Mermaid Diagram Specialist

Input

What the skill needs:

• Process/system description
• Entities and relationships (for ERDs)
• Component interactions (for sequence diagrams)
• Architecture layers (for C4 diagrams)
• States and transitions (for state diagrams)

Workflow

Step 1: Diagram Type Selection

Objective: Choose appropriate diagram type for requirements

Available Diagram Types:

1. Flowchart: Decision flows, algorithms, processes
2. Sequence Diagram: API interactions, message flows
3. ERD: Database schemas, entity relationships
4. Class Diagram: Object-oriented design
5. State Diagram: State machines, lifecycle
6. Gantt Chart: Project timelines, schedules
7. C4 Diagram: Architecture at different levels
8. Pie/Bar Charts: Data visualization
9. Git Graph: Version control flows
10. User Journey: User experience flows

Decision Matrix:

• Process with decisions → Flowchart
• API/system interactions → Sequence Diagram
• Database structure → ERD
• System architecture → C4 Diagram
• Object relationships → Class Diagram
• State transitions → State Diagram
• Project timeline → Gantt Chart

Validation:

• Diagram type matches content
• Complexity appropriate
• Audience considered
• Purpose clear

Output: Selected diagram type

Step 2: Flowchart Creation

Objective: Create process and decision flow diagrams

Syntax:

flowchart TD
    Start([Start]) --> Input[/User Input/]
    Input --> Validate{Valid?}
    Validate -->|Yes| Process[Process Data]
    Validate -->|No| Error[Show Error]
    Error --> Input
    Process --> Save[(Save to DB)]
    Save --> Success[/Success Response/]
    Success --> End([End])

Node Shapes:

• [Rectangle] - Process step
• ([Rounded]) - Start/End
• {Diamond} - Decision
• [/Parallelogram/] - Input/Output
• [(Database)] - Data storage
• ((Circle)) - Connector

Direction Options:

• TD - Top to Down
• LR - Left to Right
• BT - Bottom to Top
• RL - Right to Left

Example - Booking Flow:

flowchart TD
    Start([User Initiates Booking]) --> CheckDates[Check Date Availability]
    CheckDates --> Available{Dates Available?}
    Available -->|No| ShowError[/Show Unavailable Message/]
    ShowError --> End([End])
    Available -->|Yes| CreateBooking[Create Pending Booking]
    CreateBooking --> Payment[Process Payment]
    Payment --> PaymentSuccess{Payment Success?}
    PaymentSuccess -->|No| CancelBooking[Cancel Booking]
    CancelBooking --> ShowError
    PaymentSuccess -->|Yes| ConfirmBooking[Confirm Booking]
    ConfirmBooking --> SendEmail[/Send Confirmation Email/]
    SendEmail --> SaveDB[(Save to Database)]
    SaveDB --> Success[/Show Success/]
    Success --> End

Validation:

• All paths covered
• Decision points clear
• Start and end defined
• Flow direction logical

Output: Process flowchart

Step 3: Sequence Diagram Creation

Objective: Document API interactions and message flows

Syntax:

sequenceDiagram
    actor User
    participant Frontend
    participant API
    participant DB
    participant Payment

    User->>Frontend: Click "Book"
    Frontend->>API: POST /api/bookings
    API->>DB: Check availability
    DB-->>API: Available
    API->>Payment: Process payment
    Payment-->>API: Payment successful
    API->>DB: Create booking
    DB-->>API: Booking created
    API-->>Frontend: 201 Created
    Frontend-->>User: Show confirmation

Participant Types:

• actor - Human user
• participant - System/Service
• database - Database

Arrow Types:

• -> - Solid line (synchronous)
• --> - Dotted line (response)
• ->> - Solid arrow (async message)
• -->> - Dotted arrow (async response)

Example - Authentication Flow:

sequenceDiagram
    actor User
    participant Frontend
    participant API
    participant Clerk
    participant DB

    User->>Frontend: Enter credentials
    Frontend->>Clerk: Login request
    Clerk->>Clerk: Validate credentials
    alt Credentials valid
        Clerk-->>Frontend: JWT token
        Frontend->>API: Request with token
        API->>Clerk: Verify token
        Clerk-->>API: Token valid
        API->>DB: Fetch user data
        DB-->>API: User data
        API-->>Frontend: User session
        Frontend-->>User: Logged in
    else Credentials invalid
        Clerk-->>Frontend: Auth error
        Frontend-->>User: Show error
    end

Validation:

• All participants identified
• Message flow logical
• Return messages shown
• Alt/loop blocks used correctly

Output: Sequence diagram

Step 4: ERD Creation

Objective: Document database schema and relationships

Syntax:

erDiagram
    USER ||--o{ BOOKING : creates
    ACCOMMODATION ||--o{ BOOKING : "booked for"
    USER {
        uuid id PK
        string email UK
        string name
        timestamp created_at
    }
    BOOKING {
        uuid id PK
        uuid user_id FK
        uuid accommodation_id FK
        date check_in
        date check_out
        enum status
    }
    ACCOMMODATION {
        uuid id PK
        string name
        text description
        decimal price_per_night
    }

Relationship Types:

• ||--|| - One to one
• ||--o{ - One to many
• }o--o{ - Many to many
• ||--o| - One to zero or one

Cardinality Symbols:

• || - Exactly one
• o| - Zero or one
• }o - Zero or more
• }| - One or more

Example - Full Hospeda ERD:

erDiagram
    USER ||--o{ BOOKING : creates
    USER ||--o{ REVIEW : writes
    USER ||--o{ ACCOMMODATION : owns
    ACCOMMODATION ||--o{ BOOKING : "has bookings"
    ACCOMMODATION ||--o{ REVIEW : "has reviews"
    ACCOMMODATION }o--o{ AMENITY : includes
    BOOKING ||--|| PAYMENT : "has payment"

    USER {
        uuid id PK
        string clerk_id UK
        string email UK
        string name
        enum role
        timestamp created_at
    }

    ACCOMMODATION {
        uuid id PK
        uuid owner_id FK
        string name
        text description
        decimal price_per_night
        int max_guests
        enum status
    }

    BOOKING {
        uuid id PK
        uuid user_id FK
        uuid accommodation_id FK
        date check_in
        date check_out
        int guests
        enum status
        decimal total_price
    }

    REVIEW {
        uuid id PK
        uuid user_id FK
        uuid accommodation_id FK
        int rating
        text comment
        timestamp created_at
    }

    PAYMENT {
        uuid id PK
        uuid booking_id FK
        string mercadopago_id UK
        decimal amount
        enum status
        timestamp processed_at
    }

    AMENITY {
        uuid id PK
        string name
        string icon
    }

Validation:

• All entities defined
• Relationships accurate
• Cardinality correct
• Primary/Foreign keys marked

Output: ERD diagram

Step 5: C4 Architecture Diagrams

Objective: Document system architecture at different levels

Context Level (System in environment):

C4Context
    title System Context - Hospeda Platform

    Person(guest, "Guest", "Tourist looking for accommodation")
    Person(owner, "Owner", "Accommodation owner")
    System(hospeda, "Hospeda Platform", "Tourism booking platform")

    System_Ext(clerk, "Clerk", "Authentication provider")
    System_Ext(mercadopago, "Mercado Pago", "Payment processor")
    System_Ext(email, "Email Service", "Transactional emails")

    Rel(guest, hospeda, "Searches and books", "HTTPS")
    Rel(owner, hospeda, "Manages listings", "HTTPS")
    Rel(hospeda, clerk, "Authenticates users", "API")
    Rel(hospeda, mercadopago, "Processes payments", "API")
    Rel(hospeda, email, "Sends notifications", "SMTP")

Container Level (Applications and data stores):

C4Container
    title Container - Hospeda Platform

    Person(user, "User")

    Container(web, "Web App", "Astro + React", "Public-facing website")
    Container(admin, "Admin Panel", "TanStack Start", "Management interface")
    Container(api, "API", "Hono", "Backend services")
    ContainerDb(db, "Database", "PostgreSQL", "Stores all data")

    Rel(user, web, "Uses", "HTTPS")
    Rel(user, admin, "Manages", "HTTPS")
    Rel(web, api, "Calls", "JSON/HTTPS")
    Rel(admin, api, "Calls", "JSON/HTTPS")
    Rel(api, db, "Reads/Writes", "SQL")

Component Level (Internal structure):

C4Component
    title Components - API Application

    Container(api, "API", "Hono")

    Component(routes, "Routes", "Hono Router", "HTTP endpoints")
    Component(services, "Services", "Business Logic", "Domain operations")
    Component(models, "Models", "Data Access", "DB operations")
    Component(middleware, "Middleware", "Cross-cutting", "Auth, logging, errors")

    Rel(routes, middleware, "Uses")
    Rel(routes, services, "Calls")
    Rel(services, models, "Uses")
    Rel(models, db, "Queries")

Validation:

• Appropriate level selected
• All systems/containers shown
• Relationships clear
• External systems identified

Output: C4 architecture diagrams

Step 6: State Diagram Creation

Objective: Document state machines and lifecycles

Syntax:

stateDiagram-v2
    [*] --> Pending
    Pending --> Confirmed : Payment Success
    Pending --> Cancelled : Payment Failed
    Pending --> Cancelled : User Cancels
    Confirmed --> CheckedIn : Check-in Date
    Confirmed --> Cancelled : Cancellation Request
    CheckedIn --> CheckedOut : Check-out Date
    CheckedOut --> Reviewed : User Submits Review
    CheckedOut --> [*] : 30 Days Elapsed
    Reviewed --> [*]
    Cancelled --> [*]

Example - Booking Lifecycle:

stateDiagram-v2
    [*] --> Draft : Create Booking

    state "Pending Payment" as Pending
    state "Payment Processing" as Processing

    Draft --> Pending : Submit Booking
    Pending --> Processing : Initiate Payment

    Processing --> Confirmed : Payment Approved
    Processing --> PaymentFailed : Payment Declined

    PaymentFailed --> Pending : Retry Payment
    PaymentFailed --> Cancelled : Max Retries

    Confirmed --> Active : Check-in Date Reached
    Active --> Completed : Check-out Date Reached

    Confirmed --> CancelRequested : Cancellation Request
    CancelRequested --> RefundProcessing : Approve Cancellation
    RefundProcessing --> Cancelled : Refund Complete

    Completed --> [*]
    Cancelled --> [*]

    note right of Confirmed
        Owner notified
        Calendar blocked
    end note

    note right of Completed
        Review requested
        Payment released
    end note

Validation:

• All states defined
• Transitions logical
• Start/end states marked
• Notes explain key states

Output: State diagram

Step 7: Styling and Customization

Objective: Apply consistent styling to diagrams

Theme Application:

%%{init: {'theme':'base', 'themeVariables': {
  'primaryColor':'#3B82F6',
  'primaryTextColor':'#fff',
  'primaryBorderColor':'#2563EB',
  'lineColor':'#6B7280',
  'secondaryColor':'#10B981',
  'tertiaryColor':'#F59E0B'
}}}%%
flowchart TD
    A[Start] --> B[Process]
    B --> C[End]

Class Styling:

flowchart TD
    A[Normal] --> B[Success]
    B --> C[Error]

    classDef successClass fill:#10B981,stroke:#059669,color:#fff
    classDef errorClass fill:#EF4444,stroke:#DC2626,color:#fff

    class B successClass
    class C errorClass

Validation:

• Colors match brand
• Contrast sufficient
• Styling consistent
• Readable in both themes

Output: Styled diagrams

Output

Produces:

• Mermaid diagram code in markdown
• Multiple diagram types as needed
• Styled and themed diagrams
• Documentation-ready visualizations

Success Criteria:

• Diagram accurately represents system
• All elements properly labeled
• Relationships clear and correct
• Styling consistent with brand
• Renders correctly in markdown

Best Practices

1. Simplicity: Keep diagrams focused and uncluttered
2. Labels: Clear, descriptive labels for all elements
3. Direction: Consistent flow direction (usually top-down or left-right)
4. Grouping: Use subgraphs to group related elements
5. Colors: Use color to highlight important elements
6. Notes: Add notes to explain complex logic
7. Levels: Use appropriate abstraction level for audience
8. Updates: Keep diagrams in sync with code
9. Comments: Add comments in mermaid code for maintainability
10. Testing: Verify diagrams render in target platform

Common Patterns

API Request Flow

sequenceDiagram
    Client->>+API: GET /resource
    API->>+Service: fetchResource()
    Service->>+Model: findById()
    Model->>+DB: SELECT query
    DB-->>-Model: Row data
    Model-->>-Service: Entity
    Service-->>-API: DTO
    API-->>-Client: JSON response

Error Handling Flow

flowchart TD
    Request[Incoming Request] --> Validate{Valid?}
    Validate -->|No| ValidationError[Validation Error]
    ValidationError --> ErrorHandler[Error Handler]
    Validate -->|Yes| Process[Process Request]
    Process --> DB{DB Success?}
    DB -->|No| DBError[Database Error]
    DBError --> ErrorHandler
    DB -->|Yes| Success[Success Response]
    ErrorHandler --> LogError[Log Error]
    LogError --> ErrorResponse[Error Response]

Notes

• Mermaid renders in GitHub, GitLab, Notion, and most markdown viewers
• Live editor available at mermaid.live
• Maximum complexity: Keep under 20 nodes for readability
• Use subgraphs for grouping related nodes
• Test rendering in target platform before committing
• Keep diagram source in markdown files, not images
• Version control diagrams with code
• Update diagrams during code review