Domain Layer Design - tonglam/letletme_data GitHub Wiki

Overview

The domain layer is the core of our application, implementing business logic using Domain-Driven Design (DDD) principles and Functional Programming (FP) patterns. This document outlines the high-level architecture and design decisions.

Architecture

graph TD
    A[API Layer] --> B[Domain Layer]
    B --> C[Infrastructure Layer]
    B --> D[Repository Layer]
    B --> E[Cache Layer]
    D --> F[Database]
    E --> G[Cache]

    subgraph Domain Layer
        H[Domain Operations]
        I[Domain Types]
        J[Domain Logic]
        K[Domain Events]

        H --> I
        H --> J
        J --> K
    end

Domain Layer Structure

graph LR
    A[Domain Operations] --> B[Cache Layer]
    A --> C[Repository Layer]
    A --> D[Infrastructure Layer]
    B --> E[Cache Store]
    C --> F[Database]

    subgraph Domain Core
        G[Domain Types]
        H[Business Rules]
        I[Value Objects]
        J[Domain Events]

        G --> H
        H --> I
        I --> J
    end

Design Principles

1. Domain-Driven Design

  • Bounded Contexts: Each domain is self-contained with clear boundaries
  • Ubiquitous Language: Consistent terminology throughout the codebase
  • Value Objects: Immutable domain objects with identity
  • Domain Events: State changes represented as events

2. Functional Programming

  • Pure Functions: No side effects in core business logic
  • Immutability: All state changes create new objects
  • Type Safety: Strong typing with branded types
  • Error Handling: Explicit error handling with TaskEither

3. Layered Architecture

graph TD
    A[API Layer] --> B[Domain Layer]
    B --> C[Infrastructure Layer]

    subgraph Domain Layer
        D[Operations]
        E[Repository]
        F[Cache]
        G[Types]

        D --> E
        D --> F
        D --> G
    end

Data Flow

sequenceDiagram
    participant A as API Layer
    participant O as Domain Operations
    participant C as Cache Layer
    participant R as Repository Layer
    participant D as Database

    A->>O: Request Data
    O->>C: Check Cache
    alt Cache Hit
        C-->>O: Return Cached Data
    else Cache Miss
        O->>R: Fetch Data
        R->>D: Query Database
        D-->>R: Return Data
        R-->>O: Transform Data
        O->>C: Update Cache
    end
    O-->>A: Return Domain Model

Domain Organization

1. Core Components

  • Domain Types: Core business models and types
  • Domain Operations: Business logic implementation
  • Repository Layer: Data access abstraction
  • Cache Layer: Performance optimization

2. Cross-Cutting Concerns

  • Error Handling: Consistent error types and handling
  • Validation: Input validation and business rules
  • Type Safety: Branded types and type guards
  • Performance: Caching and optimization strategies

Implementation Strategy

1. Domain Isolation

  • Self-contained domains with explicit interfaces
  • Clear dependencies through repository pattern
  • No cross-domain knowledge leakage
  • Pure functional core with side effects at edges

2. Type Safety

  • Branded types for domain identifiers
  • Explicit validation at boundaries
  • No implicit type coercion
  • Comprehensive type definitions

3. Error Handling

  • TaskEither for all operations
  • Explicit error types and messages
  • Consistent error creation patterns
  • Error transformation at boundaries

Performance Considerations

1. Caching Strategy

graph TD
    A[Request] --> B{Cache?}
    B -- Yes --> C[Return Cached]
    B -- No --> D[Fetch Data]
    D --> E[Update Cache]
    E --> F[Return Data]

2. Data Access Patterns

  • Cache-first architecture
  • Batch operations for bulk data
  • Optimized database queries
  • Connection pooling

Testing Strategy

1. Test Pyramid

graph TD
    A[E2E Tests] --> B[Integration Tests]
    B --> C[Unit Tests]

    subgraph Test Coverage
        D[Domain Logic]
        E[Repository Layer]
        F[Cache Layer]

        D --> E
        E --> F
    end

2. Test Types

  • Unit tests for pure functions
  • Integration tests for repositories
  • E2E tests for domain operations
  • Property-based tests for validation

Maintenance and Evolution

1. Code Organization

  • Consistent file structure
  • Clear module boundaries
  • Explicit dependencies
  • Functional composition

2. Documentation

  • API documentation
  • Domain model documentation
  • Implementation guides
  • Architecture decision records

3. Monitoring

  • Performance metrics
  • Error tracking
  • Cache hit rates
  • Database performance