c4-container

C4 Container Level: Applications and Data Stores

Use this skill when

• Working on c4 container level: system deployment tasks or workflows
• Needing guidance, best practices, or checklists for c4 container level: system deployment

Do not use this skill when

• The task is unrelated to c4 container level: system deployment
• You need a different domain or tool outside this scope

Instructions

• Clarify goals, constraints, and required inputs.
• Apply relevant best practices and validate outcomes.
• Provide actionable steps and verification.

C4 containers are applications or data stores, not necessarily Docker containers or runtime nodes. Model physical instances/regions separately in a deployment diagram. Base each boundary/relationship on observed code/configuration and mark unknowns; templates below are placeholders, not discovered architecture.

Containers

[Container Name]

• Name: [Container name]
• Description: [Short description of container purpose and deployment]
• Type: [Web Application, API, Database, Message Queue, etc.]
• Technology: [Primary technologies: Node.js, Python, PostgreSQL, Redis, etc.]
• Deployment: [Docker, Kubernetes, Cloud Service, etc.]

Purpose

[Detailed description of what this container does and how it's deployed]

Components

This container deploys the following components:

• • Documentation: c4-component-name.md

Interfaces

[API/Interface Name]

• Protocol: [REST/GraphQL/gRPC/Events/etc.]
• Description: [What this interface provides]
• Specification: [Link to OpenAPI/Swagger/API Spec file]
• Endpoints:
  • GET /api/resource - [Description]
  • POST /api/resource - [Description]

Dependencies

#C4 containers are applications or data stores, not necessarily Docker containers or runtime nodes. Model physical instances/regions separately in a deployment diagram. Base each boundary/relationship on observed code/configuration and mark unknowns; templates below are placeholders, not discovered architecture.

Containers Used

• [Container Name]: [How it's used, communication protocol]

External Systems

• [External System]: [How it's used, integration type]

Infrastructure

• Deployment Config: [Link to Dockerfile, K8s manifest, etc.]
• Scaling: [Horizontal/vertical scaling strategy]
• Resources: [CPU, memory, storage requirements]

Container Diagram

Use proper Mermaid C4Container syntax:

C4Container
    title Container Diagram for [System Name]

    Person(user, "User", "Uses the system")
    System_Boundary(system, "System Name") {
        Container(webApp, "Web Application", "Spring Boot, Java", "Provides web interface")
        Container(api, "API Application", "Node.js, Express", "Provides REST API")
        ContainerDb(database, "Database", "PostgreSQL", "Stores data")
        ContainerQueue(messageQueue, "Message Queue", "RabbitMQ", "Handles async messaging")
    }
    System_Ext(external, "External System", "Third-party service")

    Rel(user, webApp, "Uses", "HTTPS")
    Rel(webApp, api, "Makes API calls to", "JSON/HTTPS")
    Rel(api, database, "Reads from and writes to", "SQL")
    Rel(api, messageQueue, "Publishes messages to")
    Rel(api, external, "Uses", "API")

Key Principles (from c4model.com):

• Show high-level technology choices (this is where technology details belong)
• Show how responsibilities are distributed across containers
• Include container types: Applications, Databases, Message Queues, File Systems, etc.
• Show communication protocols between containers
• Include external systems that containers interact with

API Specification Template

For each container API, create an OpenAPI/Swagger specification:

openapi: 3.1.0
info:
  title: [Container Name] API
  description: [API description]
  version: 1.0.0
servers:
  - url: https://api.example.com
    description: Production server
paths:
  /api/resource:
    get:
      summary: [Operation summary]
      description: [Operation description]
      parameters:
        - name: param1
          in: query
          schema:
            type: string
      responses:
        '200':
          description: [Response description]
          content:
            application/json:
              schema:
                type: object

Example Interactions

• "Synthesize all components into containers based on deployment definitions"
• "Map the API components to containers and document their APIs as OpenAPI specs"
• "Create container-level documentation for the microservices architecture"
• "Document container interfaces as Swagger/OpenAPI specifications"
• "Analyze Kubernetes manifests and create container documentation"

Key Distinctions

• vs C4-Component agent: Maps components to deployment units; Component agent focuses on logical grouping
• vs C4-Context agent: Provides container-level detail; Context agent creates high-level system diagrams
• vs C4-Code agent: Focuses on deployment architecture; Code agent documents individual code elements

Output Examples

When synthesizing containers, provide:

• Clear container boundaries with deployment rationale
• Descriptive container names and deployment characteristics
• Complete API documentation with OpenAPI/Swagger specifications
• Links to all contained components
• Mermaid container diagrams showing deployment architecture
• Links to deployment configurations (Dockerfiles, K8s manifests, etc.)
• Infrastructure requirements and scaling considerations
• Consistent documentation format across all containers

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.