles represent activities;
• diamonds represent decisions;
• bars represent the start (split) or end (join) of concurrent activities;
• a black circle represents the start (initial state) of the workflow;
• an encircled black circle represents the end (final state).

Arrows run from the start towards the end and represent the order in which activities happen.

Hence they can be regarded as a form of flowchart. Typical flowchart techniques lack constructs for expressing concurrency. However, the join and split symbols in activity diagrams only resolve this for simple cases; the meaning of the model is not clear when they are arbitrarily combined with decisions or loops.

While in UML 1.x, activity diagrams were a specialized form of state diagrams, in UML 2.x, the activity diagrams were reformalized to be based on Petri net-like semantics, increasing the scope of situations that can be modeled using activity diagrams. These changes cause many UML 1.x activity diagrams to be interpreted differently in UML 2.x

Applications

In SysML the activity diagram has been extended to indicate flows among steps that convey physical matter (e.g., gasoline) or energy (e.g., torque, pressure). Additional changes allow the diagram to better support continuous behaviors and continuous data flows.

Alternatives

In UML 1.x, an alternative to activity diagrams is the UML State diagram, in which states can be used to represent activities (namely the state of that activity being performed), and transitions represent the start or completion of activities.

This mapping of activity diagram features to state diagram were formalized in UML 1.x, leaving activity diagrams little more than the use of state machines to capture behavior and partially concurrent behavior within a work flow.

Despite the (small) advantages of using UML 1.x activity diagram over state machines, such as the ability to cover the behaviors of collaborating elements, while state machines are limited to a single element, many modelers continued using state diagrams. State machines were attractive to use as the states may also represent conditions that hold before or after the activities. Activity diagrams and state diagram had similar abilities to express concurrency, though the run-to-completion (RTC) semantics of State machines limited the expressiveness of the concurrency.

Perhaps the most important reason why state machines were more popular in UML 1.x over activity diagrams was that the tool vendors were slow to implement robust activity diagram capabilities.

With UML 2.x, the foundation of activity diagram changed from being based on state-machine semantics to now being based on Petri net semantics. This vastly expanded the number of circumstances where activity diagrams are more appropriate to capture the paths of work flow in a system. Now with the advent of robust activity diagram implementations, activity diagrams are now generally used more than state machine diagrams.

See also

• Control flow graph
• Flowchart
• Data flow diagram
• Event-driven process chain
• Petri net
• List of UML tools
• State diagram
• Pseudocode
• Business Process Modeling Notation

Unified Modeling Language Actors Organizations: Object Management Group • UML Partners Persons: Grady Booch • Ivar Jacobson • James Rumbaugh Concepts Object oriented: Object-oriented programming • Object-oriented analysis and design Structure: Actor • Attribute • Artifact • Class • Component • Interface • Object • Package Behavior: Activity • Event • Message • Method • State • Use case Relationships: Aggregation • Association • Composition • Dependency • Generalization (or Inheritance) Extensibility: Profile • Stereotype Other concepts: Multiplicity Diagrams Structural Class diagram • Component diagram • Composite structure diagram • Deployment diagram • Object diagram • Package diagram Behaviour   Activity diagram • State Machine diagram • Use case diagram Interaction Communication diagram • Sequence diagram • Interaction overview diagram • Timing diagram Derived languages Systems Modeling Language (SysML) • UML eXchange Format (UXF) • XML Metadata Interchange (XMI) Other topics Glossary of UML terms • Rational Unified Process • Comparison of UML tools • UML colors