Creating C4 Diagrams

Creating C4 Diagrams

This skill is C4-specific: model architecture using C4 abstractions and notation (not generic box-and-arrow diagrams).

Workflow: Creating a C4 diagram

1. Identify the audience and scope. Determine which diagram level is needed:

   • Level 1 — System Context: How the system fits into the world. Always create this first.
   • Level 2 — Container: Major technical building blocks and communication. Always create this second.
   • Level 3 — Component: Internal structure of a single container. Create only when it adds value.
   • Dynamic: Runtime behavior for a specific use case. Create for complex or non-obvious flows.
   • Deployment: How containers map to infrastructure. Create for production systems.
   • Skip Level 4 (Code) — auto-generate from IDE instead.

2. Choose the tool.

   • Default: Structurizr DSL — model-first, single model with multiple views, supports all diagram types. Use for any long-lived architecture documentation.
   • Alternative: Mermaid — use only for quick diagrams in Markdown files (READMEs, PRs, wikis). Limited to Context and Container levels; no dynamic or deployment diagrams.

3. Identify the elements. Map the system to C4 abstractions:

   • Person: A human user role.
   • Software System: A top-level system owned and deployed by one team.
   • Container: A separately deployable/runnable unit (app, database, queue, function). NOT a Docker container.
   • Component: A module/package within a container. Not separately deployable.
   • Apply the test: "Does this need to be running for the system to work?" → Yes = Container. No = Component.

4. Define relationships. Every arrow must have:

   • A specific action verb label: "Submits purchase orders to", "Reads customer records from", "Publishes OrderCreated events to"
   • A technology annotation for inter-process communication: HTTPS/JSON, gRPC, SQL/TCP, AMQP
   • Unidirectional direction only — never use bidirectional arrows. Draw two separate arrows instead.
   • Use the pattern: [Action verb] + [what] + [preposition]
   • Avoid weak verbs: Uses, Calls, Connects, Talks, Accesses.

5. Write the diagram code. Follow these rules:

   • Every element gets: name, type label, technology (containers/components), one-sentence responsibility.
   • Every diagram gets: title describing type and scope, key/legend, max ~20 elements.
   • Tag external systems with "External". Use tags for styling, not per-element styles.
   • In Structurizr DSL: always use !identifiers hierarchical.
   • In Mermaid: use C4Context or C4Container as diagram type.

6. Validate against the review checklist. Check all items in references/08-checklists.md.

Workflow: Reviewing an existing C4 diagram

1. Check abstraction correctness:

   • Are containers actually separately deployable? (shared libraries are components, not containers)
   • Are system boundaries aligned with team ownership?
   • Is the message broker modeled as individual topics/queues, not a single hub?
   • Are external systems shown as opaque boxes without internal details?

2. Check notation quality:

   • Does every element have a name, type label, technology, and description?
   • Are all arrows unidirectional with specific action-verb labels?
   • Are communication protocols specified on inter-process arrows?
   • Is there a title, legend, and ≤20 elements?

3. Check for common anti-patterns: vague "Uses"/"Calls" labels, bidirectional arrows, missing type metadata, color as only differentiator, 30+ elements in one diagram, invented abstraction levels. See references/03-anti-patterns.md for the full catalog.

4. Propose specific fixes with corrected code.

Workflow: Using C4 diagrams as context

When C4 diagrams are available in the project (.dsl files or Mermaid blocks), use them as architectural context for:

• Design decisions: Reference the container diagram to understand system boundaries and communication patterns before proposing changes.
• Code generation: Use component diagrams to understand module responsibilities and interfaces when generating implementation code.
• Risk analysis: Walk the container diagram to identify security boundaries, data flows, and single points of failure. See references/05-adr-and-risk-modeling.md.
• Onboarding explanations: Start with Level 1 (context), then zoom into Level 2 (containers) to explain system architecture.
• ADR context: Link architectural decisions to specific C4 elements. Reference ADRs in element descriptions.

Structurizr DSL quick reference

workspace "Name" "Description" {
    !identifiers hierarchical

    model {
        user = person "User" "Description."
        system = softwareSystem "System" "Description." {
            webapp = container "Web App" "Description." "React"
            api = container "API" "Description." "Go"
            db = container "Database" "Description." "PostgreSQL" { tags "Database" }
        }
        ext = softwareSystem "External" "Description." { tags "External" }

        user -> system.webapp "Browses via" "HTTPS"
        system.webapp -> system.api "Makes API calls to" "HTTPS/JSON"
        system.api -> system.db "Reads from and writes to" "SQL/TCP"
        system.api -> ext "Sends requests to" "HTTPS/JSON"
    }

    views {
        systemContext system "Context" { include *; autoLayout }
        container system "Containers" { include *; autoLayout }
        styles {
            element "Person" { shape person }
            element "Database" { shape cylinder }
            element "External" { background #999999; color #ffffff }
        }
    }
}

Mermaid quick reference

C4Context
    title System Context Diagram for My System
    Person(user, "User", "Description.")
    System(system, "My System", "Description.")
    System_Ext(ext, "External System", "Description.")
    Rel(user, system, "Browses via", "HTTPS")
    Rel(system, ext, "Sends requests to", "HTTPS/JSON")

C4Container
    title Container Diagram for My System
    Person(user, "User", "Description.")
    Container_Boundary(system, "My System") {
        Container(webapp, "Web App", "React", "Serves the UI.")
        Container(api, "API", "Go", "Handles business logic.")
        ContainerDb(db, "Database", "PostgreSQL", "Stores data.")
    }
    Rel(user, webapp, "Browses via", "HTTPS")
    Rel(webapp, api, "Makes API calls to", "HTTPS/JSON")
    Rel(api, db, "Reads from and writes to", "SQL/TCP")

Key decisions

• Microservices owned by one team → model as containers within one software system.
• Microservices owned by separate teams → promote each to its own software system.
• Event-driven → model individual topics/queues as containers, not the broker.
• Serverless functions → model as containers (they are separately deployable).
• Component diagrams → create only for complex containers; skip for simple microservices.

Reference material

• Fundamentals and abstractions: references/01-fundamentals-and-abstractions.md
• Notation and styling: references/02-notation-and-styling.md — relationship label pattern library
• Anti-patterns: references/03-anti-patterns.md — common mistakes and fixes
• Tooling and diagram-as-code: references/04-tooling-and-diagram-as-code.md — Structurizr DSL, Mermaid, workspace modularization
• ADR and risk modeling: references/05-adr-and-risk-modeling.md — ADR integration, risk-storming, STRIDE
• Modern patterns and adoption: references/06-modern-patterns-and-adoption.md — microservices, event-driven, serverless, DDD, team playbook
• Worked example: references/07-worked-example.md — complete PageTurn bookstore from L1 through deployment
• Checklists: references/08-checklists.md — diagram review, abstraction guide, tool selection