C4 models allow us to visualize software architecture by decomposition in containers and components.

Viewpoints are organized in hierarchical levels:

  • Context Diagrams (Level 1)

  • Container Diagrams (Level 2)

  • Component Diagrams (Level 3)

  • Code Diagrams (Level 4)

C4-PlantUML offers a variety of macros and stereotypes that make modeling fun.

An example in PlantUML:

sample.puml
@startuml
!include <c4/C4_Context.puml>
!include <c4/C4_Container.puml>

left to right direction

Person(user, "User")

System_Ext(auth, "AuthService", "Provides authentication and authorization via OIDC")

System_Boundary(zone1, "Some system boundary") {


    System(lb, "Load Balancer")

    System_Boundary(az, "App Cluster") {
        System(app, "App Servers") {
            Container(app1, "App1", "Docker", "Does stuff")
            Container(app2, "App1", "Docker", "Does stuff")

            ContainerDb(dbSess, "Session DB", "Redis")
            ContainerDb(db1, "RBMS 1", "AWS RDS Postgres")
            ContainerDb(db2, "RBMS 2", "AWS RDS Postgres")

            ' both app servers sync sessions via redis
            Rel(app1, dbSess, "Uses", "Sync Session")
            Rel(app2, dbSess, "Uses", "Sync Session")

            ' both app servers persist data in RDBMS
            Rel(app1, db1, "Uses", "Persist/query relational data")
            Rel(app2, db2, "Uses", "Persist/query relational data")
        }
    }
}

Rel(user, lb, "call")
Rel(lb, app1, "delegate")
Rel(lb, app2, "delegate")
Rel(app1, auth, "Verify", "User auth")
Rel(app2, auth, "Verify", "User auth")

SHOW_FLOATING_LEGEND()

@enduml

This renders the following visualization:

hascode system context
Figure 1. Rendered C4 system context

You can use SHOW_LEGEND() or SHOW_FLOATING_LEGEND() to generate a legend for your diagram!

Ressources: