Mermaid Diagrams

Generate diagrams in markdown that render in GitHub, GitLab, VS Code, Obsidian, Notion.

Quick Start

```mermaid
flowchart LR
    A[Start] --> B{Decision}
    B -->|Yes| C[Action]
    B -->|No| D[End]
```

Quick Decision Tree

What to visualize?
├─ Process, algorithm, decision flow    → flowchart
├─ API calls, service interactions      → sequenceDiagram
├─ Database tables, relationships       → erDiagram
├─ OOP, type hierarchy, domain model    → classDiagram
├─ State machine, lifecycle             → stateDiagram-v2
├─ System architecture, services        → flowchart + subgraphs (or C4Context)
├─ Project timeline, sprints            → gantt
├─ User experience, pain points         → journey
├─ Git branches                         → gitGraph
├─ Data distribution                    → pie
└─ Priority matrix                      → quadrantChart

Diagram Types

Type	Declaration	Best For
Flowchart	flowchart LR/TB	Processes, decisions, data flow
Sequence	sequenceDiagram	API flows, service calls
ER	erDiagram	Database schemas
Class	classDiagram	Types, domain models
State	stateDiagram-v2	State machines
Gantt	gantt	Project timelines
Journey	journey	User experience
C4	C4Context	System architecture
Git	gitGraph	Branch visualization

Common Patterns

System Architecture

flowchart LR
    subgraph Client
        Browser & Mobile
    end
    subgraph Services
        API --> Auth & Core
    end
    subgraph Data
        DB[(PostgreSQL)]
    end
    Client --> API
    Core --> DB

API Request Flow

sequenceDiagram
    autonumber
    Client->>+API: POST /orders
    API->>Auth: Validate
    Auth-->>API: OK
    API->>+DB: Insert
    DB-->>-API: ID
    API-->>-Client: 201 Created

Database Schema

erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    USER { uuid id PK; string email UK }
    ORDER { uuid id PK; uuid user_id FK }

State Machine

stateDiagram-v2
    [*] --> Draft
    Draft --> Submitted : submit()
    Submitted --> Approved : approve()
    Submitted --> Rejected : reject()
    Approved --> [*]

Syntax Quick Reference

Flowchart Nodes

[Rectangle]  (Rounded)  {Diamond}  [(Database)]  [[Subroutine]]
((Circle))   >Asymmetric]   {{Hexagon}}

Flowchart Edges

A --> B       # Arrow
A --- B       # Line
A -.-> B      # Dotted arrow
A ==> B       # Thick arrow
A -->|text| B # Labeled

Sequence Arrows

->>   # Solid arrow (request)
-->>  # Dotted arrow (response)
-x    # X end (async)
-)    # Open arrow

ER Cardinality

||--||   # One to one
||--o{   # One to many
}o--o{   # Many to many

Best Practices

1. Choose the right type — Use decision tree above
2. Keep focused — One concept per diagram
3. Use meaningful labels — Not just A, B, C
4. Direction matters — LR for flows, TB for hierarchies
5. Group with subgraphs — Organize related nodes

Reference Documentation

File	Purpose
references/FLOWCHARTS.md	Nodes, edges, subgraphs, styling
references/SEQUENCE.md	Participants, messages, activation
references/CLASS-ER.md	Classes, ER diagrams, relationships
references/STATE-JOURNEY.md	States, user journeys
references/DATA-CHARTS.md	Gantt, Pie, Timeline, Quadrant
references/ARCHITECTURE.md	C4, Block, Kanban
references/CHEATSHEET.md	All syntax quick reference

Resources

• Official Documentation: https://mermaid.js.org
• Live Editor: https://mermaid.live
• GitHub Repository: https://github.com/mermaid-js/mermaid
• GitHub Markdown Support: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams
• GitLab Markdown Support: https://docs.gitlab.com/ee/user/markdown.html#diagrams-and-flowcharts