Component Diagrams and Internal Structure Overview Introduction
Welcome to Module 4, where we zoom one level deeper into the architecture and explore Level 3: Component Diagrams — the most detailed view within the core C4 hierarchy that still remains conceptual and implementation-agnostic.
After establishing the high-level business context (Level 1) and the major technical building blocks and their interactions (Level 2), many teams find that the Container Diagram provides sufficient clarity for most purposes. However, certain situations demand a closer look inside one or more containers to understand:
- How is this container internally structured?
- What are its logical building blocks (modules, services, subsystems, layers)?
- Where are the key responsibilities, boundaries, and interfaces defined?
- How does the internal design support maintainability, testability, extensibility, or domain alignment?
This is the purpose of the Component Diagram: it reveals the logical decomposition inside a single container without diving into actual code-level classes, methods, or implementation details.
When and Why to Create Component Diagrams
Unlike Levels 1 and 2 — which are almost always created — Level 3 is applied selectively. You should zoom to Component level when:
- The container is complex, large, or contains critical business logic that needs clear modular boundaries (e.g., a monolithic backend, a domain-heavy microservice, a legacy system undergoing modernization).
- You are planning or executing refactoring, monolith decomposition, or microservices extraction and need to map conceptual groupings before touching code.
- New developers need a mental map of how responsibilities are organized inside an important service.
- You are conducting architecture reviews, security threat modeling, or dependency analysis and require visibility into internal coupling and interfaces.
- Domain-driven design (DDD) concepts — bounded contexts, aggregates, domain services, anti-corruption layers — need to be explicitly visualized.
- The container has multiple teams working on it, and clear interface contracts reduce coordination overhead.
If a container is simple (e.g., a thin API wrapper, a basic CRUD service, or a stateless worker), you can safely skip Level 3 and stay at Container level.
What a Component Diagram Shows (and Doesn’t Show)
At Level 3, each Component is:
- A logical grouping of code that has a cohesive responsibility
- A named abstraction with well-defined interfaces (not individual classes)
- Something that could theoretically be extracted or replaced independently
Common component examples inside a backend container:
- “Order Placement Service”
- “Payment Processing Orchestrator”
- “Customer Domain Model”
- “Inventory Anti-Corruption Layer”
- “Reporting Module”
- “Authentication & Authorization Service”
- “Event Publisher”
What you do not show at this level:
- Individual classes, methods, fields, or code structure (that’s Level 4: Code)
- Implementation technologies (e.g., “Spring @Service”, “Java class”)
- Runtime details like threads, connection pools, or caching strategies
- Data schemas or detailed database tables (save for Level 4 or separate ERDs)
Instead, the focus is on responsibilities, interfaces, and high-level dependencies — providing a bridge between architectural intent and actual code.
Key Benefits of Mastering Component Diagrams
- Clarifies modularity and exposes hidden coupling early
- Supports evolutionary architecture by making it easier to identify extractable modules
- Improves onboarding speed for developers joining complex services
- Enables better DDD alignment visualization (entities, value objects, domain events, etc., as named components)
- Serves as a blueprint for refactoring or splitting monoliths without guesswork
What You’ll Learn and Build in This Module
In Module 4, we will cover:
- Precisely defining what qualifies as a component (vs. container, vs. class)
- Techniques for identifying meaningful components using responsibility-driven design, DDD heuristics, or layering patterns
- Drawing clean relationships that highlight interfaces, dependencies, and protected boundaries
- Deciding when to stop at Level 3 and avoid premature over-detailing
- Avoiding common mistakes: class-level granularity, technology leakage, or flat/unstructured diagrams
- Hands-on conversational refinement — using the AI Chatbot to iteratively add, remove, rename, or regroup components, fix incorrect relationships, and ensure the diagram tells a coherent internal story
By the end of this module, you’ll know exactly when and how to apply Level 3 diagrams effectively — creating focused, high-value views that illuminate the internal structure of your most important containers without getting lost in code-level noise.
This is where architecture becomes truly actionable for developers and where the C4 Model proves its worth in supporting real design decisions and evolutionary change.
Let’s zoom inside a container, map its logical building blocks, and make the internal design crystal clear — one well-named component at a time! 🚀