Activity diagram

A process model is a formal way of representing how a business operates. An activity diagram is one method of representing a process model. It describes the behaviour of a system by depicting the sequencing of events through workflow. They illustrate what happens in workflow, what activities can be done in parallel and whether there are alternative paths through the workflow.

Before drawing an activity diagram, the following elements should be identified:

  • Activities
  • Association
  • Conditions
  • Constraints

The advantage of activity diagrams over some of the other diagram tools is that they can be used to capture flow from one system to another and include capabilities such as branding, parallel flow and guards (conditions that must be true). These diagrams are high level and model the activities (business requirements) so they are beneficial towards understanding the business but not necessarily implementation details. 

The symbols used in an activity diagram include:

  • Initial node – depicted by a solid black dot, this is the state before the activity begins.
  • Final node – an open circle with a smaller, solid black dot in the middle, this is the end of the activity.
  • Activity – a rectangle with rounded corners, activity symbols usually contain a short description the activity they represent.
  • Note – represented by a rectangle with a folded down corner, used to provide additional information.
  • Decision – a diamond shape symbol with information flow coming in and at least two option flows going out, used to show where a choice is made in the process.
  • Merge – also represented by the diamond shape symbol with information flowing in but only one option flowing out, used where a point in the process is reached where all information needed for an activity is received.
  • Guard – represented by information enclosed in square brackets, it indicates there are conditions that must be met to continue the process.
  • Fork – depicted by a solid rectangle with one flow coming in and at least two flows going out, this symbol represents activities that split into multiple activities
  • Join – depicted by a solid rectangle with multiple flows coming in and only one going out, this symbol represents activities that were separate now continuing as one activity, and
  • Repeated activity – also referred to as a loop, is indicated by a frame that surrounds the activities that repeat, there is a label within the frame that provides information about the loop (minimum, maximum or set times the loop repeats and any condition that must occur before moving to the next activity)

Example activity diagram:

Example activity diagram (PDF), see long description