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What is the C4 Model? ​

Overview ​

The C4 Model is a modern approach to software architecture diagrams, created by Simon Brown. It provides a hierarchical set of diagrams that help developers and stakeholders visualize a system from different levels of abstraction — from the big picture to the code.

Think of C4 as a map of your software:

  • Level 1 (Context) = World Map
  • Level 2 (Container) = Country Map
  • Level 3 (Component) = City Map
  • Level 4 (Code) = Street Map

Why the C4 Model Matters ​

  • Clarity: Avoids messy "big ball of boxes and arrows" diagrams.
  • Scalability: Works for startups, large enterprises, and microservices.
  • Bridges HLD & LLD: Higher levels (Context, Container) are used in HLD; lower levels (Component, Code) are used in LLD.
  • Communication: Helps both technical and non-technical stakeholders.

The Four Levels of C4 ​

1. Context Diagram (Level 1) ​

  • Shows the system as a whole and its external users/systems.
  • Answers: Who uses the system? What does it interact with?

Example:

[User] ---> [E-commerce System] ---> [Payment Gateway]

2. Container Diagram (Level 2) ​

  • Zooms into the system and shows major containers: applications, services, databases, APIs.
  • Answers: What are the main building blocks? How do they communicate?

Example:

  • Web App
  • Mobile App
  • API Server
  • Database
  • Cache

3. Component Diagram (Level 3) ​

  • Breaks down a container into internal components (e.g., modules, services, libraries).
  • Answers: How is a container structured internally?

Example (API Server split into):

  • Authentication Component
  • Order Management Component
  • Notification Component

4. Code Diagram (Level 4) ​

  • Optional. Shows the class-level detail for a single component.
  • Answers: What classes, methods, or packages exist?
  • Usually auto-generated from IDEs or UML tools.

Example: E-commerce Platform with C4 ​

  • Context: User interacts with E-commerce System, which talks to Payment Gateway and Email Service.
  • Container: System has a Web App, Mobile App, API Server, and Database.
  • Component: API Server has Auth, Order, and Payment components.
  • Code: Order component → OrderController, OrderService, OrderRepository.

C4 in HLD vs LLD ​

  • HLD: Use Context + Container diagrams.
  • LLD: Use Component + Code diagrams.

This makes C4 a unifying framework that bridges high-level architecture with low-level design.

Conclusion ​

The C4 Model gives software teams a structured way to visualize systems at different zoom levels. By mastering it, you can avoid vague architecture diagrams and instead communicate clearly with both engineers and stakeholders.