# ADR-002: Common Package Refactoring

## Status
Accepted and Implemented

## Context
The common package contained several large, monolithic files that were becoming difficult to maintain and extend. The files included:
- aggregation.py
- indicators.py
- validation.py
- transformation.py
- data_types.py

These files handled critical functionality but were growing in complexity and responsibility, making it harder to:
- Understand and maintain individual components
- Test specific functionality
- Add new features without affecting existing code
- Ensure proper separation of concerns

## Decision
We decided to refactor the common package into a more modular structure by:

1. **Splitting Large Files into Sub-packages:**
   - Created `aggregation/` package with specialized modules
   - Created `indicators/` package with focused components
   - Maintained core data types in a single, well-structured file

2. **Improving Validation:**
   - Enhanced modularity of validation system
   - Added clearer validation rules and messages
   - Maintained backward compatibility

3. **Enhancing Transformation:**
   - Added safety limits system
   - Improved error handling
   - Better separation of transformation concerns

4. **Preserving Data Types:**
   - Reviewed and verified data_types.py structure
   - Maintained as single file due to good organization
   - Documented existing patterns

## Consequences

### Positive
- Better code organization and maintainability
- Clearer separation of concerns
- Easier to test individual components
- More focused and cohesive modules
- Better safety with new limits system
- Improved documentation and examples
- Easier to extend with new features

### Negative
- Slightly more complex import paths
- Need to update existing documentation
- Initial learning curve for new structure

### Neutral
- Need to maintain more files
- More granular version control
- Different organization pattern from original

## Alternatives Considered

### Keep Monolithic Structure
Rejected because:
- Growing complexity
- Difficult maintenance
- Hard to test
- Poor separation of concerns

### Complete Microservices Split
Rejected because:
- Too complex for current needs
- Would introduce unnecessary overhead
- Not justified by current scale

### Hybrid Approach (Selected)
Selected because:
- Balances modularity and simplicity
- Maintains good performance
- Easy to understand and maintain
- Allows for future growth

## Implementation Notes

### Phase 1: Aggregation and Indicators
- Split into focused sub-packages
- Added proper interfaces
- Maintained backward compatibility
- Added comprehensive tests

### Phase 2: Validation and Transformation
- Enhanced validation system
- Added safety limits
- Improved error handling
- Updated documentation

### Phase 3: Verification
- Reviewed data types
- Ran comprehensive tests
- Updated documentation
- Verified no regressions

## Migration Guide

### For Developers
1. Update imports to use new package structure
2. Review new safety limits in transformation
3. Check validation error handling
4. Update any custom extensions

### For Maintainers
1. Familiarize with new package structure
2. Review new testing patterns
3. Understand safety limit system
4. Follow modular development pattern

## References
- Original task list: tasks/refactor-common-package.md
- Documentation standards: docs/documentation.mdc
- Test coverage reports
- Code review feedback