Component Diagrams: Detailing Internal Logic

After establishing your system context and container diagrams, the next leap is understanding how components interact within a single container. This is where the C4 component diagram shines—revealing internal structure, responsibilities, and hidden dependencies that can derail maintainability.

Think of this level as the architectural equivalent of a floor plan. It’s not just about what’s inside, but how those parts work together. A well-structured component diagram turns architectural ambiguity into clear ownership, boundaries, and communication flow.

You’ve likely run into situations where code works but feels tangled—dependencies cross-cutting, responsibilities blurred, testing blocked. That’s where the C4 model component level tutorial becomes essential. It’s not about code, but about how the system is designed to be built.

What Is a C4 Component Diagram?

The C4 component diagram is Level 3 in the C4 model’s hierarchy. It breaks down containers into meaningful, reusable components—collections of code that encapsulate a specific responsibility.

Unlike code-level diagrams, components are high enough to stay independent of implementation details yet specific enough to reveal design intent.

Each component should have a single, well-defined responsibility. If it doesn’t, you’re likely missing a layer or misdefining the scope.

Key elements include:

• Components (boxes with labels)
• Dependencies (arrows indicating runtime or compile-time relationships)
• Responsibility descriptions (often in a legend or inline)
• Clear boundaries (dashed lines or grouping)

Why This Level Matters

Too many teams skip this step, assuming that container-level diagrams are enough. But without component-level clarity, you’re left with:

• Shared responsibility confusion
• Unplanned circular dependencies
• Difficulty in testing, deployment, and refactoring

Consider this: a “User Service” container might contain a “User Authentication,” “User Profile,” and “User Notification” component. Each has a single responsibility. Mislabeling one as “User Management” can lead to a monolithic mess.

How to Model Components in C4

Creating a C4 component diagram isn’t about drawing the most boxes. It’s about making design decisions visible.

Start with a single container—say, a web application. Ask: What are the major functional areas? Each area becomes a potential component.

Use the Single Responsibility Principle as your filter: can this component be changed for only one reason? If not, split it.

Step-by-Step Process

1. Identify functional domains—e.g., “Payment Processing,” “Order Management,” “Inventory Tracking.”
2. Group related classes or modules into logical units.
3. Give each a clear, concise name—avoid ambiguous labels like “Utils” or “Helper.”
4. Draw components as rectangles with a short label and a description from the legend.
5. Map dependencies with arrows—where the arrow points is the dependent component.
6. Check for circular dependencies—they signal design issues.

Patterns That Help

Some common patterns help structure components effectively:

• Hexagonal (Ports and Adapters)—separate core logic from external frameworks.
• Event-Driven Components—when components communicate via events, not direct calls.
• Layered Architecture—components in a dependency direction (e.g., UI → Service → Repository).
• Microkernel—core component that delegates to extension points.

Common Pitfalls and How to Avoid Them

Even experienced modelers make mistakes. Here are the most frequent errors and how to correct them.

Issue	Why It’s Bad	Solution
Overlapping responsibilities	Leads to tightly coupled changes	Refactor into smaller, single-purpose components
Circular dependencies	Breaks modularity and testing	Re-evaluate component boundaries; introduce interfaces
Too many components	Creates noise, hides the forest for the trees	Group related components; use subcomponents or layers
Using implementation names	Breaks abstraction	Focus on responsibility: “Order Processor” not “OrderServiceV2”

One rule I’ve learned: if you can’t explain a component’s purpose in one sentence, it’s not well-defined.

Real-World Example: E-Commerce Platform

Let’s say we have a “Shopping Cart” container. The initial sketch might show:

• Cart Service
• Checkout Service
• Payment Gateway
• Inventory Service

But a deeper look reveals that “Cart Service” depends on “Inventory Service” to check stock. If the inventory is out-of-date, the cart might allow invalid purchases.

By modeling this dependency, we expose a risk: the cart is not resilient to inventory delays.

So we refactor: introduce a “Stock Validation” component that sits between the cart and inventory. Now the dependency pattern is clearer, and we can add fallbacks or delay handling.

Before and After

Before: Cart → Payment → Inventory — this implies the cart waits for inventory, which isn’t ideal.

After: Cart → Stock Validation → Inventory; Cart → Payment

The new structure shows that payment doesn’t depend on inventory, only stock availability does. That’s a major improvement in resilience and clarity.

Tools That Make C4 Modeling Easier

You don’t need to draw by hand. Modern tools like Visual Paradigm support C4 notation with built-in templates and libraries.

What I’ve found useful: use color coding to group related components (e.g., red for payment, blue for inventory), and always include a legend that defines the notation.

Also, keep component diagrams lightweight. No more than 10–15 components per diagram. If you exceed that, consider breaking into sub-diagrams or using a layered approach.

When to Use a C4 Component Diagram

Use this level when:

• Planning a major refactor or feature rollout
• Onboarding new developers to a project
• Reviewing architecture decisions in a team
• Identifying hidden dependencies that block deployment

It’s not for every conversation. But when you need to show how parts of a system fit together without diving into code, this is your go-to.

Frequently Asked Questions

What’s the difference between a container and a component in C4?

A container is a deployable unit—like a web app, mobile app, or database. A component is a logical piece of code within that container. For example, a web app container might have a “User Service” component and a “Payment Processing” component.

Can I have multiple component diagrams for one container?

Yes. If the container is complex, split it by domain or layer. For example, one diagram for “Order Management,” another for “Inventory.” This keeps each diagram focused and readable.

How do I decide component boundaries?

Use responsibility. Ask: “What problem does this part solve?” If two components solve the same problem, they may be over-split. If one component has more than one reason to change, split it.

Should I include data access components like repositories?

Yes—especially if they’re independent of the business logic. A “User Repository” component that handles database access is valid. But avoid naming it after the database (e.g., “MySQLUserDAO”). Use “User Data Access” or “User Repository.”

How do I represent asynchronous communication in a C4 component diagram?

Use a dashed arrow with a label like “event,” “async,” or “message.” For example: Order Service → Notification Service [event: order-placed]. This makes event-driven flows visible.

Can I model a component as a library or external SDK?

Yes. If the component is not part of your codebase but is used by your system, label it as “Third-Party” or “External.” For example: Payment Gateway (External). This helps avoid confusion with internal components.