Layered Architecture

Layered architecture is a fundamental software architecture pattern that structures applications into distinct layers of functionality. This pattern promotes separation of concerns and helps manage complexity in large software systems by establishing clear boundaries between different aspects of an application.

Core Principles

Layer Isolation

• Each layer should be cohesive and independent
• Layers communicate through well-defined interfaces
• Changes in one layer should minimally impact others
• Supports the single responsibility principle

Dependency Direction

• Dependencies flow downward only
• Upper layers depend on lower layers
• Lower layers remain unaware of upper layers
• Enables loose coupling between components

Common Layer Structure

1. Presentation Layer

   • Handles user interface and user interaction
   • Contains UI components and presentation logic
   • Implements Model-View-Controller patterns

2. Application Layer

   • Orchestrates business workflows
   • Manages application state
   • Implements use cases and business rules

3. Domain Layer

   • Contains core business logic
   • Defines domain model and business entities
   • Implements business rules and validations

4. Infrastructure Layer

   • Provides technical capabilities
   • Handles data persistence and external services
   • Implements repository pattern and technical concerns

Benefits

• Maintainability: Clear organization makes code easier to maintain
• Testability: Layers can be tested in isolation
• Flexibility: Components can be modified or replaced independently
• Scalability: Different layers can scale independently

Considerations

Layer Crossing

• Direct layer crossing should be avoided
• Use dependency injection to manage dependencies
• Consider implementing facade pattern for complex subsystems

Performance Impact

• Additional abstraction layers can impact performance
• Consider n-tier architecture for distributed systems
• Balance abstraction with performance requirements

Common Pitfalls

• Over-layering leading to unnecessary complexity
• Tight coupling between layers
• Violating the dependency rule
• circular dependency issues

Variations

1. Strict Layering

   • Each layer only communicates with adjacent layers
   • Provides maximum isolation
   • May increase complexity

2. Relaxed Layering

   • Allows skipping intermediate layers
   • More flexible but potentially more coupled
   • Requires careful management

Implementation Guidelines

• Start with clear layer boundaries
• Define explicit interfaces between layers
• Use dependency inversion where appropriate
• Consider implementing SOLID principles

Related Patterns

• Clean Architecture
• Hexagonal Architecture
• Onion Architecture
• Service-Oriented Architecture

Layered architecture remains one of the most widely used architectural patterns, providing a solid foundation for building maintainable and scalable software systems. Its principles continue to influence modern architectural approaches and best practices in software development.