Free Online Sequence Diagram Editor

Design UML sequence diagrams to visualize interactions between system components. Perfect for API documentation and system design.

Create Sequence Diagrams

Key Features

  • Visualize message exchanges between actors
  • Support for synchronous and asynchronous calls
  • Add notes and activation boxes
  • Real-time diagram rendering
  • Export to SVG or PNG
  • Share diagrams with a simple URL

Common Use Cases

  • Document API interactions
  • Design system architecture
  • Visualize authentication flows
  • Plan microservices communication
  • Create technical documentation
  • Explain user-system interactions

Example Sequence Diagram

sequenceDiagram
  participant A as Alice
  participant B as Bob
  A->>B: Hello Bob
  B-->>A: Hi Alice
Try this example

Why Use This Sequence Diagram Editor?

Our free online sequence diagram editor makes it easy to create professional diagrams using simple text-based syntax. No installation required - just start typing and see your diagram render in real-time.

Built on Mermaid.js, this editor provides instant preview, SVG/PNG export, and URL sharing capabilities. Perfect for developers, technical writers, project managers, and anyone who needs to create clear, professional diagrams quickly.

How to Create a Sequence Diagram

  1. Start with the 'sequenceDiagram' keyword to initialize your diagram
  2. Define participants (actors or systems) using the 'participant' keyword with clear, descriptive names
  3. Add message flows between participants using arrows: -> for sync messages, ->> for async, -->> for return messages
  4. Use activation boxes with +/- to show when a participant is actively processing
  5. Add notes over or beside participants to provide additional context or explanations
  6. Create loops, alternatives, and optional blocks using rect, alt, opt keywords for complex scenarios
  7. Number your messages automatically with autonumber for easier reference
  8. Review the generated diagram and refine the message sequence for clarity

Best Practices for Sequence Diagrams

  • Name participants clearly: Use descriptive names that reflect their role (e.g., 'UserService' not 'US')
  • Keep messages concise: Use brief, action-oriented labels for arrows (e.g., 'login(credentials)' not 'user tries to log in')
  • Show return values: Use dashed arrows (-->) to explicitly show what data is returned from calls
  • Use activation boxes strategically: Show activation only for significant processing periods, not every message
  • Group related interactions: Use rect boxes to group related message exchanges with colored backgrounds
  • Document error paths: Include alt/else blocks to show both success and error scenarios
  • Limit participant count: Keep to 5-7 participants maximum; break complex interactions into multiple diagrams
  • Order participants logically: Arrange from left to right based on architectural layers or typical interaction flow

Advanced Sequence Diagram Features

Parallel Interactions

Use 'par' blocks to show messages that occur simultaneously or in parallel. Essential for documenting concurrent operations, async processing, or multi-threaded scenarios.

Critical Regions

Mark critical sections that must execute atomically using 'critical' blocks. Useful for documenting thread-safe operations or database transactions.

Loops and Iterations

Show repeated message sequences with 'loop' blocks. Specify conditions or counts to document polling, retries, or batch processing patterns.

Auto-numbering Messages

Enable automatic message numbering with 'autonumber' for easy reference in documentation. You can customize the format and starting number.

Common Sequence Diagram Mistakes

  • ⚠️Forgetting to close activation boxes - every + must have a corresponding - or the diagram will look unbalanced
  • ⚠️Using synchronous arrows for asynchronous operations - use ->> for async calls, not ->
  • ⚠️Missing return messages - always show what data is returned, even if it's just an acknowledgment
  • ⚠️Too many participants - more than 7-8 participants makes the diagram hard to read
  • ⚠️Inconsistent message naming - use a consistent pattern like methodName(params) throughout
  • ⚠️Not showing error conditions - real systems have errors; document them with alt/else blocks
  • ⚠️Overusing notes - notes should add value, not repeat what's obvious from the message labels
  • ⚠️Wrong arrow direction - messages flow from caller to callee; double-check your arrows point correctly

When to Use Sequence Diagrams vs Other Diagrams

Sequence diagrams excel at showing how different parts of a system interact over time. They're perfect for documenting API calls, authentication flows, or any process where timing and message order matter. Use sequence diagrams when you need to explain 'what talks to what and in what order.'

Choose flowcharts when you need to show decision logic or process steps without focusing on which components execute them. Use state diagrams when modeling how an object's state changes in response to events. For showing the static structure of classes and their relationships, class diagrams are more appropriate.

Sequence diagrams are particularly valuable in technical documentation, especially for distributed systems, microservices architectures, and API specifications. They help developers understand complex interactions and are commonly used in design documents and architecture reviews.

Frequently Asked Questions

Common questions about using the Mermaid Viewer and creating diagrams