When designing complex software systems, clarity and communication are key. The C4 Model, created by Simon Brown, provides a simple yet powerful way to visualize and communicate software architecture. It organizes system design into four levels of abstraction, making it accessible to both technical and non-technical stakeholders.

What is the C4 Model?

The C4 Model breaks down software architecture into four hierarchical levels, each offering a different perspective of the system:

  1. Context
  2. Container
  3. Component
  4. Code (Class)
Credit to the C4 Model website by Simon Brown

Each level answers a specific question about the system, progressively diving deeper into the technical details. Let’s explore these levels.

1. Context Diagram

  • Purpose: Provides a bird’s-eye view of the system and how it interacts with external users and systems.
  • Audience: Non-technical stakeholders, business leaders, and team members.
  • Key Question: What is this system, and who/what does it interact with?

Example:


For an Online Bookstore, the context diagram would show:

  • The bookstore system.
  • Interactions with customers, payment gateways, shipping partners, etc.
[Customer] ---> [Online Bookstore System] ---> [Payment Gateway]

Tools: Tools like Lucidchart, Draw.io, or Diagrams.net are great for drawing these diagrams.

Credit to the C4 Model website by Simon Brown

2. Container Diagram

  • Purpose: Breaks the system into containers such as applications, databases, and services.
  • Audience: Architects and developers responsible for system design.
  • Key Question: What are the major pieces of this system, and how do they communicate?
Credit to the C4 Model website by Simon Brown

Example:


For the Online Bookstore, the containers might include:

  1. Frontend Web Application (handles customer interaction).
  2. Backend Service (handles business logic).
  3. Database (stores book inventory, orders, etc.).
[Web Browser] ---> [Frontend Web Application] ---> [Backend API Service] ---> [Database]

3. Component Diagram

  • Purpose: Explores the internals of a container, detailing its components and how they work together.
  • Audience: Developers and technical teams.
  • Key Question: What are the components inside a container, and how do they collaborate?
Credit to the C4 Model website by Simon Brown

Example:


For the Backend API Service, components might include:

  1. Authentication Service (manages user login).
  2. Order Management Component (handles order creation).
  3. Inventory Service (tracks book availability).
[Frontend Web Application] ---> [Order Management API] ---> [Inventory Service] ---> [Database]

4. Code (Class) Diagram

  • Purpose: Shows the detailed implementation of individual components, such as class structures and relationships.
  • Audience: Developers working on implementation.
  • Key Question: How is this component built at the code level?

Example:


For the Order Management Component, you might have:

  • Order Class: Represents an order entity.
  • OrderProcessor Class: Handles business logic for processing orders.
  • Payment Class: Manages payment transactions.
Order ---> OrderProcessor ---> Payment

Why Use the C4 Model?

  1. Simplifies Communication:
    • Stakeholders see the big picture without being overwhelmed by technical jargon.
    • Developers get clarity on system design and responsibilities.
  2. Adaptable to Audience:
    • High-level diagrams (Context/Container) for non-technical teams.
    • Low-level diagrams (Component/Code) for technical teams.
  3. Scalable Design:
    • Easily scale from a high-level view to intricate details.

How to Use the C4 Model Effectively

  1. Start with Context:
    • Map out how your system interacts with external entities.
    • Example: Show users, other systems, and primary interactions.
  2. Define Containers:
    • Identify the primary building blocks.
    • Example: Applications, microservices, and databases.
  3. Drill into Components:
    • Analyze the major parts of each container.
    • Example: APIs, services, and logical groupings.
  4. Focus on Code as Needed:
    • Use class diagrams only for complex components.
    • Example: Design class hierarchies, relationships, and methods.

Tools for C4 Modeling

  • Structurizr: Purpose-built for creating C4 diagrams.
  • PlantUML: Write diagram descriptions in code and render them.
  • Draw.io / Lucidchart: General-purpose diagramming tools.

Conclusion

The C4 Model is a structured and scalable way to document and communicate software architecture. Whether you’re discussing the system with executives or collaborating with developers, the C4 model helps bridge the gap between abstract ideas and concrete implementation.

By adopting this model, you can ensure everyone on your team—from business analysts to engineers—shares a unified understanding of your system.