Clean Architecture layers in LLD - Architecture Diagram

System Overview - Clean Architecture layers

Clean Architecture organizes software into layers to separate concerns and improve maintainability. It ensures that business rules are independent of frameworks, UI, and databases, making the system easier to test and evolve.

Architecture Diagram

User Interface (UI)
      |
      v
Application Layer
      |
      v
Domain Layer
      |
      v
Infrastructure Layer
      |
      v
External Systems (Database, Web, etc.)

Components

User Interface (UI)

layer

Handles user interactions and displays information

Application Layer

layer

Coordinates application activities and business use cases

Domain Layer

layer

Contains business rules and domain entities

Infrastructure Layer

layer

Manages external systems like databases and web services

External Systems

external_system

Databases, web services, and other external resources

Request Flow - 8 Hops

User Interface (UI) → Application Layer

Application Layer → Domain Layer

Domain Layer → Infrastructure Layer

Infrastructure Layer → External Systems

External Systems → Infrastructure Layer

Infrastructure Layer → Domain Layer

Domain Layer → Application Layer

Application Layer → User Interface (UI)

Failure Scenario

Component Fails:Infrastructure Layer

Impact:External system calls fail, causing data persistence or retrieval to break

Mitigation:Use retry mechanisms, fallback caches, and isolate failures to prevent domain logic disruption

Architecture Quiz - 3 Questions

Which layer contains the business rules and core logic?

A Application Layer

B Domain Layer

C Infrastructure Layer

D User Interface (UI)

Design Principle

Clean Architecture separates concerns by organizing code into layers that depend inwardly, keeping business rules independent from UI and infrastructure. This improves testability, flexibility, and maintainability.

Practice

1. Which layer in Clean Architecture contains the core business rules and logic?

easy

A. UI layer

B. Entities layer

C. Database layer

D. Frameworks layer

Solution

1. Step 1: Understand the role of each layer

   The Entities layer holds the core business rules and logic, independent of external concerns.

2. Step 2: Identify the correct layer for business logic

   UI, Database, and Frameworks layers handle external interactions, not core logic.

3. Final Answer:

   Entities layer -> Option B

4. Quick Check:

   Business logic = Entities layer [OK]

Hint: Core logic always lives in the Entities layer [OK]

Common Mistakes:

• Confusing UI layer with business logic
• Thinking database layer contains core rules
• Mixing frameworks with core logic

2. In Clean Architecture, which layer is responsible for adapting data from the database to the business logic?

easy

A. Entities layer

B. Use Cases (Interactor) layer

C. UI layer

D. Interface Adapters layer

Solution

1. Step 1: Identify the role of Interface Adapters

   Interface Adapters convert data from external sources like databases into a form usable by inner layers.

2. Step 2: Confirm other layers' roles

   Entities hold business rules, Use Cases orchestrate logic, UI handles presentation, so adapting data fits Interface Adapters.

3. Final Answer:

   Interface Adapters layer -> Option D

4. Quick Check:

   Data adaptation = Interface Adapters [OK]

Hint: Data conversion happens in Interface Adapters [OK]

Common Mistakes:

• Choosing Entities layer for data adaptation
• Confusing Use Cases with data conversion
• Selecting UI layer for database data handling

3. Given the following flow in Clean Architecture: UI calls Use Cases, which then call Entities. What is the correct order of dependency direction?

medium

A. UI -> Entities -> Use Cases

B. Entities -> Use Cases -> UI

C. UI -> Use Cases -> Entities

D. Use Cases -> UI -> Entities

Solution

1. Step 1: Understand dependency rule in Clean Architecture

   Dependencies always point inward, from outer layers to inner layers.

2. Step 2: Apply to given flow

   UI depends on Use Cases, which depend on Entities, so direction is UI -> Use Cases -> Entities.

3. Final Answer:

   UI -> Use Cases -> Entities -> Option C

4. Quick Check:

   Dependency direction = UI to Entities [OK]

Hint: Dependencies always point inward [OK]

Common Mistakes:

• Reversing dependency direction
• Confusing which layer calls which
• Assuming Entities depend on UI

4. A developer placed database access code directly inside the Entities layer. What is the main problem with this design?

medium

A. Entities layer should not depend on external frameworks or databases

B. Entities layer must handle UI rendering

C. Database code belongs only in the UI layer

D. Entities layer should only contain database schemas

Solution

1. Step 1: Recall Clean Architecture dependency rules

   Inner layers like Entities must be independent of external concerns like databases.

2. Step 2: Identify why database code in Entities is wrong

   It creates tight coupling and breaks separation of concerns.

3. Final Answer:

   Entities layer should not depend on external frameworks or databases -> Option A

4. Quick Check:

   Entities layer independence = true [OK]

Hint: Keep Entities free from external dependencies [OK]

Common Mistakes:

• Thinking Entities handle UI
• Placing database code in UI layer
• Confusing database schemas with business logic

5. You need to design a system where the UI can be changed without affecting business rules, and the database can be swapped easily. Which Clean Architecture principle helps achieve this?

hard

A. Dependency Rule: Inner layers do not depend on outer layers

B. UI layer directly accesses Entities for faster updates

C. Database layer contains business logic for flexibility

D. Use Cases layer handles UI rendering and database access

Solution

1. Step 1: Identify the principle for independence

   The Dependency Rule states inner layers (business rules) do not depend on outer layers (UI, database).

2. Step 2: Explain how this helps system flexibility

   This separation allows changing UI or database without impacting core business logic.

3. Final Answer:

   Dependency Rule: Inner layers do not depend on outer layers -> Option A

4. Quick Check:

   Dependency Rule ensures flexibility [OK]

Hint: Inner layers must be independent for easy changes [OK]

Common Mistakes:

• Allowing UI to access Entities directly
• Putting business logic in database layer
• Mixing UI rendering with Use Cases