Expand description
§Application Layer
The application layer orchestrates business workflows and coordinates between the domain layer and external systems. It implements the use cases of the system and serves as the entry point for all business operations.
§Architecture
The application layer follows the Clean Architecture pattern and acts as a coordination layer that:
- Receives requests from the interface layer
- Coordinates domain services and entities
- Manages transactions and cross-cutting concerns
- Transforms data between layers
- Handles application-specific business rules
§Module Structure
application/
├── commands/ # Command objects representing user intentions
├── handlers/ # Command and query handlers
├── queries/ # Query objects for data retrieval
└── services/ # Application services coordinating workflows§Commands
Commands represent user intentions and system operations. They are immutable data structures that encapsulate all the information needed to perform an action.
Characteristics:
- Immutable data structures
- Self-validating
- Express user intent
- Contain all necessary parameters
Example:
§Queries
Queries represent requests for data retrieval. They specify what data is needed and any filtering or sorting criteria.
Characteristics:
- Read-only operations
- Specify data requirements
- Support filtering and pagination
- Return DTOs or view models
Example:
§Handlers
Handlers contain the application logic for processing commands and queries. They coordinate between domain services and manage the flow of operations.
Characteristics:
- Stateless operations
- Coordinate domain services
- Manage transactions
- Handle cross-cutting concerns
Example:
§Application Services
Application services implement complex workflows that span multiple domain services or require coordination with external systems.
Characteristics:
- Orchestrate complex workflows
- Manage external dependencies
- Handle application-specific logic
- Provide transaction boundaries
Example:
§Design Principles
§Dependency Inversion
The application layer depends on domain abstractions, not concrete implementations. All external dependencies are injected through interfaces.
§Single Responsibility
Each application service, handler, and command has a single, well-defined purpose.
§Separation of Concerns
Commands handle data, handlers handle logic, services handle coordination.
§Testability
All components are designed to be easily testable with dependency injection and clear interfaces.
§Error Handling
The application layer handles errors from the domain layer and translates them into appropriate responses for the interface layer:
§Testing Strategy
Application layer components are tested with:
- Unit Tests: Test individual handlers and services with mocked dependencies
- Integration Tests: Test complete workflows with real implementations
- Contract Tests: Verify interfaces between layers