C4 Architecture Documentation

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

Workflow

• Understand scope - Determine which C4 level(s) are needed based on audience

• Analyze codebase - Explore the system to identify components, containers, and relationships

• Generate diagrams - Create Mermaid C4 diagrams at appropriate abstraction levels

• 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")                        # 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

• Every element must have: Name, Type, Technology (where applicable), and Description

• Use unidirectional arrows only - Bidirectional arrows create ambiguity

• Label arrows with action verbs - "Sends email using", "Reads from", not just "uses"

• Include technology labels - "JSON/HTTPS", "JDBC", "gRPC"

• Stay under 20 elements per diagram - Split complex systems into multiple diagrams

Clarity Guidelines

• Start at Level 1 - Context diagrams help frame the system scope

• One diagram per file - Keep diagrams focused on a single abstraction level

• Meaningful aliases - Use descriptive aliases (e.g., orderService not s1)

• Concise descriptions - Keep descriptions under 50 characters when possible

• Always include a title - "System Context diagram for [System Name]"

What to Avoid

See references/common-mistakes.md for detailed anti-patterns:

• 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

Audience

Recommended Diagrams

Executives

System Context only

Product Managers

Context + Container

Architects

Context + Container + key Components

Developers

All levels as needed

DevOps

Container + Deployment

References

• references/c4-syntax.md - Complete Mermaid C4 syntax

• references/common-mistakes.md - Anti-patterns to avoid

• references/advanced-patterns.md - Microservices, event-driven, deployment