# Design Patterns

This document establishes the design patterns consistently applied throughout jBOM's domain-centric architecture.

## Domain Service Patterns

### Constructor Configuration Pattern
Services establish behavior through constructor parameters rather than method arguments or property settings.

**Purpose**: Ensure consistent service behavior throughout its lifecycle
**Implementation**: Constructor parameters configure strategies, options, and operational behavior
**Benefits**: Immutable service configuration, clear behavioral contracts, easier testing

```
Service Creation → Configuration → Operational Use
     ↓                  ↓              ↓
Constructor         Parameters      Method Calls
Parameters          Validate        Execute with
Configure           Domain          Configured
Behavior           Constraints      Behavior
```

### Single Responsibility Pattern
Each domain service handles one bounded context with clear business purpose.

**Purpose**: Maintain clear service boundaries and prevent coupling
**Implementation**: Services focus on one area of domain expertise
**Benefits**: Testable in isolation, composable workflows, clear domain boundaries

**Service Categories**:
- **Data Extraction**: File format parsing to domain objects
- **Business Processing**: Domain transformations and calculations
- **Data Enhancement**: Augmenting domain objects with additional information

### Service Composition Pattern
Complex workflows achieved by composing single-purpose services rather than creating monolithic operations.

**Purpose**: Enable flexible workflow construction from reusable components
**Implementation**: Services use other services for specialized domain operations
**Benefits**: Reusable components, flexible workflows, clear separation of concerns

```
Workflow Composition:
SchematicReader → BOMGenerator → InventoryMatcher → OutputFormatter
      ↓               ↓               ↓               ↓
   Components    →   BOMData    → EnhancedBOM   → FormattedOutput
```

## Application Layer Patterns

### Command Handler Pattern
Application layer commands orchestrate domain services without containing business logic.

**Purpose**: Separate interface concerns from domain operations
**Implementation**: Stateless functions that translate, orchestrate, and format
**Benefits**: Clean separation, interface flexibility, testable orchestration

**Command Structure**:
```
1. Input Validation & Translation
2. Domain Service Instantiation
3. Service Orchestration
4. Output Formatting
5. Error Handling
```

### Input Translation Pattern
Convert interface-specific arguments to type-safe domain configuration objects.

**Purpose**: Bridge between interface representations and domain concepts
**Implementation**: Translation functions create domain objects from interface data
**Benefits**: Type safety, domain validation, interface independence

### Output Adaptation Pattern
Transform domain results for interface-appropriate presentation formats.

**Purpose**: Present domain data in interface-specific formats
**Implementation**: Formatting functions adapt domain objects for presentation
**Benefits**: Interface flexibility, consistent domain data, presentation separation

### Error Translation Pattern
Convert domain exceptions to user-appropriate error messages and response codes.

**Purpose**: Provide meaningful error feedback while preserving domain exception information
**Implementation**: Exception handling that maps domain errors to interface responses
**Benefits**: User-friendly errors, technical context preservation, consistent error handling

## Domain Model Patterns

### Value Object Pattern
Immutable objects representing domain concepts through their attributes rather than identity.

**Purpose**: Model domain concepts that are defined by their characteristics
**Implementation**: Frozen dataclasses with domain validation in `__post_init__`
**Benefits**: Immutability, domain validation, clear semantic meaning

**Characteristics**:
- No identity-based equality
- Immutable after creation
- Domain validation built-in
- Rich domain behavior through properties

### Entity Pattern
Objects with identity representing core business concepts that change over time.

**Purpose**: Model domain objects that have identity independent of their attributes
**Implementation**: Dataclasses with identity-based equality and hash methods
**Benefits**: Clear identity semantics, lifecycle management, domain behavior encapsulation

**Characteristics**:
- Identity-based equality and hashing
- Can change attributes while maintaining identity
- Domain behavior methods
- Business rule validation

### Pure Function Pattern
Stateless functions that perform domain calculations without side effects.

**Purpose**: Provide reusable domain logic without coupling to business process state
**Implementation**: Functions that take domain objects as input and return domain objects
**Benefits**: Testability, reusability, functional composition, no side effects

### Configuration Object Pattern
Type-safe configuration objects that capture domain intentions and validate business constraints.

**Purpose**: Ensure valid service configuration with domain-appropriate validation
**Implementation**: Frozen dataclasses with domain-specific validation logic
**Benefits**: Type safety, domain validation, configuration immutability, clear contracts

## Composition Patterns

### Composition Root Pattern
Central location for service instantiation, configuration, and dependency wiring.

**Purpose**: Manage service creation and configuration in a single, controlled location
**Implementation**: Main application entry point handles service composition
**Benefits**: Centralized configuration, clear dependencies, testable composition

### Dependency Injection Pattern
Services receive dependencies through constructor parameters rather than creating them internally.

**Purpose**: Enable flexible service composition and testability
**Implementation**: Constructor parameters for service dependencies and configuration
**Benefits**: Testability with mock dependencies, flexible composition, clear contracts

### Factory Method Pattern
Services create complex internal objects through factory methods rather than constructor logic.

**Purpose**: Encapsulate complex object creation while maintaining clean constructor interfaces
**Implementation**: Private methods that create internal processors, strategies, or utilities
**Benefits**: Clean constructors, encapsulated complexity, consistent object creation

## Integration Patterns

### Adapter Pattern
Interface layers adapt between external frameworks and internal domain concepts.

**Purpose**: Isolate framework-specific code from domain logic
**Implementation**: Interface-specific adapters that translate between frameworks and domain
**Benefits**: Framework isolation, domain protection, interface flexibility

### Strategy Pattern (Configuration-Based)
Behavior variations implemented through constructor parameters and conditional logic.

**Purpose**: Enable runtime behavior customization without complex inheritance hierarchies
**Implementation**: Constructor parameters that influence internal processing logic
**Benefits**: Runtime flexibility, simple implementation, clear behavior contracts

**Note**: jBOM uses parameter-based strategies rather than strategy object injection, keeping implementation simple while maintaining flexibility.

### Observer Pattern (Event-Based)
Domain services communicate state changes through well-defined domain events.

**Purpose**: Enable loose coupling between services while maintaining domain integrity
**Implementation**: Domain events that carry business-relevant information
**Benefits**: Loose coupling, domain event history, extensible workflows

## Testing Patterns

### Isolated Unit Testing Pattern
Domain services tested with domain objects and mock dependencies.

**Purpose**: Validate business logic in isolation from infrastructure concerns
**Implementation**: Unit tests that create domain objects and test service methods
**Benefits**: Fast execution, clear failure indication, business logic focus

### Integration Testing Pattern
Application layer tested with real domain services and mock infrastructure.

**Purpose**: Validate service orchestration and workflow logic
**Implementation**: Integration tests that use real services with controlled inputs
**Benefits**: Workflow validation, service collaboration testing, realistic scenarios

### Contract Testing Pattern
Services tested against well-defined input/output contracts.

**Purpose**: Ensure service contracts remain stable across implementation changes
**Implementation**: Tests that validate service behavior against documented contracts
**Benefits**: Contract stability, implementation flexibility, clear service boundaries

## Extension Patterns

### Plugin Interface Pattern
Well-defined extension points for adding new capabilities without modifying existing code.

**Purpose**: Enable system extension while maintaining architectural integrity
**Implementation**: Abstract interfaces with clear contracts for extension implementations
**Benefits**: Extensibility, stability, architectural consistency

### Configuration Extension Pattern
New behavior added through configuration objects rather than code modification.

**Purpose**: Add capabilities through configuration rather than code changes
**Implementation**: Configuration objects that enable new processing options
**Benefits**: Runtime flexibility, backward compatibility, minimal code impact

These design patterns provide the foundation for consistent, maintainable, and extensible software architecture throughout the jBOM system.
