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Component diagram

Following on from a Container Diagram, next you can zoom in and decompose each container further to identify the major structural building blocks and their interactions.

The Component diagram shows how a container is made up of a number of components, what each of those components are, their responsibilities and the technology/implementation details.

Example

As an example, a Component diagram for the Web Application of a simplified, fictional Internet Banking System might look something like this. In summary, it shows the components that make up the Web Application, and the relationships between them.

An example Component diagram

With Structurizr for .NET, you can create this diagram with code like the following:

Component homePageController = webApplication.AddComponent("Home Page Controller", "Serves up the home page.", "Spring MVC Controller");
Component signinController = webApplication.AddComponent("Sign In Controller", "Allows users to sign in to the Internet Banking System.", "Spring MVC Controller");
Component accountsSummaryController = webApplication.AddComponent("Accounts Summary Controller", "Provides customers with an summary of their bank accounts.", "Spring MVC Controller");
Component securityComponent = webApplication.AddComponent("Security Component", "Provides functionality related to signing in, changing passwords, etc.", "Spring Bean");
Component mainframeBankingSystemFacade = webApplication.AddComponent("Mainframe Banking System Facade", "A facade onto the mainframe banking system.", "Spring Bean");

webApplication.Components.Where(c => "Spring MVC Controller".Equals(c.Technology)).ToList().ForEach(c => customer.Uses(c, "Uses", "HTTPS"));
signinController.Uses(securityComponent, "Uses");
accountsSummaryController.Uses(mainframeBankingSystemFacade, "Uses");
securityComponent.Uses(database, "Reads from and writes to", "JDBC");
mainframeBankingSystemFacade.Uses(mainframeBankingSystem, "Uses", "XML/HTTPS");

ComponentView componentView = views.CreateComponentView(webApplication, "Components", "The components diagram for the Web Application");
componentView.AddAllContainers();
componentView.AddAllComponents();
componentView.Add(customer);
componentView.Add(mainframeBankingSystem);

See BigBankPlc.cs for the full code, and https://structurizr.com/share/36141#Components for the diagram.

Extracting components automatically

Please note that, in a real-world scenario, you would probably want to extract components automatically from a codebase with the "component finder", using static analysis and reflection techniques.