C4 Level 2: Container Diagram & Infrastructure Mapping

The Container diagram represents the high-level technical architecture (web apps, mobile apps, databases, background jobs).

🎯 Stakeholder Focus

• Architects: High-level tech decisions and API boundaries.

• Developers: High-level system structure and cross-app communication.

• Ops/DevOps: Deployment strategy and infrastructure mapping.

🛠 Infrastructure Mapping

Level 2 should ideally map to actual infrastructure components:

• Web App/Mobile App: Maps to a build artifact or deployment.

• API/Service: Maps to a Docker container or K8s deployment.

• Database: Maps to a managed cloud DB or persistent container.

• Message Broker: Maps to Kafka, RabbitMQ, or Pub/Sub.

🚫 Anti-Patterns to Guard (Level 2)

• NOT A FLOWCHART: Avoid modeling complex business logic; use a sequence diagram for that.

• NO LIBRARIES: Shared libraries (DLL, JAR, NuGet) are NOT containers.

• READABILITY: If the system has >10 containers, consider multiple Level 2 views (e.g., "Customer View", "Admin View").

🔍 Codebase Scanning (L2 Synthesis)

To identify containers in an existing codebase, scan for:

• Build manifests: package.json , pom.xml , go.mod , requirements.txt .

• Docker files: Dockerfile , docker-compose.yml .

• Infrastructure: Terraform files, k8s/ manifests.

Mermaid Template (Enhanced C4Container)

C4Container title [System Name] - Container Diagram

Person(customer, "User", "Uses the system to browse products.")

System_Boundary(my_system, "System Name") {
    Container(web_app, "Web App", "React/TypeScript", "User interface for browsing products.")
    Container(api_svc, "API Service", "Go/gRPC", "Main business logic and API gateway.")
    ContainerDb(user_db, "User DB", "PostgreSQL", "Stores user profiles and history.")
    Container(worker, "Order Processor", "NodeJS/TS", "Processes background order tasks.")
}

System_Ext(bank, "Bank API", "REST API")

Rel(customer, web_app, "Uses", "HTTPS")
Rel(web_app, api_svc, "Calls API", "JSON/HTTPS")
Rel(api_svc, user_db, "Reads/Writes", "gRPC/TCP")
Rel(api_svc, worker, "Triggers processing", "Pub/Sub")
Rel(worker, bank, "Finalizes payment", "HTTPS/REST")

Level 2 Success Criteria

• Are all containers separately deployable units?

• Are all cross-container protocols (JSON, SQL, gRPC) specified?

• Is the diagram readable and clearly bounded?

• SMART: Do the containers match build/deployment artifacts in the code?