C4 Model Skill

Purpose

This skill provides guidance on creating C4 (Context, Container, Component, Code) model diagrams using Mermaid syntax for architecture documentation. C4 models provide a hierarchical way to visualize software architecture at different levels of abstraction, following Simon Brown's C4 model methodology.

Rules

C4 Model Levels

The C4 model consists of four levels of abstraction:

Level 1: System Context - Shows how the system fits into its environment
Level 2: Container - Shows the high-level technology choices and communication between containers
Level 3: Component - Shows how containers are decomposed into components
Level 4: Code - Shows implementation details (optional, often better shown in IDEs)

When to Use Each Level

System Context Diagram - MUST USE:

When documenting a new system
To show system boundaries
To identify users and external dependencies
In architecture decision records
In system documentation home page

Container Diagram - MUST USE:

For all systems with multiple deployable units
To show technology stack
To document inter-container communication
For security architecture (trust boundaries)
In deployment documentation

Component Diagram - SHOULD USE:

For complex containers with many components
When explaining internal structure
For onboarding new developers
For technical debt analysis

Code Diagram - RARELY USE:

Only for explaining specific complex patterns
Better suited for IDE visualization
Consider UML class diagrams instead

Documentation Requirements

MUST:

Create both CURRENT and FUTURE state diagrams (ARCHITECTURE.md and FUTURE_ARCHITECTURE.md)
Include title, description, and key for each diagram
Use consistent notation across diagrams
Document relationships between elements
Include technologies used
Version diagrams when architecture changes
Keep diagrams up-to-date with code changes

MUST NOT:

Create overly detailed diagrams (focus on key elements)
Mix different abstraction levels in same diagram
Omit technology choices from Container diagrams
Use ambiguous relationship labels

Mermaid Syntax for C4 Diagrams

Mermaid supports C4 diagrams through the C4Context, C4Container, and C4Component directives.

Examples

Example 1: System Context Diagram

C4Context
    title System Context Diagram for Hack23 CIA Platform

    Person(user, "Political Analyst", "A user analyzing political data")
    Person(admin, "Administrator", "Manages the system")
    
    System(cia, "Citizen Intelligence Agency", "Provides political data analysis and visualization")
    
    System_Ext(riksdagen, "Riksdagen API", "Swedish Parliament open data")
    System_Ext(worldbank, "World Bank API", "Global economic indicators")
    System_Ext(github, "GitHub API", "Politician activity tracking")
    System_Ext(auth, "OAuth Provider", "Authentication service")
    
    Rel(user, cia, "Uses", "HTTPS")
    Rel(admin, cia, "Administers", "HTTPS")
    Rel(cia, riksdagen, "Fetches data from", "HTTPS/REST")
    Rel(cia, worldbank, "Fetches data from", "HTTPS/REST")
    Rel(cia, github, "Tracks activity via", "HTTPS/REST")
    Rel(cia, auth, "Authenticates via", "OAuth 2.0")
    
    UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

Example 2: Container Diagram

C4Container
    title Container Diagram for Hack23 CIA Platform

    Person(user, "Political Analyst", "Analyzes political data")
    
    System_Boundary(cia, "Citizen Intelligence Agency") {
        Container(web, "Web Application", "React, JavaScript", "Provides political analysis UI")
        Container(api, "API Application", "Spring Boot, Java", "Provides data via REST API")
        Container(worker, "Background Worker", "Java", "Processes data updates")
        ContainerDb(db, "Database", "PostgreSQL", "Stores political data, analytics")
        ContainerDb(cache, "Cache", "Redis", "Caches frequently accessed data")
        Container(search, "Search Engine", "Elasticsearch", "Full-text search")
    }
    
    System_Ext(riksdagen, "Riksdagen API", "Parliament data")
    System_Ext(cdn, "CloudFront CDN", "Static content delivery")
    System_Ext(auth, "Auth0", "Authentication")
    
    Rel(user, web, "Uses", "HTTPS")
    Rel(user, cdn, "Loads static assets", "HTTPS")
    Rel(web, api, "Makes API calls", "HTTPS/REST")
    Rel(web, auth, "Authenticates", "OAuth 2.0")
    Rel(api, db, "Reads/Writes", "JDBC/SSL")
    Rel(api, cache, "Reads/Writes", "Redis Protocol")
    Rel(api, search, "Queries", "HTTPS/REST")
    Rel(worker, riksdagen, "Fetches data", "HTTPS/REST")
    Rel(worker, db, "Writes", "JDBC/SSL")
    
    UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

Example 3: Component Diagram

C4Component
    title Component Diagram for CIA API Application

    Container(web, "Web Application", "React", "UI for political analysis")
    Container(worker, "Background Worker", "Java", "Data processing")
    ContainerDb(db, "Database", "PostgreSQL", "Data storage")
    
    Container_Boundary(api, "API Application") {
        Component(auth, "Authentication Controller", "Spring Security", "Handles authentication/authorization")
        Component(politician, "Politician Controller", "Spring MVC", "Manages politician data")
        Component(vote, "Vote Controller", "Spring MVC", "Manages voting records")
        Component(analytics, "Analytics Controller", "Spring MVC", "Provides analytics data")
        
        Component(politicianService, "Politician Service", "Spring Bean", "Business logic for politicians")
        Component(voteService, "Vote Service", "Spring Bean", "Business logic for votes")
        Component(analyticsService, "Analytics Service", "Spring Bean", "Calculates analytics")
        
        Component(dataAccess, "Data Access Layer", "Spring Data JPA", "Database access")
        Component(cacheLayer, "Cache Layer", "Spring Cache", "Caching abstraction")
    }
    
    Rel(web, auth, "Authenticates", "HTTPS/REST")
    Rel(web, politician, "Gets politician data", "HTTPS/REST")
    Rel(web, vote, "Gets voting records", "HTTPS/REST")
    Rel(web, analytics, "Gets analytics", "HTTPS/REST")
    
    Rel(politician, politicianService, "Uses")
    Rel(vote, voteService, "Uses")
    Rel(analytics, analyticsService, "Uses")
    
    Rel(politicianService, dataAccess, "Uses")
    Rel(voteService, dataAccess, "Uses")
    Rel(analyticsService, dataAccess, "Uses")
    
    Rel(politicianService, cacheLayer, "Uses")
    Rel(voteService, cacheLayer, "Uses")
    
    Rel(dataAccess, db, "Reads/Writes", "JDBC")
    Rel(worker, dataAccess, "Writes data")
    
    UpdateLayoutConfig($c4ShapeInRow="4", $c4BoundaryInRow="1")

Example 4: Simple Homepage Container Diagram

C4Container
    title Container Diagram for Hack23 Homepage

    Person(visitor, "Visitor", "Someone browsing the website")
    
    System_Boundary(homepage, "Hack23 Homepage") {
        Container(web, "Static Website", "HTML/CSS/JavaScript", "Corporate website")
    }
    
    System_Ext(s3, "AWS S3", "Static file storage")
    System_Ext(cloudfront, "AWS CloudFront", "CDN for global delivery")
    System_Ext(github, "GitHub Actions", "CI/CD pipeline")
    
    Rel(visitor, cloudfront, "Visits", "HTTPS")
    Rel(cloudfront, s3, "Fetches content", "HTTPS")
    Rel(github, s3, "Deploys to", "AWS SDK")
    
    UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

Example 5: Security Architecture with Trust Boundaries

C4Container
    title Security Architecture for CIA Platform

    Person(publicUser, "Public User", "Unauthenticated visitor")
    Person(authUser, "Authenticated User", "Registered analyst")
    Person(admin, "Administrator", "System administrator")
    
    System_Boundary(dmz, "DMZ - Public Access") {
        Container(cdn, "CloudFront CDN", "AWS", "Static content delivery")
        Container(waf, "Web Application Firewall", "AWS WAF", "Threat protection")
        Container(web, "Web Application", "React/S3", "Public UI")
    }
    
    System_Boundary(app, "Application Tier - Authenticated") {
        Container(api, "API Gateway", "AWS API Gateway", "API management, rate limiting")
        Container(lambda, "API Functions", "AWS Lambda", "Business logic")
        Container(auth, "Authentication", "AWS Cognito", "User authentication")
    }
    
    System_Boundary(data, "Data Tier - Restricted") {
        ContainerDb(db, "Database", "RDS PostgreSQL", "Encrypted data storage")
        ContainerDb(cache, "Cache", "ElastiCache Redis", "Session and data cache")
    }
    
    Rel(publicUser, cdn, "HTTPS", "Public access")
    Rel(cdn, waf, "Filtered traffic", "HTTPS")
    Rel(waf, web, "Protected traffic", "HTTPS")
    
    Rel(authUser, api, "HTTPS + JWT", "Authenticated access")
    Rel(api, auth, "Validates token", "AWS IAM")
    Rel(api, lambda, "Invokes", "AWS IAM")
    
    Rel(admin, api, "HTTPS + JWT + MFA", "Admin access")
    
    Rel(lambda, db, "JDBC over SSL", "VPC private subnet")
    Rel(lambda, cache, "Redis over TLS", "VPC private subnet")
    
    UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

Example 6: Future State Architecture

C4Container
    title Future Container Diagram for Hack23 CIA Platform (2026)

    Person(user, "Analyst", "Political data analyst")
    
    System_Boundary(cia, "CIA Platform - Microservices Architecture") {
        Container(web, "Web Application", "Next.js/React", "Modern responsive UI")
        Container(mobile, "Mobile App", "React Native", "iOS/Android app")
        
        Container(gateway, "API Gateway", "Kong", "API management, auth")
        
        Container(politicianSvc, "Politician Service", "Go", "Politician data microservice")
        Container(voteSvc, "Vote Service", "Go", "Voting records microservice")
        Container(analyticsSvc, "Analytics Service", "Python", "ML-powered analytics")
        Container(notificationSvc, "Notification Service", "Node.js", "Real-time notifications")
        
        ContainerDb(politicianDb, "Politician DB", "PostgreSQL", "Politician data")
        ContainerDb(voteDb, "Vote DB", "PostgreSQL", "Voting records")
        ContainerDb(analyticsDb, "Analytics DB", "TimescaleDB", "Time-series analytics")
        
        Container(eventBus, "Event Bus", "Apache Kafka", "Event streaming")
        Container(cache, "Cache", "Redis Cluster", "Distributed cache")
        Container(search, "Search", "Elasticsearch", "Full-text search")
    }
    
    System_Ext(riksdagen, "Riksdagen API", "Parliament data")
    System_Ext(ml, "ML Platform", "TensorFlow Serving", "ML predictions")
    
    Rel(user, web, "Uses", "HTTPS")
    Rel(user, mobile, "Uses", "HTTPS")
    
    Rel(web, gateway, "API calls", "HTTPS/REST")
    Rel(mobile, gateway, "API calls", "HTTPS/REST")
    
    Rel(gateway, politicianSvc, "Routes", "gRPC")
    Rel(gateway, voteSvc, "Routes", "gRPC")
    Rel(gateway, analyticsSvc, "Routes", "gRPC")
    Rel(gateway, notificationSvc, "Routes", "WebSocket")
    
    Rel(politicianSvc, politicianDb, "Reads/Writes", "SQL")
    Rel(voteSvc, voteDb, "Reads/Writes", "SQL")
    Rel(analyticsSvc, analyticsDb, "Reads/Writes", "SQL")
    
    Rel(politicianSvc, eventBus, "Publishes events", "Kafka")
    Rel(voteSvc, eventBus, "Publishes events", "Kafka")
    Rel(analyticsSvc, eventBus, "Subscribes to events", "Kafka")
    Rel(notificationSvc, eventBus, "Subscribes to events", "Kafka")
    
    Rel(politicianSvc, cache, "Uses", "Redis")
    Rel(voteSvc, cache, "Uses", "Redis")
    Rel(analyticsSvc, ml, "Inference", "HTTPS/REST")
    
    UpdateLayoutConfig($c4ShapeInRow="4", $c4BoundaryInRow="1")

Best Practices

Naming Conventions

MUST:

Use clear, descriptive names for elements
Use consistent terminology across diagrams
Specify technology/framework in Container diagrams
Include protocols in relationship labels

Visual Clarity

SHOULD:

Limit elements per diagram (max 15-20)
Group related elements
Use consistent colors for element types
Arrange elements logically (left-to-right flow for data)

Documentation Structure

MUST:

Include diagram title
Add description explaining purpose
Provide key/legend if using custom notation
Document assumptions and constraints
Link to related diagrams

Recommended File Structure:

docs/
  architecture/
    ARCHITECTURE.md              # Current state C4 diagrams
    FUTURE_ARCHITECTURE.md       # Future state C4 diagrams
    decisions/                   # Architecture Decision Records
      ADR-001-microservices.md
      ADR-002-database-choice.md

Related ISMS Policies

Secure Development Policy - Architecture documentation requirements
Information Security Policy - Security architecture

Tool Support

Mermaid Live Editor

URL: https://mermaid.live/
Use for creating and testing diagrams

VS Code Extensions

Mermaid Preview
Markdown Preview Mermaid Support

Rendering in GitHub

GitHub natively renders Mermaid diagrams in Markdown

C4 Model References

Official C4 Model: https://c4model.com/
Mermaid C4 Syntax: https://mermaid.js.org/syntax/c4.html
Simon Brown's Architecture Resources: https://www.architectis.je/

c4-modeling

C4 Model Skill

Purpose

Rules

C4 Model Levels

When to Use Each Level

Documentation Requirements

Mermaid Syntax for C4 Diagrams

Examples

Example 1: System Context Diagram

Example 2: Container Diagram

Example 3: Component Diagram

Example 4: Simple Homepage Container Diagram

Example 5: Security Architecture with Trust Boundaries

Example 6: Future State Architecture

Best Practices

Naming Conventions

Visual Clarity

Documentation Structure

Related ISMS Policies

Related Documentation

Tool Support

Mermaid Live Editor

VS Code Extensions

Rendering in GitHub

C4 Model References