/c4-architecture

name: c4-architecture
description: "Generate architecture documentation using C4 model Mermaid diagrams. Use when asked to create architecture diagrams, document system architecture, visualize software structure, or generate context/container/component/deployment diagrams."
domain: engineering
supports: [claude-code, cursor, codex, generic]
version: 0.1.0

C4 Architecture Documentation

Generate architecture documentation using C4 model Mermaid diagrams. Use when asked to create architecture diagrams, document system architecture, visualize software structure, create C4 diagrams, or generate context/container/component/deployment diagrams. Triggers include "architecture diagram", "C4 diagram", "system context", "container diagram", "component diagram", "deployment diagram", "document architecture", "visualize architecture".

Workflow

1. **Understand scope** — Determine which C4 level(s) are needed based on audience 2. **Analyze codebase** — Explore the system to identify components, containers, and relationships 3. **Generate diagrams** — Create Mermaid C4 diagrams at appropriate abstraction levels 4. **Document** — Write diagrams to markdown files with explanatory context

C4 Diagram Levels

Select the appropriate level based on the documentation need:

| Level | Diagram Type | Audience | Shows | When to Create | |-------|-------------|----------|-------|----------------| | 1 | **C4Context** | Everyone | System + external actors | Always (required) | | 2 | **C4Container** | Technical | Apps, databases, services | Always (required) | | 3 | **C4Component** | Developers | Internal components | Only if adds value | | 4 | **C4Deployment** | DevOps | Infrastructure nodes | For production systems | | - | **C4Dynamic** | Technical | Request flows (numbered) | For complex workflows |

**Key Insight:** "Context + Container diagrams are sufficient for most software development teams." Only create Component/Code diagrams when they genuinely add value.

Quick Start Examples

System Context (Level 1)

C4Context
  title System Context - Workout Tracker

  Person(user, "User", "Tracks workouts and exercises")
  System(app, "Workout Tracker", "Vue PWA for tracking strength and CrossFit workouts")
  System_Ext(browser, "Web Browser", "Stores data in IndexedDB")

  Rel(user, app, "Uses")
  Rel(app, browser, "Persists data to", "IndexedDB")

Container Diagram (Level 2)

C4Container
  title Container Diagram - Workout Tracker

  Person(user, "User", "Tracks workouts")

  Container_Boundary(app, "Workout Tracker PWA") {
    Container(spa, "SPA", "Vue 3, TypeScript", "Single-page application")
    Container(pinia, "State Management", "Pinia", "Manages application state")
    ContainerDb(indexeddb, "IndexedDB", "Dexie", "Local workout storage")
  }

  Rel(user, spa, "Uses")
  Rel(spa, pinia, "Reads/writes state")
  Rel(pinia, indexeddb, "Persists", "Dexie ORM")

Component Diagram (Level 3)

C4Component
  title Component Diagram - Workout Feature

  Container(views, "Views", "Vue Router pages")

  Container_Boundary(workout, "Workout Feature") {
    Component(useWorkout, "useWorkout", "Composable", "Workout execution state")
    Component(useTimer, "useTimer", "Composable", "Timer state machine")
    Component(workoutRepo, "WorkoutRepository", "Dexie", "Workout persistence")
  }

  Rel(views, useWorkout, "Uses")
  Rel(useWorkout, useTimer, "Controls")
  Rel(useWorkout, workoutRepo, "Saves to")

Dynamic Diagram (Request Flow)

C4Dynamic
  title Dynamic Diagram - User Sign In Flow

  ContainerDb(db, "Database", "PostgreSQL", "User credentials")
  Container(spa, "Single-Page App", "React", "Banking UI")

  Container_Boundary(api, "API Application") {
    Component(signIn, "Sign In Controller", "Express", "Auth endpoint")
    Component(security, "Security Service", "JWT", "Validates credentials")
  }

  Rel(spa, signIn, "1. Submit credentials", "JSON/HTTPS")
  Rel(signIn, security, "2. Validate")
  Rel(security, db, "3. Query user", "SQL")

  UpdateRelStyle(spa, signIn, $textColor="blue", $offsetY="-30")

Deployment Diagram

C4Deployment
  title Deployment Diagram - Production

  Deployment_Node(browser, "Customer Browser", "Chrome/Firefox") {
    Container(spa, "SPA", "React", "Web application")
  }

  Deployment_Node(aws, "AWS Cloud", "us-east-1") {
    Deployment_Node(ecs, "ECS Cluster", "Fargate") {
      Container(api, "API Service", "Node.js", "REST API")
    }
    Deployment_Node(rds, "RDS", "db.r5.large") {
      ContainerDb(db, "Database", "PostgreSQL", "Application data")
    }
  }

  Rel(spa, api, "API calls", "HTTPS")
  Rel(api, db, "Reads/writes", "JDBC")

Element Syntax

People and Systems

Person(alias, "Label", "Description")
Person_Ext(alias, "Label", "Description")       # External person
System(alias, "Label", "Description")
System_Ext(alias, "Label", "Description")       # External system
SystemDb(alias, "Label", "Description")         # Database system
SystemQueue(alias, "Label", "Description")      # Queue system

Containers

Container(alias, "Label", "Technology", "Description")
Container_Ext(alias, "Label", "Technology", "Description")
ContainerDb(alias, "Label", "Technology", "Description")
ContainerQueue(alias, "Label", "Technology", "Description")

Components

Component(alias, "Label", "Technology", "Description")
Component_Ext(alias, "Label", "Technology", "Description")
ComponentDb(alias, "Label", "Technology", "Description")

Boundaries

Enterprise_Boundary(alias, "Label") { ... }
System_Boundary(alias, "Label") { ... }
Container_Boundary(alias, "Label") { ... }
Boundary(alias, "Label", "type") { ... }

Relationships

Rel(from, to, "Label")
Rel(from, to, "Label", "Technology")
BiRel(from, to, "Label")                        # Bidirectional
Rel_U(from, to, "Label")                        # Upward
Rel_D(from, to, "Label")