Activity Diagrams

Activity diagrams are essentially advanced flowcharts. They model the flow of control from one activity to another, supporting parallel behavior and complex logic.

1. Basic Symbols

Start Node: A solid black circle.
End Node: A bullseye (circle with a dot).
Activity/Action: Rounded rectangle.
Control Flow: Solid arrow.

2. Control Nodes

Decision Diamond: One input, multiple outputs with [guards].
Merge Diamond: Multiple inputs, one output (ends a decision).
Fork (Split): A thick horizontal or vertical bar. One input splits into multiple parallel paths.
Join: A thick bar where parallel paths merge back into one.

3. Swimlanes (Partitions)

Swimlanes group activities based on who performs them (e.g., “User”, “System”, “Database”).

4. Example: Order Processing

PlantUML Code:

|Customer|
start
:Place Order;
|Order System|
:Validate Order;
if (Order Valid?) then (yes)
  fork
    :Process Payment;
  fork again
    :Prepare Shipping;
  end fork
  :Ship Goods;
  stop
else (no)
  :Notify Error;
  stop
endif

System Diagram

Decisions: Activity vs. State?

Activity: Focuses on the flow of work (what happens). Use when describing a process/algorithm.
State: Focuses on the lifecycle of an object (what it is). Use when an object’s behavior changes based on its current condition.