C4 Model - The Basics
Software Architecture is all about developing systems that scale and are maintainable. Clear visualizations will help teams to communicate the design effectively. The C4 model comes in handy for this task! But what is C4, and why should you care?
What is the C4 model?
The C4 model is a framework that enables the documentation and visualization of software architecture. It gives a structured means of creating diagrams showing how different software systems interact with each other. While some people are quite satisfied with other solutions such as UML or general-purpose tools like draw.io or Excalidraw, C4 can come across as simpler and more standardized, while keeping your diagrams from bloating and being over-engineered.
While there are no fixed rules on how to build your software architecture diagrams, C4 attempts to bring clarity and standardization into this process. It is versatile enough to cover a wide range of detail levels, from high-level overviews to more technical and in-depth diagrams.
Why Use C4?
The value of C4 is in its simplicity and flexibility. You can have very basic diagrams that give someone a general sense of your system architecture but then go into more detailed diagrams, such as deployment models.
If you opt-out by using the diagram-as-code path, you can easily have this source controlled, which means that you see our architecture growing and evolving as time goes by.
4 Levels of the C4 Model
The C4 Model explains the architecture of a system from four levels of abstraction: context, container, component, and code. Each of them depicts the same system architecture in greater detail than the previous one.
Let's dive into each level and what it tries to tell.
Context Diagram
The Context Diagram provides a very high-level overview of the software system and its relationship to external dependencies. This diagram is about showing where your software would sit within the larger landscape, and it will highlight the external systems it interfaces with, such as external APIs, databases, or third-party services like Stripe or an e-mail service.
Purpose: High-level understanding of how the system interacts with the outside world.
Audience: Both technical and nontechnical stakeholders.
Important Details: Would a non-technical person get it instantly from this diagram? If not try to further simplify it.
Here’s an example of a context diagram:
Container Diagram
The Container Diagram zooms in one level further from the overall system into the different containers making up your system. Containers are essentially executables: for example, backend services, front-end applications, or databases. This level helps tear down how various containers are interconnected, showing what their individual responsibilities will be.
Purpose: Understand the building blocks of your system.
Audience: More technical stakeholders who require further details than what the Context Diagram can give them.
Important Details: More often than not, in this layer, one could introduce more technical information, like the programming languages being used and the types of databases. Where the Context Diagram usually tries to convey the whole, big picture, the Container Diagram goes into the internal structure of the system.
Here’s an example of a container diagram:
Component Diagram
The Component Diagram goes even further, as it zooms into the details inside of each container and the internal components that make it up. For instance, if one container is a web application, this diagram can go down to controllers, services, or even repositories inside an application and how they might talk with one another.
Purpose: Breaking down each container to show how internal components work together.
Audience: Technical teams for closer views of the inner details.
Important Details: The level of detail is higher here; hence, it is best used for teams that are directly working on the software.
Here’s an example of a component diagram:
Code Diagram
In that case, the Code Diagram focuses primarily on the internal structure that components usually contain, classes, or objects. Not every situation demands this amount of detail, and such diagrams are frequently created automatically by tools, such as Integrated Development Environments (IDEs). Since C4 doesn't define the notations at this level, other modeling notations, such as UML, can be used to represent code structure.
Purpose: The implementation level details will be depicted, such as class structures.
Audience: All the developers that need to see code-level details.
Important Details: Often this level can be generated automatically with tools, hence it might not be necessary to create these diagrams manually.
Here’s an example of a code diagram, taken from the C4 docs:
When to Use Different Levels of C4
Another strength of the C4 Model is that it's so flexible, meaning you don't have to use all four levels all of the time. For many systems, drawing a Context Diagram and a Container Diagram will often be sufficient to communicate what a team needs to know about the system. You can often convey the largest part of the information for most stakeholders using just these two levels.
If you're working on a very complex system, or if you need to document more about the system, then that's the time to introduce Component and Code Diagrams. Remember, the deeper the level of detail you want, like using a Code Diagram, the more things change and the more you have to adjust this diagram, and this is an added cost on its own in terms of maintenance.
Architecture vs. Design
Where Does C4 Fit? This interesting debate discusses whether a very detailed diagram, like a Code Diagram, must be taken into consideration in software architecture, or if it is more about software design.
Architecture is basically concerned with higher-level decisions on the structure and the behavior of the system, while design in general covers the implementation of the single components.
This debate emphasizes the flexibility of the C4 Model—whether you use it strictly for architectural purposes or extend it into design is totally up to your team's needs.
Pros and Cons of Using the C4 Model
As with any approach, there are strengths and weaknesses associated with using the C4 Model. Here's a quick rundown:
Pros:
Standardized Approach: The C4 provides a lucid, consistent means to visualize the architecture, which can be quite helpful while onboarding new members into a team.
Flexible: You can choose how much detail to include, from high-level architecture to low-level code structures.
Clear Communication: Non-technical stakeholders can understand the high-level diagrams, thus enhancing communication across teams.
Cons:
Learning Curve: The learning curve may be involved in getting started if your team is not comfortable with the C4 Model.
Maintenance: Detailed diagrams-such as the Code or Component diagrams-may require frequent updates because the system is always evolving, hence adding to more maintenance work.
Conclusion
The C4 Model is a very useful tool for clear and structured visualization of software architecture. Whether it is for high-level overviews when starting or deep insights into the system's internal structure, C4 supports documenting architecture in a way that is understandable and maintainable.
With a small learning curve, the payoffs of using C4 far outweigh the initial setup effort. This is a great investment to maintain clarity and healthy software systems.
Resources
If you are more of a visual learner, I’ve created two YouTube videos to cover C4, one more focused on theory and another focused on practice.
You can find the first 3 diagrams here.
You can also check these amazing resources: