Mermaid Diagrams

Mermaid Diagramming

Create professional software diagrams using Mermaid's text-based syntax. Mermaid renders diagrams from simple text definitions, making diagrams version-controllable, easy to update, and maintainable alongside code.

Core Syntax Structure

All Mermaid diagrams follow this pattern:

diagramType
  definition content

Key principles:

• First line declares diagram type (e.g., classDiagram, sequenceDiagram, flowchart)
• Use %% for comments
• Line breaks and indentation improve readability but aren't required
• Unknown words break diagrams; parameters fail silently

Diagram Type Selection Guide

Choose the right diagram type:

1. Class Diagrams - Domain modeling, OOP design, entity relationships

   • Domain-driven design documentation
   • Object-oriented class structures
   • Entity relationships and dependencies

2. Sequence Diagrams - Temporal interactions, message flows

   • API request/response flows
   • User authentication flows
   • System component interactions
   • Method call sequences

3. Flowcharts - Processes, algorithms, decision trees

   • User journeys and workflows
   • Business processes
   • Algorithm logic
   • Deployment pipelines

4. Entity Relationship Diagrams (ERD) - Database schemas

   • Table relationships
   • Data modeling
   • Schema design

5. C4 Diagrams - Software architecture at multiple levels

   • System Context (systems and users)
   • Container (applications, databases, services)
   • Component (internal structure)
   • Code (class/interface level)

6. State Diagrams - State machines, lifecycle states

7. Git Graphs - Version control branching strategies

8. Gantt Charts - Project timelines, scheduling

9. Pie/Bar Charts - Data visualization

Quick Start Examples

Class Diagram (Domain Model)

classDiagram
    Title -- Genre
    Title *-- Season
    Title *-- Review
    User --> Review : creates
    
    class Title {
        +string name
        +int releaseYear
        +play()
    }
    
    class Genre {
        +string name
        +getTopTitles()
    }

Sequence Diagram (API Flow)

sequenceDiagram
    participant User
    participant API
    participant Database
    
    User->>API: POST /login
    API->>Database: Query credentials
    Database-->>API: Return user data
    alt Valid credentials
        API-->>User: 200 OK + JWT token
    else Invalid credentials
        API-->>User: 401 Unauthorized
    end

Flowchart (User Journey)

flowchart TD
    Start([User visits site]) --> Auth{Authenticated?}
    Auth -->|No| Login[Show login page]
    Auth -->|Yes| Dashboard[Show dashboard]
    Login --> Creds[Enter credentials]
    Creds --> Validate{Valid?}
    Validate -->|Yes| Dashboard
    Validate -->|No| Error[Show error]
    Error --> Login

ERD (Database Schema)

erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    PRODUCT ||--o{ LINE_ITEM : includes
    
    USER {
        int id PK
        string email UK
        string name
        datetime created_at
    }
    
    ORDER {
        int id PK
        int user_id FK
        decimal total
        datetime created_at
    }

Detailed References

For in-depth guidance on specific diagram types, see:

• references/class-diagrams.md - Domain modeling, relationships (association, composition, aggregation, inheritance), multiplicity, methods/properties
• references/sequence-diagrams.md - Actors, participants, messages (sync/async), activations, loops, alt/opt/par blocks, notes
• references/flowcharts.md - Node shapes, connections, decision logic, subgraphs, styling
• references/erd-diagrams.md - Entities, relationships, cardinality, keys, attributes
• references/c4-diagrams.md - System context, container, component diagrams, boundaries
• references/advanced-features.md - Themes, styling, configuration, layout options

Best Practices

1. Start Simple - Begin with core entities/components, add details incrementally
2. Use Meaningful Names - Clear labels make diagrams self-documenting
3. Comment Extensively - Use %% comments to explain complex relationships
4. Keep Focused - One diagram per concept; split large diagrams into multiple focused views
5. Version Control - Store .mmd files alongside code for easy updates
6. Add Context - Include titles and notes to explain diagram purpose
7. Iterate - Refine diagrams as understanding evolves

Configuration and Theming

Configure diagrams using frontmatter:

---
config:
  theme: base
  themeVariables:
    primaryColor: "#ff6b6b"
---
flowchart LR
    A --> B

Available themes: default, forest, dark, neutral, base

Layout options:

• layout: dagre (default) - Classic balanced layout
• layout: elk - Advanced layout for complex diagrams (requires integration)

Look options:

• look: classic - Traditional Mermaid style
• look: handDrawn - Sketch-like appearance

Exporting and Rendering

Native support in:

• GitHub/GitLab - Automatically renders in Markdown
• VS Code - With Markdown Mermaid extension
• Notion, Obsidian, Confluence - Built-in support

Export options:

• Mermaid Live Editor - Online editor with PNG/SVG export
• Mermaid CLI - npm install -g @mermaid-js/mermaid-cli then mmdc -i input.mmd -o output.png
• Docker - docker run --rm -v $(pwd):/data minlag/mermaid-cli -i /data/input.mmd -o /data/output.png

Common Pitfalls

• Breaking characters - Avoid {} in comments, use proper escape sequences for special characters
• Syntax errors - Misspellings break diagrams; validate syntax in Mermaid Live
• Overcomplexity - Split complex diagrams into multiple focused views
• Missing relationships - Document all important connections between entities

When to Create Diagrams

Always diagram when:

• Starting new projects or features
• Documenting complex systems
• Explaining architecture decisions
• Designing database schemas
• Planning refactoring efforts
• Onboarding new team members

Use diagrams to:

• Align stakeholders on technical decisions
• Document domain models collaboratively
• Visualize data flows and system interactions
• Plan before coding
• Create living documentation that evolves with code

Rendering and sending in mimir

The mimirbot container ships mmdc (@mermaid-js/mermaid-cli) on PATH. Workflow for sending a rendered diagram in a Discord/Slack reply:

# 1. Write the mmd source under the outbound attachments dir.
mkdir -p "$MIMIR_HOME/attachments/outbound"
cat > "$MIMIR_HOME/attachments/outbound/architecture.mmd" <<'EOF'
flowchart LR
    A[Client] --> B[API]
    B --> C[(DB)]
EOF

# 2. Render to PNG in the same dir (so <send-file> can resolve it).
mmdc -i "$MIMIR_HOME/attachments/outbound/architecture.mmd" \
     -o "$MIMIR_HOME/attachments/outbound/architecture.png"

Then send via the directive — single send_message call:

Here's the shape I have in mind.

<actions>
  <send-file path="architecture.png" kind="image" caption="Architecture sketch" cleanup="true" />
</actions>

cleanup="true" deletes the rendered PNG after a successful send so the attachments dir doesn't accumulate stale renders.