c4-architecture

Imported from zzafergok/algopit (.agent/skills/c4-architecture/SKILL.md). Install upstream with npx skills add zzafergok/algopit --skill c4-architecture. Copyright stays with the author.

C4 Architecture Documentation

Generate software architecture documentation using C4 model diagrams in Mermaid syntax.

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:

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")                        # Downward
Rel_L(from, to, "Label")                        # Leftward
Rel_R(from, to, "Label")                        # Rightward

Deployment Nodes

Deployment_Node(alias, "Label", "Type", "Description") { ... }
Node(alias, "Label", "Type", "Description") { ... }  # Shorthand

Styling and Layout

Layout Configuration

UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

• $c4ShapeInRow - Number of shapes per row (default: 4)
• $c4BoundaryInRow - Number of boundaries per row (default: 2)

Element Styling

UpdateElementStyle(alias, $fontColor="red", $bgColor="grey", $borderColor="red")

Relationship Styling

UpdateRelStyle(from, to, $textColor="blue", $lineColor="blue", $offsetX="5", $offsetY="-10")

Use $offsetX and $offsetY to fix overlapping relationship labels.

Best Practices

Essential Rules

1. Every element must have: Name, Type, Technology (where applicable), and Description
2. Use unidirectional arrows only - Bidirectional arrows create ambiguity
3. Label arrows with action verbs - "Sends email using", "Reads from", not just "uses"
4. Include technology labels - "JSON/HTTPS", "JDBC", "gRPC"
5. Stay under 20 elements per diagram - Split complex systems into multiple diagrams

Clarity Guidelines

1. Start at Level 1 - Context diagrams help frame the system scope
2. One diagram per file - Keep diagrams focused on a single abstraction level
3. Meaningful aliases - Use descriptive aliases (e.g., orderService not s1)
4. Concise descriptions - Keep descriptions under 50 characters when possible
5. Always include a title - "System Context diagram for [System Name]"

What to Avoid

• Confusing containers (deployable) vs components (non-deployable)
• Modeling shared libraries as containers
• Showing message brokers as single containers instead of individual topics
• Adding undefined abstraction levels like "subcomponents"
• Removing type labels to "simplify" diagrams

Microservices Guidelines

Single Team Ownership

Model each microservice as a container (or container group):

C4Container
  title Microservices - Single Team


  System_Boundary(platform, "E-commerce Platform") {
    Container(orderApi, "Order Service", "Spring Boot", "Order processing")
    ContainerDb(orderDb, "Order DB", "PostgreSQL", "Order data")
    Container(inventoryApi, "Inventory Service", "Node.js", "Stock management")
    ContainerDb(inventoryDb, "Inventory DB", "MongoDB", "Stock data")
  }

Multi-Team Ownership

Promote microservices to software systems when owned by separate teams:

C4Context
  title Microservices - Multi-Team


  Person(customer, "Customer", "Places orders")
  System(orderSystem, "Order System", "Team Alpha")
  System(inventorySystem, "Inventory System", "Team Beta")
  System(paymentSystem, "Payment System", "Team Gamma")


  Rel(customer, orderSystem, "Places orders")
  Rel(orderSystem, inventorySystem, "Checks stock")
  Rel(orderSystem, paymentSystem, "Processes payment")

Event-Driven Architecture

Show individual topics/queues as containers, NOT a single "Kafka" box:

C4Container
  title Event-Driven Architecture


  Container(orderService, "Order Service", "Java", "Creates orders")
  Container(stockService, "Stock Service", "Java", "Manages inventory")
  ContainerQueue(orderTopic, "order.created", "Kafka", "Order events")
  ContainerQueue(stockTopic, "stock.reserved", "Kafka", "Stock events")


  Rel(orderService, orderTopic, "Publishes to")
  Rel(stockService, orderTopic, "Subscribes to")
  Rel(stockService, stockTopic, "Publishes to")
  Rel(orderService, stockTopic, "Subscribes to")

Output Location

Write architecture documentation to docs/architecture/ with naming convention:

• c4-context.md - System context diagram
• c4-containers.md - Container diagram
• c4-components-{feature}.md - Component diagrams per feature
• c4-deployment.md - Deployment diagram
• c4-dynamic-{flow}.md - Dynamic diagrams for specific flows

Audience-Appropriate Detail