Agenda

• Fundamentals of Activity Diagrams

• Advanced Constructs and Applications

What Are Activity Diagrams?

Definition

Activity diagrams describe the flow of control and data between actions in a process.

Use Cases

• Model algorithms, business process, and workflows
• Detail use case realizations
• Similar to flowcharts but supporting concurrent behavior

Comparison

Core Elements

• Action Nodes – steps or tasks.
• Control Flows – arrows showing execution order.
• Decision Nodes – conditional branching.
• Merge Nodes – combine alternative flows.
• Fork / Join Nodes – manage parallel execution.
• Initial / Final Nodes – start and end points.

Example: Order Processing

Decomposing an Action as a subactivity

An action can be implemented as:

• a method in a class
• as a sub-activity (shown using the rake symbol)

Partitions (swim lanes) – who does what?

Which actions one class or organization unit carries out

Interactive Exercise 1: Course Registration Process

Scenario

A student registers for a course.

Steps

1. Log in
2. Check prerequisites
3. If eligible → register for course
4. Update records & send confirmation (in parallel)

Swimlanes

Purpose: Show which actor or subsystem performs each action.

Notation: Vertical or horizontal lanes representing responsibilities.

Example: Online Purchase Process

Parallel Flows and Synchronization

Fork/Join Example:

Discussion: What happens if one parallel branch fails or is delayed?

Tokens in Activity Diagrams

Purpose of tokens

• Represent the flow of control or flow of data.

• Enable the execution of actions when they arrive

  at input pins or control flows.

Types of tokens

• Control tokens: Indicate progression of execution.
• Object tokens: Carry data values or objects along object flows.

Token behavior

• Actions consume incoming and produce outgoing tokens.
• Fork multiplies tokens (one for each outgoing flow).
• Join synchronizes incoming tokens (one for each incoming flow).
• Decisions route tokens based on guard conditions.

Signals

• Allow to specify entry points corresponding to events
  • receiving a signal produces one token
• Activity receives an event from an outside process
  • Time signals – triggered by the passing of time
  • Accept signals – triggered by other events
  

Emiting signals and flowing into signals

One can also send signals

• e.g., send a message, waiting for a reply

Notes

• the two flows are racing: first to end completes the activity

• Flow going into an accept signal means:

  Start listening only when flow enters the signal

Data Flow and Object Nodes

• Connectors Allow splitting the draw of an edge in two parts
• Object nodes represent data produced/consumed by activities.
  • correspond to object tokens
  • Used to visualize data movement alongside control flow.
  • Can also be specified using pins

Pins: argument passing and transformation

• Pins allow showing information about parameters
• Output parameters of action should match input of next
  • One can use transformations to ensure that
• With pins we can have multiple flows entering same action

• In business modelling, pins show resources

Expansion regions: multiple action invocations

• Area where actions ocurr once for each item in a collection
  • useful for mapping and filtering collections

Mapping actions to a collection

Shorthand for single action map

Flow final: stop a flow without ending entire activity

Filter and Map actions to a collection

Join specifications: conditions attached to a join node

Comparison to Petri Nets

Conceptual similarity

• Both use tokens to represent the dynamic state of the system.
• Movement of tokens represents progress in execution.

Key differences

• Activity Diagrams: object tokens; semiformal semantics.
• Petri Nets: tokens (usually) indistinguishable; formal semantics

Execution correspondence

• Activity Diagram actions ~ Petri Net transitions.
• Activity Diagram edges / pins ~ Petri Net places.
• Token flow in both describes enabling and firing of behaviors.

Why the comparison matters

• Petri Nets provide a formal foundation to analyze concurrency, conflicts, and reachability.
• Activity Diagrams can be mapped to Petri Nets for verification or simulation.

Interactive Exercise 2: Airport Check-in Process

Create an activity diagram with two swimlanes and one parallel branch.

Actors: Passenger, System

Requirements

• Passenger checks in.
• System verifies ticket.
• If baggage overweight → pay fee.
• In parallel: print boarding pass + update database.

Wrap-Up

Key Takeaways

• Activity Diagrams model workflows and concurrent processes.
• Ideal for requirements and process-level modeling.
• Useful bridge from analysis to system design.