diagrams-mermaid

name	diagrams:mermaid
description	Produce a Mermaid diagram (flowchart, sequence, class, state, ER, gantt, pie, mindmap) inside a ```mermaid fenced code block for rendering in Markdown, docs, or the GitHub/GitLab viewer. Use when the user asks for a mermaid diagram, a flowchart/sequence/class/state/ER/gantt diagram, or a diagram to embed in Markdown.
disable-model-invocation	false

Create Professional Mermaid Diagrams for Software Development

Objective

Generate clean, maintainable Mermaid diagram code that renders professional software architecture diagrams, flowcharts, sequence diagrams, and more. Diagrams should be version-controllable, easy to update, and clearly communicate system design.

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⚠️ Critical Requirements

1. Start with diagram type declaration on the first line (e.g., flowchart TD, sequenceDiagram, classDiagram).
2. Use %% for comments to explain complex sections.
3. Use self-explanatory identifiers (not A, B, C) for nodes.
4. Keep diagrams focused: one concept per diagram; split large diagrams.
5. Maximum complexity: ~15-20 nodes before splitting into multiple diagrams.
6. Store as .mmd files alongside code for version control.

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Diagram Type Selection Guide

Use Case	Diagram Type	Direction
System architecture, component relationships	flowchart	TD (top-down) or LR (left-right)
API interactions, message flows	sequenceDiagram	Automatic (time-based)
Domain models, OOP design	classDiagram	Automatic
Database schemas	erDiagram	Automatic
State machines, lifecycle	stateDiagram	Automatic
Project timelines	gantt	Automatic

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Step-by-Step Instructions

Step 1: Define Diagram Purpose

Choose one:

• System architecture (components + connections)
• API/request flow (sequence diagram)
• Data model (class/ER diagram)
• Process workflow (flowchart)
• State transitions (state diagram)

Step 2: Choose Diagram Type & Direction

%% Flowchart directions:
flowchart TD    %% Top-down (most common)
flowchart LR    %% Left-right (good for processes)
flowchart BT    %% Bottom-top
flowchart RL    %% Right-left

%% Other diagrams auto-direction:
sequenceDiagram   %% Time-based (left to right)
classDiagram      %% Automatic hierarchy
erDiagram         %% Automatic relationships

Step 3: Define All Nodes First (Best Practice)

✅ DO — Introduce nodes with descriptive identifiers:

flowchart TD
  WEBPORTAL[Web Portal]
  WEBEXPAPI[Web Portal Experience API]
  MOBILEAPP[Mobile App]
  MOBEXPAPI[Mobile App Experience API]
  MICROSERVICES[Microservices]
  DB[Database]

  %% Now define connections
  WEBPORTAL-->WEBEXPAPI
  MOBILEAPP-->MOBEXPAPI

❌ DON'T — Mix definition with connections:

flowchart TD
  WEBPORTAL[Web Portal]-->WEBEXPAPI[Web Portal Experience API]
  MOBILEAPP[Mobile App]-->MOBEXPAPI[Mobile App Experience API]

Step 4: Use Self-Explanatory Identifiers

✅ DO:

flowchart TD
  AUTHSERVICE[Auth Service]-->DATABASE[(Database)]
  USERSERVICE[User Service]-->DATABASE[(Database)]

❌ DON'T:

flowchart TD
  A[Auth Service]-->B[(Database)]
  C[User Service]-->B[(Database)]

Step 5: Define Connections with Labels

%% Basic connection
A --> B

%% With label (use pipes)
A -->|label text| B

%% Different arrow styles
A --> B      %% Solid arrow
A -.-> B     %% Dotted line
A ==> B      %% Thick arrow
A -.- B      %% Dotted without arrow

%% With text label
A -->|POST /login| B
A -->|200 OK| B

Step 6: Use Node Shapes for Clarity

flowchart TD
  A[Hard text]        %% Rectangle (default)
  B(Round edge)       %% Rounded rectangle
  C([Stadium])        %% Stadium shape
  D{Decision}         %% Diamond (for decisions)
  E[(Database)]       %% Cylinder (database)
  F[/Input or Output/] %% Parallelogram
  Start([Start])      %% Circle (start/end)

Step 7: Add Subgraphs for Logical Grouping

flowchart TD
  subgraph CLIENTS["Client Layer"]
    WEBPORTAL[Web Portal]
    MOBILEAPP[Mobile App]
  end

  subgraph BACKEND["Backend Layer"]
    subgraph API["API Gateway"]
      WEBEXPAPI[Web API]
      MOBEXPAPI[Mobile API]
    end

    MICROSERVICES[Microservices]
    DB[(Database)]
  end

  WEBPORTAL-->WEBEXPAPI
  MOBILEAPP-->MOBEXPAPI
  WEBEXPAPI-->MICROSERVICES
  MOBEXPAPI-->MICROSERVICES
  MICROSERVICES-->DB

Step 8: Add Comments for Complex Flows

flowchart TD
  %% Mobile App authentication flow
  MOBILEAPP[Mobile App]-->AUTHAPI[Auth API]

  %% Auth API validates against Identity Provider
  AUTHAPI-->IDENTITYPROVIDER[Identity Provider]

  %% On success, return JWT token
  IDENTITYPROVIDER-->|JWT token| MOBILEAPP

Step 9: Handle Shared Nodes Properly

✅ DO — Define shared node on its own line:

flowchart TD
  WEBPORTAL[Web Portal]-->MICROSERVICES[Microservices]
  MOBILEAPP[Mobile App]-->MICROSERVICES[Microservices]
  MICROSERVICES[Microservices]-->DB[(Database)]

❌ DON'T — Repeat shared node:

flowchart TD
  WEBPORTAL[Web Portal]-->MICROSERVICES[Microservices]-->DB[(Database)]
  MOBILEAPP[Mobile App]-->MICROSERVICES[Microservices]

---

Diagram Type Syntax Guides

A. Flowchart (System Architecture)

flowchart TD
  %% Define all nodes first
  CLIENT[Client Application]
  APIGATEWAY[API Gateway]
  AUTHSERVICE[Auth Service]
  USERSERVICE[User Service]
  DATABASE[(User Database)]
  CACHE[(Redis Cache)]

  %% Define connections with labels
  CLIENT -->|HTTPS Request| APIGATEWAY
  APIGATEWAY -->|Validate JWT| AUTHSERVICE
  APIGATEWAY -->|Get User| USERSERVICE

  %% Service dependencies
  USERSERVICE -->|Read| DATABASE
  USERSERVICE -->|Check| CACHE

  %% Horizontal alignment for clarity
  APIGATEWAY -->|Route| USERSERVICE
  USERSERVICE -->DATABASE

B. Sequence Diagram (API Flow)

sequenceDiagram
  %% Define participants
  participant User
  participant Client as Client App
  participant API as API Gateway
  participant Auth as Auth Service
  participant DB as Database

  %% Message flow
  User->>Client: Enter credentials
  Client->>API: POST /login
  API->>Auth: Validate credentials
  Auth->>DB: Query user
  DB-->>Auth: Return user data

  %% Alt/else blocks for conditions
  alt Valid credentials
    Auth-->>API: 200 OK + JWT
    API-->>Client: 200 OK + token
    Client-->>User: Show dashboard
  else Invalid credentials
    Auth-->>API: 401 Unauthorized
    API-->>Client: 401 Error
    Client-->>User: Show error
  end

C. Class Diagram (Domain Model)

classDiagram
  %% Define classes with attributes/methods
  class User {
    +string id
    +string email
    +string name
    +register()
    +login()
  }

  class Title {
    +string name
    +int releaseYear
    +play()
  }

  class Genre {
    +string name
    +getTopTitles()
  }

  class Review {
    +string content
    +datetime createdAt
    +submit()
  }

  %% Define relationships
  User --> Review : creates
  Title *-- Season : has
  Title *-- Review : has
  Title -- Genre : belongs to

  %% Relationship types
  %% *-- : Composition (strong ownership)
  %% --> : Association (weak relationship)
  %% -- : Aggregation
  %% <|-- : Inheritance

D. Entity Relationship Diagram (Database Schema)

erDiagram
  %% Define entities with keys
  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
  }

  LINE_ITEM {
    int id PK
    int order_id FK
    int product_id FK
    int quantity
    decimal price
  }

  PRODUCT {
    int id PK
    string name
    decimal price
    int stock
  }

  %% Cardinality
  %% ||--o{ : One-to-many (optional)
  %% ||--|{ : One-to-many (mandatory)
  %% }o--o{ : Many-to-many

E. State Diagram (State Machine)

stateDiagram-v2
  %% Define states
  [*] --> Idle

  Idle --> Loading: Start
  Loading --> Processing: Data ready
  Processing --> Complete: Success
  Processing --> Error: Failure

  Error --> Idle: Retry
  Complete --> [*]

  %% Nested states
  state Authentication {
    [*] --> Unauthenticated
    Unauthenticated --> Authenticating: Login
    Authenticating --> Authenticated: Success
    Authenticating --> Unauthenticated: Fail
  }

  %% Notes
  note right of Processing
    Validate input
    Update database
  end note

---

Styling & Configuration

Add Theme Configuration

---
config:
  theme: base
  themeVariables:
    primaryColor: "#ff6b6b"
    secondaryColor: "#4ecdc4"
    tertiaryColor: "#ffe66d"
  look: classic
---
flowchart TD
  A[Node A] --> B[Node B]

Available Themes

• default — Standard blue/grey
• forest — Green tones
• dark — Dark mode
• neutral — Grey tones
• base — Customizable

Style Specific Nodes

flowchart TD
  classDef success fill:#4ecdc4,stroke:#333,stroke-width:2px
  classDef error fill:#ff6b6b,stroke:#333,stroke-width:2px

  A[Success] --> B[Error]
  class A success
  class B error

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Quality Checklist

Before finalizing, verify:

• ✅ Diagram type declared on first line
• ✅ All nodes defined before connections
• ✅ Self-explanatory identifiers (not A, B, C)
• ✅ Comments (%%) explain complex sections
• ✅ Subgraphs used for logical grouping
• ✅ ~15-20 nodes max (split if larger)
• ✅ Labels on arrows describe actions
• ✅ Test renders in Mermaid Live Editor

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Common Pitfalls

❌ Wrong	✅ Right
A --> B with unclear A, B	AUTHSERVICE[Auth Service] --> DATABASE[(Database)]
No comments	%% Auth flow validates JWT token
One giant diagram	Split into architecture.mmd, api-flow.mmd, data-model.mmd
Missing node definitions	Define all nodes first, then connections
Using # in comments	Use %% not # for comments

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When to Create Mermaid Diagrams

Always diagram when:

• Starting new projects/features
• Documenting complex systems
• Explaining architecture decisions
• Designing database schemas
• Onboarding new team members

Reserve for:

• Non-obvious flows
• Multi-service interactions
• Failure modes worth documenting
• Complex data relationships

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Now create your Mermaid diagram following these instructions. Start by choosing the right diagram type, define all nodes with descriptive identifiers, then add connections with clear labels. Use comments and subgraphs to improve readability.