Sequence Diagrams
📌 What is a Sequence Diagram?
A Sequence Diagram is a type of interaction diagram in UML (Unified Modeling Language) that models the flow of messages/events between system components (objects, actors) over time. It is used to show how different parts of a system interact with each other in a specific sequence to accomplish a process or functionality.
Purpose:
Visualize the order of interactions in a process.
Show how objects and components collaborate over time.
Model use case scenarios and system behaviors in detail.
🎯 Key Elements of Sequence Diagrams
Element
Description
Notation
Lifeline
Represents an object, class, actor, or system component
Vertical dashed line
Actor
External user or system interacting with the system
Stick figure
Message
Communication between lifelines (method calls, signals)
Solid arrow (→)
Return Message
Response to a message
Dashed arrow (--> )
Activation/Execution Bar
Time period when an object performs an operation
Thin rectangle on lifeline
Creation Message
Creates a new object
Arrow with "new" label
Destruction Message
Destroys an object
Cross mark (X) on lifeline
Alt / Opt / Loop Fragments
Conditional and looping behaviors
Boxed frames with label
✅ When to Use Sequence Diagrams
To model scenarios of a use case or business process.
To visualize the flow of interactions between system components.
To specify system behavior and logic.
To analyze and design the dynamic aspects of a system.
To document communication patterns in software engineering.
🧭 How to Create a Sequence Diagram (Step-by-Step)
Identify the scenario/use case to be modeled.
List all actors and objects involved in the process.
Determine the order of interactions (messages) exchanged.
Draw lifelines for each participant.
Add messages and return messages between lifelines.
Use activation bars to show periods of activity.
Add conditional/loop fragments if needed.
Review and validate with stakeholders.
📊 Example of a Sequence Diagram
Scenario: User Login Process
Participants:
User (Actor)
Login Page (UI)
Authentication Service
Database
Step-by-Step Flow:
User enters credentials on Login Page.
Login Page sends credentials to Authentication Service.
Authentication Service queries Database for user data.
Database returns user data.
Authentication Service verifies credentials.
Authentication Service sends login success/failure to Login Page.
Login Page displays result to User.
Diagram Representation (Text format)
pgsqlCopyEditUser Login Page Auth Service Database
| | | |
|-- Login --> | | |
| |-- Auth Request ->| |
| | |-- Query User -->|
| | |<-- User Data ---|
| |<-- Auth Result --| |
|<-- Result ---| | |💡 Explanation of Flow:
Horizontal axis: Participants (User, Login Page, Services).
Vertical axis: Time flow from top (start) to bottom (end).
Solid arrows: Messages/requests.
Dashed arrows: Return messages.
Activation bars: Indicate active period of execution (optional in text).
🚀 Benefits of Sequence Diagrams
Benefit
Explanation
Clarifies communication flow
Shows how components exchange messages.
Captures system behavior
Visualizes object interactions for scenarios.
Reveals process logic
Makes implicit flows explicit for review.
Improves collaboration
Helps business and technical teams align understanding.
Useful for design and testing
Guides development and validation of processes.
🔑 Best Practices for Sequence Diagrams
Focus on a single scenario to keep diagrams simple.
Name messages clearly (use descriptive method names).
Use activation bars to highlight when objects are active.
Organize lifelines logically from left to right.
Use fragments (alt, opt, loop) for conditional/looped flows.
Keep diagrams clean and uncluttered for readability.
🎯 Summary of Sequence Diagrams
Aspect
Details
Purpose
Visualize message flow over time between objects
Key Elements
Lifelines, Messages, Activations, Fragments, Actors
Use Cases
Modeling scenarios, system interactions, communication flows
Main Benefits
Clarifies communication, documents system behavior, improves understanding
📚 Common Fragments and Notations
Fragment
Purpose
Notation
alt (Alternative)
Represents conditional flow (if/else)
Box labeled alt with conditions
opt (Optional)
Represents optional flow (if condition met)
Box labeled opt
loop
Repeating interaction flow
Box labeled loop with condition
par
Parallel flows
Box labeled par
🚀 Example of a Loop Fragment (Login Retries)
pgsqlCopyEditloop [Attempts < 3]
User -> Login Page: Enter credentials
Login Page -> Auth Service: Authenticate
Auth Service -> Login Page: Auth result
end loopLast updated