What is a Sequence Diagram?
A Sequence Diagram shows how objects interact with each other in a specific scenario, arranged in time order (top to bottom). It's the most common interaction diagram and is excellent for visualizing the flow of a particular use case or algorithm.
Think of it as a movie script — it shows who does what, in what order, and what happens as a result. Each object is a "character" and the messages are their "lines."
Sequence Diagram Notation
Basic Elements:
1. Lifeline (dashed vertical line)
──────────────────────────────────
Represents an object existing over time.
┌─────┐
│ obj │
└──┬──┘
│
│ ← lifeline (dashed line)
│
│
2. Activation Bar (thin rectangle on lifeline)
─────────────────────────────────────────────
Shows when an object is actively processing.
┌─────┐
│ obj │
└──┬──┘
│
│ ┌──┐
│ │ │ ← activation bar
│ │ │
│ └──┘
│
3. Message Types:
──────────────
────────▶ Synchronous call (object waits for response)
- - - - -▶ Asynchronous call (object continues immediately)
────────▶ Return message (dashed arrow, optional)
4. Combined Fragment:
──────────────────
┌─────────────────────┐
│ alt / opt / loop │ ← Special constructs
│ │
└─────────────────────┘Reading a Sequence Diagram
Let's walk through a simple example — a user logging into a system:
┌───────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐
│ User │ │LoginForm │ │ Auth │ │ Database │
└───┬───┘ └────┬─────┘ └────┬─────┘ └────┬─────┘
│ │ │ │
│ 1. enter │ │ │
│ credentials │ │ │
│───────────────▶│ │ │
│ │ │ │
│ │ 2. validate() │ │
│ │──────────────────▶│ │
│ │ │ │
│ │ │ 3. findUser() │
│ │ │──────────────────▶│
│ │ │ │
│ │ │ 4. user data │
│ │ │◀──────────────────│
│ │ │ │
│ │ 5. valid/invalid │ │
│ │◀──────────────────│ │
│ │ │ │
│ 6. success/ │ │ │
│ error message │ │ │
│◀───────────────│ │ │
│ │ │ │The diagram reads top-to-bottom, left-to-right. The User enters credentials (1), the form validates them (2), the auth system queries the database (3-4), returns validation result (5), and the form shows success/error (6).
Combined Fragments
Combined fragments handle complex logic like conditions, loops, and alternatives:
alt (Alternative — if/else)
┌────────────────────────────────────┐
│ alt │
│ │
│ [password valid] │
│ ───────────────▶ │
│ show dashboard │
│ │
├────────────────────────────────────┤
│ else │
│ │
│ [password invalid] │
│ ───────────────▶ │
│ show error │
│ │
└────────────────────────────────────┘
opt (Optional — if condition is true)
┌────────────────────────────────────┐
│ opt [rememberMe checked] │
│ │
│ save token to cookie │
│ │
└────────────────────────────────────┘
loop (Loop — repeat)
┌────────────────────────────────────┐
│ loop [for each item in cart] │
│ │
│ calculate item total │
│ add to grand total │
│ │
└────────────────────────────────────┘
ref (Reference — reuse another diagram)
┌────────────────────────────────────┐
│ ref │
│ Login Use Case │
└────────────────────────────────────┘Complex Example: Online Shopping
┌──────────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐
│ Customer │ │ Cart │ │ Payment │ │ Inventory│ │ Email │
└────┬─────┘ └────┬─────┘ └────┬─────┘ └────┬─────┘ └────┬─────┘
│ │ │ │ │
│ checkout() │ │ │ │
│──────────────▶│ │ │ │
│ │ │ │ │
│ │ getTotal() │ │ │
│ │──────────────▶│ │ │
│ │ │ │ │
│ │ total │ │ │
│ │◀──────────────│ │ │
│ │ │ │ │
│ │ processPayment() │ │
│ │──────────────────────────────▶│ │
│ │ │ │ │
│ │ ┌────────────────────────────────────────┐ │
│ │ │ alt [payment successful] │ │
│ │ │ │ │
│ │ │ charge card │ │
│ │ │ decreaseStock() │ │
│ │ │───────────────────────────────────────▶│ │
│ │ │ │ │
│ │ │ stock updated │ │
│ │ │◀───────────────────────────────────────│ │
│ │ │ │ │
│ │ │ sendConfirmation() │ │
│ │ │─────────────────────────────────────────────────▶│
│ │ │ │ │
│ │ │ confirmation sent │ │
│ │ │◀─────────────────────────────────────────────────│
│ │ │ │ │
│ │ ├────────────────────────────────────────┤ │
│ │ │ else [payment failed] │ │
│ │ │ │ │
│ │ │ show error │ │
│ │ │ │ │
│ │ └────────────────────────────────────────┘ │
│ │ │ │ │
│ orderComplete │ │ │ │
│◀──────────────│ │ │ │
│ │ │ │ │When to Use Sequence Diagrams
Use Sequence Diagrams When:
✓ You want to show the step-by-step flow of a specific scenario
✓ You need to understand the order of method calls
✓ You're documenting a complex algorithm or workflow
✓ You need to identify which objects participate in a use case
✓ You're debugging interaction issues between components
Sequence Diagrams Are Less Useful When:
✗ You need to show the overall system structure (use Class Diagram)
✗ You want to show many scenarios at once (gets too cluttered)
✗ You need to show concurrent activities (use Activity Diagram)
✗ You want to show state changes (use State Diagram)