# Mermaid Diagram Guide

## Overview

This guide helps you choose the right Mermaid diagram type for your documentation and provides examples for common use cases.

## Quick Reference

| Diagram Type | Use For | Complexity |
|----------------------|------------------------|---------------------------|
| **Flowchart** | Workflows, decision trees | |
| **Sequence Diagram** | API interactions, request flows | |
| **Class Diagram** | Code structure, patterns | |
| **Graph** | System architecture, dependencies | |
| **ER Diagram** | Database schema | |
| **Gantt** | Timeline, project schedule | |
| **C4 Diagram** | System context, containers | |

---

## 1. Flowcharts

### When to Use

Use flowcharts for:
- Step-by-step guides and workflows (e.g., **"Onboarding process"**)
- Decision trees and conditional logic (e.g., **"Discount calculation"**)
- Process flows with multiple branches (e.g., **"Order fulfillment"**)
- Troubleshooting procedures (e.g., **"Login issue diagnosis"**)

### Basic Flowchart

```mermaid
flowchart TD
 Start([Start]) --> Input[Get Input]
 Input --> Check{Valid?}
 Check -->|Yes| Process[Process Data]
 Check -->|No| Error[Show Error]
 Process --> Output[Return Result]
 Output --> End([End])
 Error --> End

 style Start fill:#2C3E50,stroke:#34495E,stroke-width:2px,color:#ECF0F1
 style End fill:#27AE60,stroke:#229954,stroke-width:2px,color:#ECF0F1
 style Check fill:#F39C12,stroke:#D68910,stroke-width:2px,color:#ECF0F1
 style Error fill:#C0392B,stroke:#A93226,stroke-width:2px,color:#ECF0F1
 style Process fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style Input fill:#34495E,stroke:#2C3E50,stroke-width:2px,color:#ECF0F1
 style Output fill:#34495E,stroke:#2C3E50,stroke-width:2px,color:#ECF0F1
```

**Explanation**: A basic flowchart starting with an input, followed by a validation check. If valid, it proceeds to processing and returns a result; otherwise, it shows an error and ends.

**Code**:
````markdown
```mermaid
flowchart TD
 Start([Start]) --> Input[Get Input]
 Input --> Check{Valid?}
 Check -->|Yes| Process[Process Data]
 Check -->|No| Error[Show Error]
 Process --> Output[Return Result]
 Output --> End([End])
 Error --> End

 style Start fill:#2C3E50,stroke:#34495E,stroke-width:2px,color:#ECF0F1
 style End fill:#27AE60,stroke:#229954,stroke-width:2px,color:#ECF0F1
 style Check fill:#F39C12,stroke:#D68910,stroke-width:2px,color:#ECF0F1
 style Error fill:#C0392B,stroke:#A93226,stroke-width:2px,color:#ECF0F1
```
````

### Advanced Flowchart with Subgraphs

```mermaid
flowchart LR
 A[Client Request] --> B{Auth?}
 B -->|No| C[401 Unauthorized]
 B -->|Yes| D[Process]

 subgraph Processing["Request Processing"]
 D --> E[Validate Input]
 E --> F[Execute Logic]
 F --> G[Format Response]
 end

 G --> H[Return 200 OK]
 C --> I[End]
 H --> I

 style Processing fill:#34495E,stroke:#2C3E50,stroke-width:2px,color:#ECF0F1
 style A fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style B fill:#F39C12,stroke:#D68910,stroke-width:2px,color:#ECF0F1
 style C fill:#C0392B,stroke:#A93226,stroke-width:2px,color:#ECF0F1
 style D fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style E fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style F fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style G fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style H fill:#27AE60,stroke:#229954,stroke-width:2px,color:#ECF0F1
 style I fill:#2C3E50,stroke:#34495E,stroke-width:2px,color:#ECF0F1
```

**Explanation**: A flowchart featuring an authorization check and a dedicated **Subgraph** for detailed request processing steps.

---

## 2. Sequence Diagrams

### When to Use

Use sequence diagrams for:
- API communication flows (e.g., **"New order creation API"**)
- Authentication/authorization flows (e.g., **"OAuth2 login flow"**)
- Service-to-service interactions (e.g., **"Microservices sync/async calls"**)
- Request/response cycles (e.g., **"Checkout process"**)

### Basic Sequence Diagram

```mermaid
sequenceDiagram
 participant Client
 participant API
 participant Service
 participant DB

 Client->>API: POST /login
 API->>Service: authenticate(credentials)
 Service->>DB: findUser(email)
 DB-->>Service: user
 Service->>Service: verifyPassword()
 Service-->>API: JWT token
 API-->>Client: 200 OK {token}
```

**Explanation**: A login sequence illustrating the interaction between the Client, API, Service Layer, and Database, including password verification.

**Code**:
````markdown
```mermaid
sequenceDiagram
 participant Client
 participant API
 participant Service
 participant DB

 Client->>API: POST /login
 API->>Service: authenticate(credentials)
 Service->>DB: findUser(email)
 DB-->>Service: user
 Service->>Service: verifyPassword()
 Service-->>API: JWT token
 API-->>Client: 200 OK {token}
```
````

### Advanced with Alt/Opt/Loop

```mermaid
sequenceDiagram
 participant Client
 participant API
 participant Cache
 participant DB

 Client->>API: GET /users/:id
 API->>Cache: get(key)

 alt Cache Hit
 Cache-->>API: cached data
 API-->>Client: 200 OK (from cache)
 else Cache Miss
 Cache-->>API: null
 API->>DB: SELECT * FROM users
 DB-->>API: user data
 API->>Cache: set(key, data, ttl)
 API-->>Client: 200 OK (from DB)
 end
```

**Explanation**: A data retrieval sequence using **Alt/Else** blocks to handle both Cache Hit and Cache Miss scenarios.

---

## 3. Class Diagrams

### When to Use

Use class diagrams for:
- Design patterns and code structure (e.g., **"Singleton Pattern for Logger"**)
- Object-oriented architecture (e.g., **"Domain-Driven Design (DDD) models"**)
- Inheritance and relationships (e.g., **"Repository base and concrete classes"**)
- Module dependencies (e.g., **"Service layer dependencies"**)

### Basic Class Diagram

```mermaid
classDiagram
 class BaseRepository {
 #prisma: PrismaClient
 #modelName: string
 +findById(id: string) T
 +findAll(options: QueryOptions) T[]
 +create(data: CreateDto) T
 +update(id: string, data: UpdateDto) T
 +delete(id: string) void
 }

 class UserRepository {
 +findByEmail(email: string) User
 +findByUsername(username: string) User
 }

 class FeatureRepository {
 +findByName(name: string) Feature
 +toggleStatus(id: string) Feature
 }

 BaseRepository <|-- UserRepository
 BaseRepository <|-- FeatureRepository
```

**Explanation**: A class diagram showing inheritance relationships between a generic `BaseRepository` and specific repository implementations.

---

## 4. Graph Diagrams

### When to Use

Use graph diagrams for:
- System architecture overview (e.g., **"Microservices Architecture"**)
- Component relationships (e.g., **"Service-to-database mapping"**)
- Data flow diagrams (e.g., **"Request processing pipeline"**)
- Dependency graphs (e.g., **"Package to package dependencies"**)

### System Architecture

```mermaid
graph TD
 Client[Web Client] --> Gateway[Traefik Gateway]
 Gateway --> Auth[Auth Service]
 Gateway --> IAM[IAM Service]
 Gateway --> User[User Service]

 Auth --> DB[(PostgreSQL)]
 IAM --> DB
 User --> DB

 User --> Cache[(Redis Cache)]

 style Client fill:#34495E,stroke:#2C3E50,stroke-width:2px,color:#ECF0F1
 style Gateway fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style Auth fill:#8E44AD,stroke:#7D3C98,stroke-width:2px,color:#ECF0F1
 style IAM fill:#8E44AD,stroke:#7D3C98,stroke-width:2px,color:#ECF0F1
 style User fill:#8E44AD,stroke:#7D3C98,stroke-width:2px,color:#ECF0F1
 style DB fill:#F39C12,stroke:#D68910,stroke-width:2px,color:#ECF0F1
 style Cache fill:#E67E22,stroke:#CA6F1E,stroke-width:2px,color:#ECF0F1
```

**Explanation**: A high-level view of the system architecture showing how the Gateway routes requests to different services, all connected to a shared Database and Cache.

### Data Flow

```mermaid
graph LR
 Input[User Input] --> Validation[Zod Validation]
 Validation --> Controller[Controller]
 Controller --> Service[Service Layer]
 Service --> Repository[Repository]
 Repository --> Prisma[Prisma ORM]
 Prisma --> DB[(Database)]

 Service --> Cache[(Redis Cache)]

 style Input fill:#34495E,stroke:#2C3E50,stroke-width:2px,color:#ECF0F1
 style Validation fill:#27AE60,stroke:#229954,stroke-width:2px,color:#ECF0F1
 style Controller fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style Service fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style Repository fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style Prisma fill:#8E44AD,stroke:#7D3C98,stroke-width:2px,color:#ECF0F1
 style DB fill:#F39C12,stroke:#D68910,stroke-width:2px,color:#ECF0F1
 style Cache fill:#E67E22,stroke:#CA6F1E,stroke-width:2px,color:#ECF0F1
```

**Explanation**: A detailed data flow showing Input going through Validation -> Controller -> Service -> Repository -> ORM -> DB, while also interacting with the Cache.

---

## 5. ER Diagrams

### When to Use

Use ER diagrams for:
- Database schema documentation (e.g., **"User and IAM tables"**)
- Entity relationships (e.g., **"One-to-many between User and Sessions"**)
- Data modeling (e.g., **"Designing new feature entities"**)
- Prisma schema visualization (e.g., **"Mapping code entities to DB tables"**)

### Database Schema

```mermaid
erDiagram
 User ||--o{ Session : has
 User ||--o{ RefreshToken : has
 User ||--o{ UserRole : has
 Role ||--o{ UserRole : has
 Role ||--o{ RolePermission : has
 Permission ||--o{ RolePermission : has

 User {
 string id PK
 string email UK
 string passwordHash
 boolean mfaEnabled
 datetime createdAt
 datetime updatedAt
 }

 Session {
 string id PK
 string userId FK
 string token UK
 string deviceId
 string ipAddress
 datetime expiresAt
 }

 Role {
 string id PK
 string name UK
 string description
 datetime createdAt
 }

 Permission {
 string id PK
 string code UK
 string resource
 string action
 string scope
 }
```

**Explanation**: An Entity-Relationship diagram illustrating a typical IAM schema with Users, Sessions, Roles, and Permissions.

---

## 6. Gantt Charts

### When to Use

Use Gantt charts for:
- Project timelines
- Implementation phases
- Migration schedules
- Deployment plans

### Project Timeline

```mermaid
gantt
 title Documentation Update Project Timeline
 dateFormat YYYY-MM-DD
 section Phase 1
 Analysis & Research :done, p1, 2024-01-01, 1d
 section Phase 2
 Templates & Strategy :active, p2, 2024-01-02, 0.5d
 section Phase 3
 High Priority Docs :p3, 2024-01-03, 2d
 section Phase 4
 Remaining Docs :p4, 2024-01-05, 3d
 section Phase 5
 QA & Verification :p5, 2024-01-08, 1d
```

---

## 7. C4 Diagrams

### When to Use

Use C4 diagrams for:
- System context (highest level)
- Container diagrams (services, databases)
- Component diagrams (modules within services)
- Code diagrams (classes, functions)

### System Context

```mermaid
C4Context
 title System Context Diagram for IAM System

 Person(user, "User", "System user needing authentication")
 Person(admin, "Admin", "System administrator")

 System(iam, "IAM System", "Identity and Access Management")

 System_Ext(email, "Email Service", "SendGrid/AWS SES")
 System_Ext(oauth, "OAuth Providers", "Google, Facebook, GitHub")

 Rel(user, iam, "Authenticates with", "HTTPS")
 Rel(admin, iam, "Manages users and permissions", "HTTPS")
 Rel(iam, email, "Sends emails", "SMTP/API")
 Rel(iam, oauth, "OAuth login", "OAuth 2.0")
```

---

## Color Palette Reference

### Dark Theme Color System

Our diagrams use a consistent dark color palette based on professional design principles:

```mermaid
graph LR
 subgraph Primary[" Primary Colors"]
 A1["Dark Blue<br/>#2980B9"]
 A2["Purple<br/>#8E44AD"]
 A3["Green<br/>#27AE60"]
 end

 subgraph Secondary[" Secondary Colors"]
 B1["Orange<br/>#E67E22"]
 B2["Yellow<br/>#F39C12"]
 B3["Red<br/>#C0392B"]
 end

 subgraph Neutrals[" Neutral Colors"]
 C1["Dark Gray<br/>#2C3E50"]
 C2["Medium Gray<br/>#34495E"]
 C3["Light Text<br/>#ECF0F1"]
 end

 style A1 fill:#2980B9,stroke:#1F618D,stroke-width:3px,color:#ECF0F1
 style A2 fill:#8E44AD,stroke:#7D3C98,stroke-width:3px,color:#ECF0F1
 style A3 fill:#27AE60,stroke:#229954,stroke-width:3px,color:#ECF0F1

 style B1 fill:#E67E22,stroke:#CA6F1E,stroke-width:3px,color:#ECF0F1
 style B2 fill:#F39C12,stroke:#D68910,stroke-width:3px,color:#ECF0F1
 style B3 fill:#C0392B,stroke:#A93226,stroke-width:3px,color:#ECF0F1

 style C1 fill:#2C3E50,stroke:#1C2833,stroke-width:3px,color:#ECF0F1
 style C2 fill:#34495E,stroke:#2C3E50,stroke-width:3px,color:#ECF0F1
 style C3 fill:#ECF0F1,stroke:#BDC3C7,stroke-width:3px,color:#2C3E50
```

### Color Usage Guidelines

| Color | Hex Code | Use For | Example |
|-------|----------|---------|---------|
| **Dark Blue** | `#2980B9` | Services, Processing, APIs | User Service, API Gateway |
| **Purple** | `#8E44AD` | Core Components, ORM | Prisma, IAM Service |
| **Green** | `#27AE60` | Success, Validation, End States | Success Flow, Validated Data |
| **Orange** | `#E67E22` | Cache, Warning States | Redis Cache, Warnings |
| **Yellow** | `#F39C12` | Database, Important Decisions | PostgreSQL, Decision Nodes |
| **Red** | `#C0392B` | Errors, Failures, Alerts | Error States, Failed Operations |
| **Dark Gray** | `#2C3E50` | Start/End, Neutral Elements | Start Node, End Node |
| **Medium Gray** | `#34495E` | Subgraphs, Background | Processing Groups, Containers |
| **Light Text** | `#ECF0F1` | Text Color (always use) | All node text |

### Standard Style Pattern

Always include these three properties for professional appearance:

```markdown
style NodeName fill:#HexColor,stroke:#DarkerShade,stroke-width:2px,color:#ECF0F1
```

**Components:**
- `fill`: Main background color
- `stroke`: Border color (typically darker shade of fill)
- `stroke-width`: Border thickness (use `2px` for standard, `3px` for emphasis)
- `color`: Text color (always use `#ECF0F1` for dark backgrounds)

### Node Type Color Patterns

| Node Type | Fill Color | Stroke Color | Use Case |
|-----------|------------|--------------|----------|
| **Services** | `#2980B9` | `#1F618D` | Auth Service, User Service |
| **API/Gateway** | `#2980B9` | `#1F618D` | Traefik, API Gateway |
| **Database** | `#F39C12` | `#D68910` | PostgreSQL, MySQL |
| **Cache** | `#E67E22` | `#CA6F1E` | Redis, Memcached |
| **ORM/Framework** | `#8E44AD` | `#7D3C98` | Prisma, TypeORM |
| **Success/End** | `#27AE60` | `#229954` | Success Flow, Completion |
| **Error/Failure** | `#C0392B` | `#A93226` | Error States, Failures |
| **Decision** | `#F39C12` | `#D68910` | Validation, Conditionals |
| **Start** | `#2C3E50` | `#1C2833` | Entry Point |
| **Process** | `#34495E` | `#2C3E50` | General Processing |

### Quick Color Selection Guide

**Choose colors based on node function:**

 **Blue (`#2980B9`)** → Services, APIs, Controllers
 **Purple (`#8E44AD`)** → Core Systems, ORM, Frameworks
 **Green (`#27AE60`)** → Success, Validation, Completion
 **Orange (`#E67E22`)** → Cache, Memory, Storage
 **Yellow (`#F39C12`)** → Databases, Decisions, Important Actions
 **Red (`#C0392B`)** → Errors, Failures, Critical Issues
 **Dark Gray (`#2C3E50`)** → Start/End, Neutral States
 **Medium Gray (`#34495E`)** → Containers, Subgraphs, Groups

---

## Best Practices

### Guidelines

1. **Keep it Simple**: Don't overcomplicate diagrams
2. **Use Consistent Styling**: Apply color scheme consistently
3. **Add Legends**: Explain symbols and colors when needed
4. **Limit Complexity**: Break into multiple diagrams if too complex
5. **Test Rendering**: Always test diagrams render correctly
6. **Dark Theme**: Always use dark color palette for professional appearance
7. **Text Contrast**: Always use `color:#ECF0F1` for text on dark backgrounds

---

## Common Pitfalls

### Too Complex

```mermaid
graph TD
 A --> B
 A --> C
 B --> D
 B --> E
 C --> F
 C --> G
 D --> H
 E --> H
 F --> I
 G --> I
 H --> J
 I --> J
```

### Simplified with Subgraphs

```mermaid
graph TD
 A[Start] --> B[Process A]
 B --> C[Process B]

 subgraph Detailed["Detailed Processing"]
 C --> D[Step 1]
 D --> E[Step 2]
 end

 E --> F[End]

 style Detailed fill:#34495E,stroke:#2C3E50,stroke-width:2px,color:#ECF0F1
 style A fill:#2C3E50,stroke:#1C2833,stroke-width:2px,color:#ECF0F1
 style F fill:#27AE60,stroke:#229954,stroke-width:2px,color:#ECF0F1
 style B fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style C fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style D fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
 style E fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1
```

---

## Testing Diagrams

Always test your diagrams before committing:

```bash
# Install mermaid-cli
npm install -g @mermaid-js/mermaid-cli

# Test render (SVG)
mmdc -i your-doc.md -o test-output.svg

# Render high-quality PNG with dark theme
mmdc -i your-doc.md -o test-output.png -b black -t dark -s 3

# Render ALL diagrams in a markdown file
mmdc -i your-doc.md

# Parameter Explanations:
# -i: Input file (.md or .mmd)
# -o: Output file (format based on extension .svg, .png, .pdf)
# -b: Background color (hex code or color names like black, white, transparent)
# -t: Theme (default, forest, dark, neutral)
# -s: Scale (increase resolution, e.g., 3 for sharper images)
```

---

## Quick Tips

### Mermaid Common Issues

**Issue**: Diagram not rendering
- Check for syntax errors (missing arrows, brackets)
- Ensure proper indentation in subgraphs
- Validate special characters are escaped

**Issue**: Colors not applying
- Place style commands after all node definitions
- Use exact node IDs in style statements
- Ensure hex colors include `#` prefix

**Issue**: Text overlapping
- Use shorter labels or `<br/>` for line breaks
- Adjust diagram direction (TD, LR, RL, BT)
- Increase spacing between nodes

### Color Pattern Quick Reference

```markdown
// Services & APIs
style Node fill:#2980B9,stroke:#1F618D,stroke-width:2px,color:#ECF0F1

// Database
style Node fill:#F39C12,stroke:#D68910,stroke-width:2px,color:#ECF0F1

// Cache
style Node fill:#E67E22,stroke:#CA6F1E,stroke-width:2px,color:#ECF0F1

// Success/End
style Node fill:#27AE60,stroke:#229954,stroke-width:2px,color:#ECF0F1

// Error/Failure
style Node fill:#C0392B,stroke:#A93226,stroke-width:2px,color:#ECF0F1

// Decision Points
style Node fill:#F39C12,stroke:#D68910,stroke-width:2px,color:#ECF0F1

// Start/Neutral
style Node fill:#2C3E50,stroke:#1C2833,stroke-width:2px,color:#ECF0F1
```

### Visual Indicators

Use emojis to enhance diagram readability:
- Services & APIs
- Core Systems
- Success Paths
- Databases
- Cache/Storage
- Errors
- Neutral/Start/End
- Warnings
- Validated
- Failed

---

## Resources

- [Mermaid Official Documentation](https://mermaid.js.org/) - Complete reference
- [Mermaid Live Editor](https://mermaid.live/) - Test diagrams online
- [Mermaid CheatSheet](https://jojozhuang.github.io/tutorial/mermaid-cheat-sheet/) - Quick reference

---

**Last Updated**: 2026-01-08