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Activity Diagrams

An activity diagram is one of the five diagrams in the Unified Modelling Language which is essentially used to show the flow of control from one activity to another activity happening in the system through system perspective and not actor perspective. An activity diagram is drawn to model how the system should function in order to achieve-

  1. Business objectives
  2. Business functionality
  3. Business logic

Each activity diagram is derived from a single-use case capturing the detailed business logic. The activity diagram represents the sequential flow but in turn is not a flow diagram, primarily to understand the activities by the actor their interaction with the system. It is for better visualizing the dynamic nature of a system.

Elements in the Activity Diagram:

  1. Star node – Represents the start of the workflow
  2. Control flow – Represents the direction of the workflow from one activity to another
  3. End node – Represents where the workflow completes usually from the final activity to the end node
  4. Activity – Each activity represents the unit of work done at a single step i.e. one activity at a time
  5. Decision box – When there is a probability of alternate flow of direction decision box is used
  6. Guard condition – Condition attached to the control flow which comes out of a decision symbol and can be named the respective actor performing within the system
  7. Event – An event triggers the control flow by forcing the previous activity to end, an event must occur for the flow to move along the control flow
  8. Connector – Connectors are used for 1-many and many-1 associations, it has multiple inputs and outputs
  9. Continuation node – Continuity is used to indicate navigation to the next page if the activity is big
  10. Branch and merge – When more the two flow are leading to the same activity all the branches of those activities are merged connect them

Ex-Admin required to add/delete/modify the customer’s data, here branch and merge is used as the admin can either add or delete or modify data one at a time.

  1. Fork and join – It is used to represent parallel activities that may or may not begin at the same time are joined which indicated a no of activities may begin in any order and a join indicates the workflow may commence only once the parallel activities that flow into it have all been completed.

Ex – System generates alerts, here fork and join is used as the system will parallelly generate SMS alert and email alert

  1. Nested loop activity – when there is an activity inside another activity nested loops are used

Step to draw an activity diagram

  • List all the actors, activities, conditions and events
  • Start with an activity which with first activity who interacts with the system in the first place
  • Followed by the series of activities in the sequential flow
  • Use fork & join for parallel activities and branch & merge for or conditions
  • Ensure all the activities i.e. all the interactions with the system are listed in the diagram
  • Conclude with the final activity such as acknowledgement etc.

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