The Visual Language of Software
Sequence Diagrams: The Most Powerful Tool
in Software Design
From the pioneers at Bell Labs in the 1990s to modern distributed systems, sequence diagrams remain the gold standard for visualizing how systems communicate. Build, simulate, and share yours here.
Live Example: User Login Flow
Step 0 of 6 - Click "Simulate" to watch the flow
The Origin Story
From telecommunications research to universal software design standard
Message Sequence Charts
Born at ITU-T (International Telecommunication Union) for documenting telecommunications protocols. Engineers needed a way to visualize how signals flow between network components.
UML Standardization
The Object Management Group (OMG) incorporated sequence diagrams into the Unified Modeling Language. Grady Booch, Ivar Jacobson, and James Rumbaugh unified competing notations.
Universal Standard
From microservices to mobile apps, sequence diagrams are the lingua franca of software architecture. They bridge the gap between technical implementation and business requirements.
Where Sequence Diagrams Shine
Every interaction between components is a sequence diagram waiting to be drawn
API Design
Document REST/GraphQL endpoints and their interactions
System Architecture
Visualize microservices communication patterns
User Flows
Map out user journeys through your application
Database Operations
Design transaction flows and query patterns
Error Handling
Document exception flows and recovery strategies
Async Messaging
Design event-driven and pub/sub systems
The Vocabulary
Master these six core elements and you can describe any system interaction
Participant
Objects or actors that interact in the sequence
Message
Communication between participants
Lifeline
Vertical line showing participant existence over time
Activation
Rectangle showing when a participant is processing
Fragment
Combined fragments for control flow
Note
Annotations and comments on the diagram
Combined Fragments
Control flow operators that add logic to your sequences
alt AlternativeIf/else branching logic
Example: [x > 0] / [else]
opt OptionalExecute if condition is true
Example: [user.isAdmin]
loop LoopRepeated execution
Example: [for each item]
par ParallelConcurrent execution
Example: parallel processing
break BreakExit enclosing fragment
Example: [error occurred]
critical CriticalAtomic operation
Example: transaction block
Best Practices
Tips from years of real-world software architecture
Keep It Simple
Focus on one scenario per diagram. Split complex flows into multiple diagrams.
Name Clearly
Use descriptive names for participants and messages that match your codebase.
Show Return Values
Always show return messages to complete the interaction picture.
Use Fragments Sparingly
Only use combined fragments when control flow is essential to understanding.
Maintain Order
Arrange participants left-to-right in the order they appear in the flow.
Add Context
Use notes to explain non-obvious behavior or business rules.
Design Together, Build Together
Create collaborative sequence diagrams with your team. Share a link, see each other's cursors in real-time, and build complex systems together.
Share a Link
Generate a share link and invite anyone to collaborate. No sign-up required for teammates.
Live Cursors
See your teammates' cursors and selections in real-time. Follow their view or work independently.
Offline First
Works offline with automatic sync when reconnected. Your work is always saved locally.
Your Projects
Ready to Build Your First Diagram?
Start with the single-user editor or create a collaborative project for team design.
Participant types
Message types
Combined fragments
Team collaboration
Press ⌘K or Ctrl+K for quick actions