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L2 Container Diagram Example

The L2 Container diagram zooms into a single system and reveals its internal applications, services, databases, and message buses — including the technology choices and communication protocols.

See Example C4 Diagrams for an overview of all diagram types and when to use each.

When to Use

  • Planning or reviewing a system's internal architecture
  • Communicating technology choices to technical teams
  • Identifying integration points and data flows between containers
  • Does not show code-level detail — that belongs in L3

Example: Order Management System Internal Structure

Preview Mermaid Code
C4Container title Order Management System - L2 Container Diagram Person(customer, "Customer", "Places orders via web or mobile") Boundary(order_system, "Order Management System") { Container(web_app, "Web Application", "React.js", "Displays orders and order history. Allows customers to place and track orders.") Container(mobile_app, "Mobile App", "React Native", "Allows customers to place orders on their phone") Container(admin_portal, "Admin Portal", "ASP.NET Core", "Internal tool for support agents to manage orders and refunds") Container(order_api, "Order API", "ASP.NET Core REST", "Provides order management capabilities to web, mobile, and admin applications") Container(fulfillment_svc, "Fulfillment Service", "Node.js", "Coordinates order fulfillment with warehouse ERP") ContainerDb(order_db, "Order Database", "SQL Server", "Stores customers, orders, and line items") ContainerDb(inventory_cache, "Inventory Cache", "Redis", "Caches current inventory levels") ContainerQueue(message_bus, "Message Bus", "Azure Service Bus", "Asynchronous event processing for order fulfillment") } System_Ext(payment_gateway, "Payment Gateway", "Processes payments") System_Ext(email_service, "Email Service", "Sends notifications") System_Ext(warehouse_erp, "Warehouse ERP", "Manages physical inventory") Rel(customer, web_app, "Uses", "HTTPS") Rel(customer, mobile_app, "Uses", "HTTPS") Rel(web_app, order_api, "Calls", "REST/JSON") Rel(mobile_app, order_api, "Calls", "REST/JSON") Rel(admin_portal, order_api, "Calls", "REST/JSON") Rel(order_api, order_db, "Reads/Writes", "SQL") Rel(order_api, inventory_cache, "Reads", "Redis Protocol") Rel(order_api, message_bus, "Publishes Order Placed event", "AMQP") Rel(fulfillment_svc, message_bus, "Subscribes to events", "AMQP") Rel(fulfillment_svc, warehouse_erp, "Sends fulfillment requests", "REST/JSON") Rel(order_api, payment_gateway, "Processes payment", "REST/JSON") Rel(order_api, email_service, "Sends notifications", "SMTP") 
C4Container
    title Order Management System - L2 Container Diagram

    Person(customer, "Customer", "Places orders via web or mobile")

    Boundary(order_system, "Order Management System") {
        Container(web_app, "Web Application", "React.js", "Displays orders and order history. Allows customers to place and track orders.")
        Container(mobile_app, "Mobile App", "React Native", "Allows customers to place orders on their phone")
        Container(admin_portal, "Admin Portal", "ASP.NET Core", "Internal tool for support agents to manage orders and refunds")

        Container(order_api, "Order API", "ASP.NET Core REST", "Provides order management capabilities to web, mobile, and admin applications")
        Container(fulfillment_svc, "Fulfillment Service", "Node.js", "Coordinates order fulfillment with warehouse ERP")

        ContainerDb(order_db, "Order Database", "SQL Server", "Stores customers, orders, and line items")
        ContainerDb(inventory_cache, "Inventory Cache", "Redis", "Caches current inventory levels")

        ContainerQueue(message_bus, "Message Bus", "Azure Service Bus", "Asynchronous event processing for order fulfillment")
    }

    System_Ext(payment_gateway, "Payment Gateway", "Processes payments")
    System_Ext(email_service, "Email Service", "Sends notifications")
    System_Ext(warehouse_erp, "Warehouse ERP", "Manages physical inventory")

    Rel(customer, web_app, "Uses", "HTTPS")
    Rel(customer, mobile_app, "Uses", "HTTPS")

    Rel(web_app, order_api, "Calls", "REST/JSON")
    Rel(mobile_app, order_api, "Calls", "REST/JSON")
    Rel(admin_portal, order_api, "Calls", "REST/JSON")

    Rel(order_api, order_db, "Reads/Writes", "SQL")
    Rel(order_api, inventory_cache, "Reads", "Redis Protocol")
    Rel(order_api, message_bus, "Publishes Order Placed event", "AMQP")

    Rel(fulfillment_svc, message_bus, "Subscribes to events", "AMQP")
    Rel(fulfillment_svc, warehouse_erp, "Sends fulfillment requests", "REST/JSON")

    Rel(order_api, payment_gateway, "Processes payment", "REST/JSON")
    Rel(order_api, email_service, "Sends notifications", "SMTP")

Test Data Setup

To recreate this diagram in Nodinite, create the following entities.

Domains (swimlanes):

  • Experience APIs
  • Process APIs
  • System APIs

Services in Experience APIs:

  • Web Application (C4ContainerType: WebApp, C4Technology: React.js)
  • Mobile App (C4ContainerType: MobileApp, C4Technology: React Native)
  • Admin Portal (C4ContainerType: WebApp, C4Technology: ASP.NET Core)

Services in Process APIs:

  • Order API (C4ContainerType: RestApi, C4Technology: ASP.NET Core REST)
  • Fulfillment Service (C4ContainerType: Service, C4Technology: Node.js)

Services in System APIs:

  • Order Database (C4ContainerType: Database, C4Technology: SQL Server)
  • Inventory Cache (C4ContainerType: Cache, C4Technology: Redis)
  • Message Bus (C4ContainerType: MessageBus, C4Technology: Azure Service Bus)

Next Step

Zoom further into the Order API with the L3: Component Diagram Example to see its internal modules.