mermaid-diagrams
Comprehensive guide for creating software diagrams using Mermaid syntax. Use when users need to create, visualize, or document software through diagrams including class diagrams (domain modeling, object-oriented design), sequence diagrams (application flows, API interactions, code execution), flowcharts (processes, algorithms, user journeys), entity relationship diagrams (database schemas), C4 architecture diagrams (system context, containers, components), state diagrams, git graphs, pie charts, gantt charts, or any other diagram type. Triggers include requests to "diagram", "visualize", "model", "map out", "show the flow", or when explaining system architecture, database design, code structure, or user/application flows.
What this skill does
# 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:
```mermaid
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)
```mermaid
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)
```mermaid
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)
```mermaid
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)
```mermaid
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](references/class-diagrams.md)** - Domain modeling, relationships (association, composition, aggregation, inheritance), multiplicity, methods/properties
- **[references/sequence-diagrams.md](references/sequence-diagrams.md)** - Actors, participants, messages (sync/async), activations, loops, alt/opt/par blocks, notes
- **[references/flowcharts.md](references/flowcharts.md)** - Node shapes, connections, decision logic, subgraphs, styling
- **[references/erd-diagrams.md](references/erd-diagrams.md)** - Entities, relationships, cardinality, keys, attributes
- **[references/c4-diagrams.md](references/c4-diagrams.md)** - System context, container, component diagrams, boundaries
- **[references/advanced-features.md](references/advanced-features.md)** - Themes, styling, configuration, layout options
## Important Notes
- Avoid `{}` in comments — they break diagrams. Use proper escape sequences for special characters.
- Unknown words and misspellings break diagrams silently; parameters fail silently.
- Use `%%` for comments.
- For theming, styling, and layout configuration, see [references/advanced-features.md](references/advanced-features.md).
- **Export with CLI:** `npm install -g @mermaid-js/mermaid-cli` then `mmdc -i input.mmd -o output.png`
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