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What is an Activity Diagram? ​

Overview ​

An Activity Diagram is a type of UML (Unified Modeling Language) diagram used to model the workflow of a process. It shows the sequence of steps, decisions, and parallel flows in a system.

Think of it as a flowchart on steroids — Activity Diagrams go beyond simple flows to model concurrent processes, synchronization, and business logic.

Why Activity Diagrams Matter ​

  • Clarify workflows: Visualize how tasks proceed step-by-step.
  • Spot bottlenecks: Identify slow or complex decision points.
  • Team communication: Provide a non-technical view for stakeholders.
  • LLD interviews: Commonly used to show request flows, order lifecycles, or API usage.

Key Elements of Activity Diagrams ​

  • Initial Node: Black circle → starting point.
  • Activity/Action: Rounded rectangle → a step/task.
  • Decision Node: Diamond → branching based on conditions.
  • Merge Node: Diamond → merging flows back together.
  • Fork/Join: Bars → represent parallelism and synchronization.
  • Final Node: Black circle with a ring → end of the workflow.

Example: Online Order Workflow ​

Here’s how an e-commerce order process might be represented:

[Start]
   |
   v
[User Adds Item to Cart]
   |
   v
[Checkout]
   |
   v
+----------------+
| Payment Done?  |
+----------------+
   |Yes                 |No
   v                    v
[Generate Order]   [Cancel Order]
   |
   v
[Send Confirmation Email]
   |
   v
[End]

When to Use Activity Diagrams ​

  • Business processes (e.g., order fulfillment, loan approval).
  • System workflows (e.g., API request handling, microservice communication).
  • Interviews: To explain how components interact at a workflow level.

Conclusion ​

Activity Diagrams are a powerful UML tool for modeling workflows and decision-making processes. By mastering them, you can clearly communicate how data and tasks flow across a system, both in real projects and system design interviews.

Next Step: Explore Sequence Diagrams to learn how to model interactions between objects and services.