C4 Model: Importance, Use Cases, and Examples

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What Is the C4 Model? 

The C4 model, created by Simon Brown, is an abbreviation for Context, Container, Component, and Code — the four levels of this model. It is a lean graphical notation technique designed to depict the architecture of software systems. The C4 model addresses the need for software architecture visualization at varying levels of detail, from high-level system context down to the level of code.

This model provides a way for software developers, architects, and stakeholders to comprehend a software system without getting lost in the details. It is specifically designed to express the architecture of software systems in a way that is easy to understand by all involved parties, thus bridging the communication gap between technical and non-technical stakeholders.

One of the key features of the C4 model is its simplicity. Unlike other complex architecture models, the C4 model is easy to understand and implement. It doesn't require specialist knowledge or training to understand or use it. Additionally, it does not rely on a specific technology or programming language, making it extremely versatile.

This is part of a series of articles about code visualization.

Importance of the C4 Model in Software Architecture 

Clear Communication

In software projects, misunderstandings and misinterpretations can lead to costly mistakes and project delays. The C4 model enhances communication between team members by providing a common language for describing the architecture of a system.

By using a clear, graphical notation, the C4 model allows for easy visualization of the system's architecture. This clarity promotes better understanding among team members and stakeholders. Moreover, it offers an effective way to onboard new team members, as they can quickly comprehend the system's architecture using the visualizations.

Improved Software Design

The C4 model contributes significantly to improving software design. By breaking down the architecture into four levels — context, container, component, and code — it helps to identify potential design issues early in the development process.

The detailed visualization offered by the C4 model allows developers to gain a comprehensive understanding of the system. This understanding facilitates the identification and resolution of design issues, ensuring a robust and efficient system.

Facilitates Documentation

Documentation is an essential aspect of software development. However, it can be a challenging task, especially for complex systems. The C4 model simplifies this task by providing a clear and concise framework for documentation.

With its four-level approach, the C4 model ensures that all aspects of the system's architecture are covered. It facilitates the creation of a comprehensive documentation that serves as a valuable resource for the development team and stakeholders.

Efficient System Maintenance and Evolution

As applications continuously adapt to new requirements and grow over time, their underlying architecture needs to remain both flexible and easily maintainable. Here, the C4 model demonstrates its significance.

With its layered representation, spanning from the broad Context to the intricate Code level, the C4 model offers a clear view of the intricate interconnections between components and containers. Such clarity is important when developers aim to introduce modifications or new features, allowing them to identify potential impacts or dependencies.

Use Cases and Examples of the C4 Model 

Quality Assurance and Testing

The C4 model can be useful for quality assurance (QA) and testing. It provides a clear understanding of the system's architecture, enabling QA teams to design comprehensive test plans and ensure all components are adequately tested.

The Context and Container levels can help identify the areas that interact with external systems, thus needing more rigorous testing. The Component level allows for the creation of detailed test plans for individual components, ensuring they function as intended.

Finally, the Code level provides the nitty-gritty details necessary for unit testing and code reviews.

Microservices Architecture Review

Microservices architecture is a design approach where a single application is developed as a suite of small services, each running its own process and communicating with lightweight mechanisms. The C4 model can greatly assist in understanding and documenting these architectures.

When reviewing a microservices architecture, the C4 model's layered approach comes into play. The Context level provides a high-level view of the system, showing how the individual microservices interact with each other and external systems. The Container level dives deeper, illustrating how each microservice is composed, which could include databases, web servers, or other infrastructure.

The Component level goes even further, detailing the internals of each microservice. This could include classes, interfaces, or data models. Finally, the Code level provides the implementation details, such as code snippets or method signatures. The C4 model effectively simplifies the complexity of microservices architecture, making it easier to review and understand.

Migration or Refactoring Projects

In migration or refactoring projects, the C4 model serves as an excellent tool for visualizing the current state of a system and planning the desired state. The Context and Container levels can assist in understanding the overall architecture and identifying areas that need improvement.

At the Component level, you can identify dependencies and potential bottlenecks that need to be addressed during refactoring. When migrating to a new technology stack, the Component level can help in planning the new architecture and ensuring all necessary components are included.

Finally, the Code level is useful for developers during the actual refactoring or migration process. It provides the detailed information needed to understand and modify the codebase. The C4 model, therefore, serves as a roadmap guiding the successful completion of migration or refactoring projects.

Onboarding New Team Members

The C4 model is an effective tool for onboarding new team members. Its layered approach allows newcomers to understand the system at their own pace, starting from a high-level overview and gradually delving into the details.

The Context level offers a bird's-eye view of the system, providing a general understanding of how different parts interact. This helps new team members grasp the overall system architecture and their role within it.

As they grow more comfortable, they can delve into the Container and Component levels, understanding the details of specific services or features. The Code level, finally, offers detailed insights into the codebase, preparing them to contribute to the project. By using the C4 model, the onboarding process becomes more structured and efficient, accelerating the time it takes for new team members to become productive.

Security Audits

The C4 model can help security teams understand the architecture of a system, identify potential vulnerabilities, and plan mitigation strategies.

The Context level presents a high-level view of the system, showing how different parts interact. This can help identify potential attack vectors, such as unsecured communication channels or external systems with weak security measures.

The Container and Component levels offer a more detailed view of the system, highlighting areas where sensitive data is stored or processed. This can help identify potential vulnerabilities, such as unprotected databases or components with outdated security measures.

Finally, the Code level provides a detailed view of the implementation, enabling security teams to review the code for potential security flaws.

Learn more in our detailed guide to C4 model tools.

The 4 Levels of C4 Model Architecture 

Let’s review the four levels of the C4 model.

Context Diagram

The Context diagram provides a bird's eye view of the system, showing how it interacts with its external entities, including users and other systems. The context diagram is meant to be simple, focusing on the overall shape and boundaries of the system rather than its internal details.

To illustrate, in the case of a banking application, the context diagram would show the application as a single box interacting with users, such as bank employees and customers, and external systems like payment gateways. This diagram provides a general understanding of the system's purpose and scope, making it accessible even to non-technical stakeholders.

Container Diagram

The Container diagram breaks down the system into its major building blocks, or containers. A container in the C4 model is an executable unit that encapsulates a portion of the system's functionality. Examples of containers include web servers, databases, desktop applications, and mobile apps.

In our banking application example, the container diagram would elaborate on the system's internal structure. It might depict a web server for handling customer requests, a database for storing account information, and a background service for processing transactions. The container diagram provides a more detailed view of the system, making it useful for architects and senior developers.

Component Diagram

The Component diagram shows the internal structure of each container, breaking it down into its constituent components. Components in the C4 model are smaller units of software that perform specific roles within a container.

For instance, the component diagram of the banking application's web server might show components for user authentication, account management, and transaction processing. This level of detail is beneficial for developers, providing a clear understanding of the responsibilities and interactions of various parts of the system.

Code Diagram

The Code diagram is the most detailed, showing the implementation of individual components. The code diagram is typically represented by UML class diagrams or similar representations.

In the context of our example, the code diagram for the user authentication component might show classes for LoginController, UserValidator, and SessionManager. The code diagram is primarily of interest to developers, providing a detailed view of the system's codebase.

Learn more in our detailed guide to C4 diagram.

Creating C4 Model Diagrams with CodeSee

Creating C4 diagrams helps understand your software at multiple levels, and with CodeSee visualizing your codebase becomes simple. 

The core CodeSee codebase map is effectively a C4 diagram that encompasses the Code, Components, and Containers aspects of the C4 model. So to get started all you have to do is add your repository to CodeSee. CodeSee will then automatically generate a comprehensive codebase map:

This not only reveals the software’s design patterns but also each file and function within it. You can pinpoint every functional interaction, a feat tedious if done manually with other C4 diagramming tools. Because it is pulling data directly from the codebase, you can also add more information into this visualization:

  • Lines of code. How long each file is can tell you whether it is a candidate for refactoring.
  • Engineering hotspots. This is where your team is working the most, and can tell you both what are the most important pieces of your code, and also the most neglected
  • Creation data. When each file was created can tell you where the codebase is growing.

CodeSee also allows annotations and tours, beneficial for categorizing code segments and onboarding new team members, amplifying the context of C4 diagrams. This is fundamentally what C4 diagrams are about, disseminating information. 

Further context can be gleaned in CodeSee through service maps. Service maps portray how services within an application interrelate. Such maps, unlike codebase maps, focus on the holistic view of services, illustrating service segments, external dependencies, and databases in use. 

Integration with platforms like Datadog further enriches these diagrams, showing real-time data flow and system operations, offering stakeholders an in-depth understanding of system workings.

Learn more about codebase maps and service maps in our documentation, and get an immediate C4 diagram of your code by adding your repo to CodeSee.

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