Documenting Architecture with C4 Model

The C4 model is a framework for visualizing and documenting software architecture. It was developed by Simon Brown to address the need for clear, scalable, and easy-to-understand diagrams that explain complex systems. The model helps stakeholders—from developers to business leaders—better understand the structure, components, and interactions within a system. The primary goal of the C4 model is to make software architecture documentation more accessible, reducing the complexity often associated with traditional documentation practices.

The C4 model focuses on four hierarchical levels of abstraction that describe a system from different perspectives. These levels are:

1. Context Diagram (Level 1)

2. Container Diagram (Level 2)

3. Component Diagram (Level 3)

4. Code Diagram (Level 4)

Each of these diagrams provides a different level of detail and is designed to serve the needs of different audiences, including business executives, developers, and system administrators. In this article, we will explore each of the four levels of the C4 model and discuss how to effectively use them in documenting software architecture.

Level 1: Context Diagram

The Context Diagram is the highest-level diagram in the C4 model and provides a broad overview of the system. It identifies the system’s boundaries, its external entities, and the relationships between the system and those entities. External entities typically include users, other systems, and external services that the system interacts with.

Key elements of a Context Diagram:

• System: The focal point of the diagram, representing the system being documented.

• External entities: These are systems, users, or services that interact with the system being documented.

• Relationships: The arrows or lines represent the interactions between the system and its external entities, showing data flows or communication patterns.

The Context Diagram is ideal for high-level stakeholders who need to understand what the system does and how it fits into the larger ecosystem. It answers key questions like:

• Who are the primary users of the system?

• What other systems does the system communicate with?

• What are the major data flows between the system and external entities?

A typical example could be a e-commerce platform that communicates with external payment gateways, inventory systems, and user interfaces.

Level 2: Container Diagram

The Container Diagram zooms in on the system to illustrate its high-level architecture. It breaks the system down into major “containers” that encapsulate specific functionality or services. Containers can be applications, databases, or microservices. The diagram highlights how these containers interact and communicate with each other.

Key elements of a Container Diagram:

• Containers: These represent the key applications, services, or databases that make up the system. Each container typically represents a major runtime environment (e.g., web application, mobile app, database, microservice, etc.).

• Relationships: The interactions between containers are shown, including data flows, communication protocols, or API calls.

• Technology stack: The diagram can also include information about the technology stack used in each container, such as programming languages, frameworks, and databases.

The Container Diagram is a good fit for technical stakeholders, such as software architects, developers, and system administrators, as it provides a clearer picture of how the system is structured and how its major components interact. It answers questions like:

• What are the major components of the system?

• How do these components communicate with one another?

• What technologies are being used in each component?

For example, a banking system could consist of a web application container, a mobile app container, and a backend microservice container.

Level 3: Component Diagram

The Component Diagram drills down further into individual containers, focusing on the components that reside within each container. A container may consist of several components that work together to provide specific functionality. The Component Diagram outlines how these components interact with each other within the container.

Key elements of a Component Diagram:

• Components: These are the key parts or modules within a container, such as classes, services, or subsystems.

• Interfaces: Components often communicate via well-defined interfaces, such as APIs or shared data models.

• Relationships: The interactions between components are shown, usually with lines or arrows to represent data flow, method calls, or other forms of communication.

The Component Diagram is especially helpful for developers who need to understand the inner workings of a system or container. It is an essential part of technical documentation that describes the internal architecture of each major part of the system. It answers questions like:

• What components are part of each container?

• How do these components communicate and interact?

• What dependencies exist between the components?

For example, the web application container in an e-commerce platform might have components like user authentication, shopping cart, and product catalog, each with its own responsibilities.

Level 4: Code Diagram

The Code Diagram provides the most detailed view, showing how individual classes, methods, or code structures fit together. While this level is not always necessary for high-level system understanding, it can be crucial for developers who need to understand the technical details of how specific components are implemented.

Key elements of a Code Diagram:

• Classes and methods: This includes the specific classes, methods, or functions that implement the logic of a component.

• Relationships: This could show how classes depend on each other, inheritance relationships, or method calls.

• Technology-specific details: The diagram may also show the actual technology used to implement the classes or components.

Code diagrams are typically useful during the development phase or for teams that are actively working on the system’s codebase. They provide detailed insights into the implementation of the architecture and answer questions like:

• How are the components actually implemented?

• What methods, classes, or services are involved in each component?

• How does the code within a component interact with other parts of the system?

For instance, a payment gateway component in an e-commerce system might have code classes like PaymentProcessor, TransactionValidator, and CardInfoManager, each with specific responsibilities and methods.

Best Practices for Documenting Architecture Using C4

1. Consistency: Ensure that the diagrams follow a consistent style and notation throughout the levels. This makes it easier for the audience to understand and navigate the different diagrams.

2. Audience Awareness: Tailor the level of detail to your audience. High-level diagrams should be used for business stakeholders, while more technical diagrams are useful for developers.

3. Keep it Updated: Software architectures evolve over time, so make sure to keep the documentation up to date with any changes to the system.

4. Use of Tools: There are several tools available for creating C4 diagrams, such as Structurizr, Lucidchart, and PlantUML. These tools can help generate consistent and professional diagrams.

5. Keep Diagrams Simple: Avoid overcrowding the diagrams with unnecessary details. Focus on the most important aspects of the system to maintain clarity.

6. Iterate: Just like software, architecture documentation should be iteratively refined. Don’t try to capture everything at once—start with a simple diagram and add more details as the system evolves.

Conclusion

The C4 model offers a powerful and scalable approach for documenting software architecture. By breaking the system down into four levels of abstraction—Context, Containers, Components, and Code—you can provide clarity at every stage of the system’s lifecycle. This hierarchical approach allows you to tailor the documentation to different audiences, ensuring that both technical and non-technical stakeholders can easily understand the system’s structure and functionality. Whether you are designing a new system or documenting an existing one, the C4 model can be an invaluable tool for creating clear, organized, and maintainable architectural documentation.