Sequence Diagrams

Sequence diagrams visualize how objects interact over time. They show the order of messages exchanged between participants to accomplish a specific task or scenario.

Core Concepts

Participants

Participants represent the objects or actors involved in the interaction. Each participant has a lifeline - a vertical dashed line extending downward that represents time.

Messages

Messages are the communications between participants, shown as horizontal arrows between lifelines. Time flows from top to bottom.

Activation Boxes

Activation boxes (also called execution specifications) are thin rectangles on a lifeline that show when a participant is actively processing.

Creating Participants

Using the Participant Tool

Press P to activate the Participant tool, then click on the canvas to place participants.

Participant Types

Participants can represent different types of entities:

• Actor - A user or external system (stick figure icon)
• Object - An instance of a class (rectangle)
• Component - A system component (rectangle with stereotype)
• Boundary - UI or system boundary
• Control - Process or controller logic
• Entity - Data or domain object

Configure the participant type in the Inspector panel.

Creating Messages

Using the Message Tool

Press M to activate the Message tool, then:

1. Click on the source participant's lifeline
2. Drag to the target participant's lifeline
3. Release to create the message

Message Types

Message Labels

Double-click on a message to edit its label. Common formats:

• methodName() - Simple method call
• methodName(param1, param2) - Call with parameters
• result := methodName() - Call with return assignment
• [condition] methodName() - Conditional message (guard)

Activation Boxes

Activation boxes show the period during which a participant is executing or waiting for a response.

Adding Activation Boxes

1. Select a message
2. In the Inspector, enable "Show Activation"
3. The activation box appears on the receiving participant's lifeline

Nested Activations

When a participant calls itself (recursive call) or handles multiple nested calls, activation boxes can be stacked.

Fragments

Fragments group messages to show conditional logic, loops, or other control flow.

Fragment Types

Creating Fragments

1. Select the messages you want to group
2. Right-click and choose "Add Fragment" or use the Fragment tool
3. Select the fragment type
4. Add conditions/guards as needed

Fragment Guards

Guards are conditions shown in square brackets:

• [x > 0] - Numeric condition
• [user.isAdmin] - Property check
• [else] - Default/else branch in alt fragments

Reordering Elements

Moving Participants

Drag participants horizontally to reorder them. Connected messages will follow.

Moving Messages

Drag messages vertically to change their order in the sequence. This affects the timing/order of the interaction.

Best Practices

1. Start with the main flow - Model the happy path first, then add alternatives
2. Keep it readable - Limit to 5-7 participants per diagram
3. Use meaningful names - Label messages with actual method names or descriptions
4. Show return messages - Especially for synchronous calls where the return value matters
5. Use fragments sparingly - Too many nested fragments reduce readability
6. Add notes - Use note elements to explain complex logic

Example: User Login Flow

A typical login sequence might include:

1. User enters credentials
2. Client sends login request to Server
3. Server validates with Database
4. Database returns user record
5. Server creates session
6. Server returns success/failure to Client
7. Client updates UI

This flow demonstrates synchronous calls, return messages, and potential alt fragments for success/failure paths.