Activity Diagrams - Unified Modeling Language (UML)

An Activity Diagram is a behavioral UML diagram used to represent workflows, processes, and the sequence of activities within a system. It shows how actions are connected and how control flows from one activity to another.

• Illustrates sequential and concurrent activities, along with decision paths and control flow.
• Starts with an initial node and ends with a final node, helping model system behavior over time.

Example: An Online Shopping System activity diagram showing steps such as Browse Products-> Add to Cart -> Make Payment -> Order Confirmation.

Uses of Activity Diagram

Activity diagrams are used to visualize workflows, processes, and the dynamic behavior of a system.

• Modeling Workflows or Processes: Represents the sequence of activities in a business process or system workflow.
• Concurrent or Parallel Processing: Shows multiple activities that can execute simultaneously.
• Understanding Dynamic Behavior: Illustrates how a system behaves and responds to events over time.
• Clarifying Complex Logic: Simplifies decision-making and branching processes for better understanding.
• System Design and Analysis: Helps analyze, design, and communicate system behavior effectively.
• Describing Use Cases: Visualizes the flow of actions and interactions within a use case.

Activity Diagram Notations

1. Initial State

Represents the starting point of an activity or process.

• Shown as a filled black circle in the diagram.
• A process typically has only one initial state unless nested activities are used.

Example: Here the initial state of the system before the application is opened.

2. Action or Activity State

Represents the execution of an action or task within a process.

• Shown as a rectangle with rounded corners.
• Represents any action, operation, or event that occurs in the workflow.

Example: Consider the previous example of opening an application, opening the application is an activity state in the activity diagram.

3. Action Flow or Control flows

Represents the transition and sequence of activities within a process.

• Shown as a line with an arrowhead indicating the direction of flow.
• An activity can have multiple incoming and outgoing control flows, with conditions specified on the arrows if required.

Example: Here both the states transit into one final state using action flow symbols i.e. arrows.

4. Decision node and Branching

Represents a point where a decision is made to determine the next flow of control.

• Shown with multiple outgoing paths based on conditions or guard expressions.
• Always has two or more outgoing control flow arrows.

Example: We apply the conditions on input number to display the result :

• If number is odd then display the number.
• If number if even then display the error.

5. Guard

Represents a condition that must be satisfied for a particular flow to be followed.

• Written next to a decision path, often within square brackets.
• Helps control the direction of flow based on specific conditions.

6. Fork

Represents a point where a workflow is split into multiple parallel activities.

• Shown as a solid rectangular bar with one incoming arrow and multiple outgoing arrows.
• Allows several activities to execute simultaneously.

Example: In the example below, the activity of making coffee can be split into two concurrent activities and hence we use the fork notation.

7. Join

Represents a point where multiple parallel activities merge into a single flow.

• Shown with two or more incoming edges and one outgoing edge.
• Synchronizes concurrent activities before proceeding to the next step.

Example: When both activities i.e. steaming the milk and adding coffee get completed, we converge them into one final activity.

8. Merge or Merge Event

Represents a point where multiple alternative paths combine into a single flow.

• Merges two or more workflow paths into one outgoing path.
• The process continues regardless of which incoming path was followed.

Example: In the diagram below: we can't have both sides executing concurrently, but they finally merge into one. A number can't be both odd and even at the same time.

9. Swimlanes

Represents a grouping of activities based on roles, responsibilities, or processes.

• Shown as a rectangular row or column in the activity diagram.
• Helps identify who is responsible for each activity and improves workflow clarity.

Example: Here different set of activities are executed based on if the number is odd or even. These activities are grouped into a swimlane.

10. Time Event

Represents an event that pauses the workflow for a specific period of time.

• Shown using an hourglass symbol in the activity diagram.
• Used when an activity or event requires a time delay before continuing.

Example: Let us assume that the processing of an image takes a lot of time. Then it can be represented as shown below.

11. Final State or End State

Represents the point where a process or activity reaches completion.

• Shown as a filled circle inside another circle.
• A system or process can have multiple final states.

Differences between an Activity diagram and a Flowchart

An activity diagram is very similar to a flowchart. So let us understand if activity diagrams or flowcharts are any different.

Flow Chart

An algorithm is like a set of clear instructions to solve a problem, and a flowchart is a picture that shows those instructions.

• Flowcharts help programmers visualize the steps of an algorithm and plan the logic required to solve a problem before coding.
• They are also used to model business workflows and decision-making processes, making them a simple precursor to activity diagrams.

Example: A manufacturer uses a flow chart to explain and illustrate how a particular product is manufactured.