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What is a Component Diagram? ​

Overview ​

A Component Diagram is a type of UML (Unified Modeling Language) diagram that shows the modular structure of a system. It focuses on how different components (services, modules, APIs) interact with each other via interfaces and dependencies.

Think of it as a blueprint of system modules — while a Class Diagram goes deep into attributes and methods, a Component Diagram zooms out to show how modules are wired together.

Why Component Diagrams Matter ​

  • Modularity: Visualize system components and their responsibilities.
  • Communication: Show how different modules/services depend on one another.
  • Architecture View: Useful for both HLD (system architecture) and LLD (module design).
  • Interview Relevance: Often used to describe microservices, APIs, or modular monoliths.

Key Elements of Component Diagrams ​

  • Component: Represented as a rectangle with the keyword <<component>>.
  • Interface: Small circle (provided) or half-circle (required) showing contracts.
  • Dependency: Dashed arrow showing one component depends on another.

Example: E-Commerce Platform ​

Here’s how an e-commerce platform might be shown in a component diagram:

+-------------------+       +------------------+
| <<component>>     |       | <<component>>    |
|  User Service     |<----->|  Order Service   |
+-------------------+       +------------------+
         |                           |
         v                           v
+-------------------+       +------------------+
| <<component>>     |       | <<component>>    |
|  Auth Service     |       |  Payment Service |
+-------------------+       +------------------+
  • User Service communicates with Auth Service for login.
  • Order Service depends on Payment Service for transactions.
  • All are independent components that can be developed/tested separately.

When to Use Component Diagrams ​

  • System design interviews: To show how services interact.
  • Microservices architecture: Mapping service-to-service communication.
  • API design: Illustrating dependencies between modules.
  • Documentation: For teams to understand system structure quickly.

Conclusion ​

Component Diagrams provide a high-level view of modular structure, making them ideal for explaining system dependencies in both real-world systems and interviews. By mastering them, you’ll be able to communicate system architecture clearly and concisely.

Next Step: Explore Activity Diagrams to learn how to represent workflows inside these components.