Problem:

The code between eventlet and threading backends contained significant duplication in loopingcall.py and threadgroup.py modules, making maintenance difficult and increasing the risk of inconsistencies.
Solution:
Extract common logic into a new _common/ package
Create abstract base classes for LoopingCallBase and ThreadGroupBase
Keep backend-specific implementations in their respective modules
Preserve all existing public APIs

Benefits:

✅ Reduce ~1166 lines of duplicated code
✅ Centralize shared behavior for easier maintenance
✅ Clearly define common interfaces
✅ Simplify future backend changes
✅ Improve consistency between backends

Impact:

• No impact on public APIs
• Full backward compatibility maintained
• More maintainable architecture

Complexity:

• Significant refactoring affecting 7 files
• Common logic extraction requiring deep understanding of both backends
• Testing required to ensure compatibility
• But no API changes, so not maximum complexity