Implementation Strategy

Refactoring a large codebase to follow Domain-Driven Design principles is a significant undertaking. This document outlines a strategy for implementing the changes in a gradual, controlled manner.

Challenges

1. Maintaining System Stability: The system must continue to function during the refactoring process
2. Minimizing Risk: Changes should be made incrementally to reduce the risk of introducing bugs
3. Team Coordination: The team must be aligned on the refactoring approach
4. Testing: Ensuring that the refactored code works as expected

Phased Approach

We recommend a phased approach to implementing the DDD refactoring:

Phase 1: Preparation and Planning

1. Team Education:
2. Ensure all team members understand DDD concepts
3. Conduct workshops on DDD principles

4. Review the refactoring plan with the team

5. Identify Bounded Contexts:

6. Analyze the domain and identify bounded contexts
7. Document the bounded contexts and their relationships

8. Define the ubiquitous language for each bounded context

9. Create a Detailed Refactoring Plan:

10. Prioritize bounded contexts for refactoring
11. Define milestones and timelines

12. Establish success criteria

13. Set Up Testing Infrastructure:

14. Ensure comprehensive test coverage of existing functionality
15. Set up automated testing pipelines
16. Define testing strategy for refactored code

Phase 2: Start with a Single Bounded Context

1. Select a Bounded Context:
2. Choose a bounded context that is relatively isolated
3. Start with a context that has clear boundaries

4. Consider a context with high business value

5. Create Domain Models:

6. Implement rich domain models for the selected context
7. Define value objects and entities

8. Implement domain services

9. Implement Repository Interfaces:

10. Define repository interfaces in the domain layer
11. Implement repository implementations in the infrastructure layer

12. Create adapters between the new domain model and existing infrastructure

13. Implement Application Services:

14. Create application services for the bounded context
15. Refactor controllers to use the new application services

16. Implement domain events for the bounded context

17. Test and Validate:

18. Write unit tests for the domain model
19. Write integration tests for the application services
20. Validate that the refactored code works as expected

Phase 3: Implement the Strangler Pattern

The Strangler Pattern is a technique for gradually replacing an existing system with a new one:

1. Create Facade Services:
2. Implement facade services that delegate to either the old or new implementation

3. Use feature flags to control which implementation is used

4. Gradually Replace Functionality:

5. Refactor one feature at a time
6. Test each refactored feature thoroughly

7. Enable the new implementation for a subset of users

8. Monitor and Validate:

9. Monitor the performance and behavior of the new implementation
10. Compare results with the old implementation
11. Gradually increase the usage of the new implementation

Phase 4: Expand to Other Bounded Contexts

1. Prioritize Remaining Contexts:
2. Evaluate which bounded context to tackle next
3. Consider dependencies between contexts

4. Focus on high-value, high-impact contexts

5. Implement Each Context:

6. Follow the same approach as in Phase 2
7. Ensure proper integration between contexts

8. Use domain events for cross-context communication

9. Refactor Shared Kernel:

10. Identify and implement shared concepts
11. Ensure consistency across bounded contexts
12. Avoid tight coupling between contexts

Phase 5: Clean Up and Optimize

1. Remove Legacy Code:
2. Once all functionality has been migrated, remove the old implementation
3. Clean up any temporary adapters or facades

4. Refactor any remaining code that doesn't follow DDD principles

5. Optimize Performance:

6. Identify and address any performance issues
7. Optimize database queries and data access

8. Consider caching strategies

9. Documentation and Knowledge Transfer:

10. Update documentation to reflect the new architecture
11. Conduct knowledge transfer sessions
12. Ensure all team members understand the new architecture

Implementation Guidelines

Use Feature Flags

Feature flags allow you to toggle between old and new implementations:

// Example of using feature flags
class RequisitionController {
  constructor({ featureFlags, oldRequisitionService, newRequisitionApplicationService }) {
    this.featureFlags = featureFlags;
    this.oldService = oldRequisitionService;
    this.newService = newRequisitionApplicationService;
  }

  async createRequisition(request, reply) {
    if (this.featureFlags.isEnabled('use-ddd-requisition')) {
      // Use new DDD implementation
      return this._createRequisitionDDD(request, reply);
    } else {
      // Use old implementation
      return this._createRequisitionLegacy(request, reply);
    }
  }

  async _createRequisitionDDD(request, reply) {
    try {
      const { body, userFromToken } = request;

      const requisition = await this.newService.createRequisition(
        body,
        userFromToken.id
      );

      return reply.status(201).send(requisition);
    } catch (error) {
      return reply.status(400).send({ error: error.message });
    }
  }

  async _createRequisitionLegacy(request, reply) {
    // Original implementation
    return this.oldService.createRequisition(request, reply);
  }
}

Use Adapters

Adapters can help bridge between the old and new implementations:

// Example of an adapter between old and new domain models
class RequisitionAdapter {
  static toNewDomain(oldRequisition) {
    return new Requisition({
      id: oldRequisition.id,
      rsNumber: oldRequisition.rsNumber,
      rsLetter: oldRequisition.rsLetter,
      companyCode: oldRequisition.companyCode,
      createdBy: oldRequisition.createdBy,
      status: oldRequisition.status,
      // ... map other properties
    });
  }

  static toOldDomain(newRequisition) {
    return {
      id: newRequisition.id,
      rsNumber: newRequisition.rsNumber,
      rsLetter: newRequisition.rsLetter,
      companyCode: newRequisition.companyCode,
      createdBy: newRequisition.createdBy,
      status: newRequisition.status.value,
      // ... map other properties
    };
  }
}

Implement Unit of Work

The Unit of Work pattern can help manage transactions:

// src/infrastructure/database/UnitOfWork.js
class UnitOfWork {
  constructor(sequelize) {
    this.sequelize = sequelize;
  }

  async execute(callback) {
    const transaction = await this.sequelize.transaction();

    try {
      const result = await callback(transaction);
      await transaction.commit();
      return result;
    } catch (error) {
      await transaction.rollback();
      throw error;
    }
  }
}

module.exports = UnitOfWork;

Testing Strategy

1. Unit Tests:
2. Test domain entities and value objects in isolation
3. Verify that business rules are enforced

4. Use mocks for dependencies

5. Integration Tests:

6. Test application services with real repositories
7. Verify that the system works end-to-end

8. Use test databases

9. Comparison Tests:

10. Compare the output of old and new implementations
11. Ensure that the refactored code produces the same results
12. Identify and address any discrepancies

Monitoring and Rollback Plan

1. Monitoring:
2. Monitor application performance
3. Track error rates

4. Compare metrics between old and new implementations

5. Rollback Plan:

6. Have a clear plan for rolling back changes if issues arise
7. Use feature flags to quickly disable new implementations
8. Maintain the old implementation until the new one is proven

Conclusion

Refactoring to Domain-Driven Design is a significant undertaking, but with a careful, phased approach, it can be done with minimal disruption to the system. By focusing on one bounded context at a time and using techniques like the Strangler Pattern and feature flags, you can gradually transform the codebase while maintaining system stability.