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Architecture Decision Record (ADR)

Context and Problem Statement

The application requires a scalable, maintainable, and well-structured backend architecture that supports: - Multi-tenant data isolation - Role-based access control - Domain-driven design principles - Clean separation of concerns - Testable and maintainable code

Design Decisions

1. Domain-Driven Architecture

Decision: Implement a domain-driven design with clear bounded contexts

Rationale: - Better organization of business logic - Clear separation of concerns - Improved maintainability - Scalable structure for future growth

Consequences: - More initial setup required - Steeper learning curve - Better long-term maintainability - Clearer business logic representation

2. Layer Separation

Decision: Implement four distinct layers: API, Service, Domain, and Infrastructure

Rationale: - Clear separation of concerns - Independent testing of each layer - Easier maintenance and updates - Better code organization

Structure: 

app/
├── api/          # API Layer
├── service/      # Service Layer
├── domain/       # Domain Layer
└── infrastructure/ # Infrastructure Layer

3. Repository Pattern

Decision: Use the repository pattern with domain-specific repositories

Rationale: - Abstracts data persistence - Enables easier testing - Supports different data sources - Maintains domain isolation

Implementation: - Repository interfaces in domain layer - Implementations in infrastructure layer - Unit of Work pattern for transactions - Query specifications for complex queries

4. Dependency Injection

Decision: Implement a custom dependency injection container

Rationale: - Loose coupling between components - Easier testing and mocking - Flexible configuration - Clear service lifecycles

Features: - Service registration - Lifetime management - Scoped containers - Testing support

5. Multi-Tenant Architecture

Decision: Implement schema-based multi-tenancy

Rationale: - Strong data isolation - Flexible tenant-specific customization - Simplified backup and restore - Better security model

Implementation: - Tenant middleware - Schema routing - Connection pooling - Tenant-specific configurations

6. Security Model

Decision: Implement layered security with RBAC

Rationale: - Fine-grained access control - Tenant-specific permissions - Audit capability - Flexible role management

Components: - Permission registry - Role resolver - Access decision maker - Audit logging

Technical Stack

Core Framework

  • FastAPI for API development
  • Pydantic for data validation
  • SQLModel for ORM
  • Alembic for migrations

Storage

  • PostgreSQL for primary database
  • Redis for caching
  • S3 for file storage

Authentication

  • JWT for tokens
  • Bcrypt for password hashing
  • Role-based authorization

Testing

  • Pytest for testing framework
  • Coverage for test coverage
  • Factory Boy for test data

Implementation Guidelines

1. Code Organization

  • Each domain in its own module
  • Clear interface definitions
  • Consistent file naming
  • Documentation requirements

2. Testing Strategy

  • Unit tests for domain logic
  • Integration tests for APIs
  • Performance tests
  • Security tests

3. Error Handling

  • Domain-specific exceptions
  • Global error handler
  • Structured error responses
  • Detailed logging

4. Performance Considerations

  • Query optimization
  • Caching strategy
  • Connection pooling
  • Async operations

Migration Strategy

Phase 1: Foundation

  1. Set up basic project structure
  2. Implement core infrastructure
  3. Configure basic authentication

Phase 2: Domain Implementation

  1. Set up first domain module
  2. Implement repository pattern
  3. Add service layer

Phase 3: Multi-tenancy

  1. Implement tenant middleware
  2. Set up schema routing
  3. Add tenant-specific configurations

Phase 4: Security

  1. Implement RBAC
  2. Add permission system
  3. Set up audit logging

Monitoring and Maintenance

1. Metrics

  • Request/response times
  • Error rates
  • Database performance
  • Cache hit rates

2. Logging

  • Structured logging
  • Error tracking
  • Audit trails
  • Performance monitoring

3. Documentation

  • API documentation
  • Architecture documentation
  • Deployment guides
  • Development guidelines

Success Criteria

  1. Performance
  2. Response times < 200ms
  3. 99.9% uptime

  4. Efficient resource usage

  5. Maintainability

  6. Clear code organization
  7. Comprehensive documentation
  8. Automated testing

  9. Easy deployment

  10. Scalability

  11. Horizontal scaling capability
  12. Efficient multi-tenant support

  13. Resource isolation

  14. Security

  15. Strong data isolation
  16. Audit capability
  17. Access control
  18. Secure authentication

These architectural decisions provide a solid foundation for building a scalable, maintainable, and secure application while ensuring clear guidelines for implementation and future development.