Diagram Knowledge Base

Quick reference for technical diagrams, Mermaid syntax, and C4 model.

Diagram Types Overview

Type	Use Case	When to Use
C4 Context	System boundaries	External actors, systems
C4 Container	Deployable units	Apps, databases, services
C4 Component	Internal structure	Classes, modules in container
Sequence	Interactions	Request flows, protocols
Class	Structure	Domain model, relationships
ER	Data	Database schema
Flowchart	Process	Algorithms, decisions
State	Lifecycle	Entity states, transitions

Mermaid Basics

Flowchart

flowchart TD
    A[Start] --> B{Decision}
    B -->|Yes| C[Action 1]
    B -->|No| D[Action 2]
    C --> E[End]
    D --> E

Syntax:

flowchart TD|TB|BT|LR|RL
    id[Rectangle]
    id(Rounded)
    id{Diamond}
    id([Stadium])
    id[[Subroutine]]
    id[(Database)]
    id((Circle))

Sequence Diagram

sequenceDiagram
    participant C as Client
    participant A as API
    participant D as Database

    C->>A: POST /users
    A->>D: INSERT user
    D-->>A: user_id
    A-->>C: 201 Created

Syntax:

->>   Solid arrow (sync)
-->>  Dashed arrow (async/response)
-)    Open arrow
--)   Dashed open arrow
Note right of A: Note text
loop Loop name
    actions
end
alt Condition
    actions
else Other
    actions
end

Class Diagram

classDiagram
    class Order {
        -OrderId id
        -OrderStatus status
        +confirm() void
        +cancel() void
    }
    class OrderItem {
        -ProductId productId
        -int quantity
    }
    Order "1" *-- "*" OrderItem : contains

Relationships:

<|-- Inheritance
*--  Composition
o--  Aggregation
-->  Association
--   Link
..>  Dependency
..|> Implementation

State Diagram

stateDiagram-v2
    [*] --> Pending
    Pending --> Confirmed : confirm()
    Pending --> Cancelled : cancel()
    Confirmed --> Shipped : ship()
    Confirmed --> Cancelled : cancel()
    Shipped --> Delivered : deliver()
    Delivered --> [*]
    Cancelled --> [*]

ER Diagram

erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ ORDER_ITEM : contains
    ORDER_ITEM }o--|| PRODUCT : references

    USER {
        uuid id PK
        string email UK
        string name
    }
    ORDER {
        uuid id PK
        uuid user_id FK
        string status
    }

Cardinality:

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

C4 Model

Level 1: Context Diagram

Shows system and external actors.

flowchart TB
    subgraph boundary[System Boundary]
        S[("🖥️ E-Commerce System")]
    end

    U[("👤 Customer")]
    PS[("💳 Payment Service")]
    ES[("📧 Email Service")]

    U -->|"Browse, Order"| S
    S -->|"Process payment"| PS
    S -->|"Send notifications"| ES

Level 2: Container Diagram

Shows deployable units.

flowchart TB
    subgraph boundary[E-Commerce System]
        WA[("🌐 Web App\nReact")]
        API[("⚙️ API\nPHP/Symfony")]
        DB[("🗄️ Database\nPostgreSQL")]
        CACHE[("💾 Cache\nRedis")]
        MQ[("📬 Queue\nRabbitMQ")]
    end

    WA -->|"REST/JSON"| API
    API -->|"SQL"| DB
    API -->|"Cache"| CACHE
    API -->|"Publish"| MQ

Level 3: Component Diagram

Shows internal structure.

flowchart TB
    subgraph api[API Container]
        direction TB
        subgraph presentation[Presentation]
            AC[Action]
            RS[Responder]
        end
        subgraph application[Application]
            UC[UseCase]
            SV[Service]
        end
        subgraph domain[Domain]
            EN[Entity]
            VO[ValueObject]
            RP[Repository Interface]
        end
        subgraph infra[Infrastructure]
            RI[Repository Impl]
            AD[Adapter]
        end
    end

    AC --> UC
    UC --> EN
    UC --> RP
    RI -.-> RP

Best Practices

Diagram Guidelines

Principle	Description	Example
7±2 Rule	Max 5-9 elements	Aggregate related items
Clear labels	Descriptive names	"User Service" not "S1"
Consistent style	Same shapes = same type	Rectangles for services
Flow direction	Top-down or left-right	Pick one per diagram
Context first	Start high-level	C4 Context → Container

Naming Conventions

✅ Good:
- "Payment Service" (descriptive)
- "PostgreSQL Database" (specific)
- "POST /orders" (action-based)

❌ Bad:
- "Service A" (meaningless)
- "DB" (ambiguous)
- "Process" (vague)

Layout Tips

# Top-down flow (recommended for hierarchies)
flowchart TD

# Left-right (recommended for timelines)
flowchart LR

# Subgraphs for grouping
subgraph name[Label]
    content
end

# Styling
style id fill:#f9f,stroke:#333
classDef className fill:#f9f
class id1,id2 className

Common Antipatterns

Antipattern	Problem	Fix
Spaghetti	Too many crossing lines	Reorder, use subgraphs
Kitchen sink	Everything in one diagram	Split by level/aspect
Mystery meat	Cryptic labels	Use full names
Outdated	Doesn't match code	Automate from code
No legend	Unknown symbols	Add legend/key
Invisible boundaries	Unclear scope	Add subgraphs

Tool Comparison

Tool	Type	Best For	Pros	Cons
Mermaid	Text-based	Documentation-as-code	Git-friendly, embeds in MD, live preview	Limited styling, complex layouts hard
PlantUML	Text-based	UML diagrams	Full UML support, more diagram types	Requires Java, slower rendering
Draw.io	GUI	Quick prototypes, business diagrams	Free, intuitive, many templates	Binary files, merge conflicts
Excalidraw	GUI	Sketches, whiteboarding	Hand-drawn style, collaborative	Less precise, limited exports
Lucidchart	GUI	Enterprise, presentations	Professional output, integrations	Paid, not text-based

Tool Selection Guide

Scenario	Recommended Tool
Code documentation (README, docs/)	Mermaid
Strict UML compliance required	PlantUML
Quick whiteboard session	Excalidraw
Stakeholder presentations	Draw.io or Lucidchart
CI/CD pipeline diagrams	Mermaid (auto-generate)
Living documentation (auto-update)	Mermaid + code generation

Tool Features Matrix

Feature	Mermaid	PlantUML	Draw.io	Excalidraw
Version control friendly	✅	✅	❌	⚠️ JSON
GitHub/GitLab rendering	✅	❌	❌	❌
No install required	✅	❌	✅	✅
Offline support	⚠️	✅	✅	✅
C4 model support	✅	✅	Manual	Manual
Export to PNG/SVG	✅	✅	✅	✅
Real-time collaboration	❌	❌	✅	✅

Diagram Selection Guide

What are you documenting?
│
├─ System overview → C4 Context
│
├─ Deployment units → C4 Container
│
├─ Internal structure → C4 Component / Class
│
├─ Data flow
│  ├─ Request/Response → Sequence
│  └─ Data processing → Flowchart
│
├─ Data structure
│  ├─ Domain model → Class
│  └─ Database → ER
│
└─ Behavior
   ├─ State machine → State
   └─ Algorithm → Flowchart

PHP-Specific Diagrams

DDD Layers

flowchart TB
    subgraph presentation[Presentation Layer]
        direction LR
        A[Action]
        R[Responder]
    end

    subgraph application[Application Layer]
        direction LR
        UC[UseCase]
        DTO[DTO]
    end

    subgraph domain[Domain Layer]
        direction LR
        E[Entity]
        VO[Value Object]
        DS[Domain Service]
        RI[Repository Interface]
    end

    subgraph infrastructure[Infrastructure Layer]
        direction LR
        RImpl[Repository]
        Adapter[Adapter]
    end

    presentation --> application
    application --> domain
    infrastructure -.-> domain

CQRS Pattern

flowchart LR
    subgraph commands[Write Side]
        CMD[Command] --> CH[CommandHandler]
        CH --> AR[Aggregate]
        AR --> EV[Event]
    end

    subgraph queries[Read Side]
        Q[Query] --> QH[QueryHandler]
        QH --> RM[ReadModel]
    end

    EV -.-> RM

References

For detailed information, load these reference files:

• references/mermaid-syntax.md — Complete Mermaid syntax reference
• references/c4-model.md — C4 model detailed guide
• references/sequence-patterns.md — Common sequence diagram patterns
• references/diagram-tools.md — Tools and automation