# Scenario 05: Gradual Migration

**Target Audience**: Maintainers of legacy codebases, tech leads
**Difficulty**: Intermediate to Advanced
**Keywords**: migration, refactoring, legacy code, technical debt, incremental change

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## 📋 The Problem

Migrating large sync codebases to async is risky:

**Big Bang Rewrite**:
- Rewrite entire codebase at once
- High risk of bugs and regressions
- Long feature freeze
- All-or-nothing deployment
- Team must learn async all at once

**Parallel Maintenance**:
- Maintain old sync and new async versions
- Feature parity challenges
- Double the work
- Confusing for contributors
- Delayed deprecation of old code

**Common blockers**:
- Mixed sync/async dependencies
- Can't change public API (breaking change)
- Some modules easy to convert, others hard
- Need gradual rollout for safety
- Team has varying async experience

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## 💡 Solution with SmartAsync

**Incremental migration path**:

1. **Phase 1: Add decorator to existing code**
   - No behavior change
   - No breaking API changes
   - Sync code still works

2. **Phase 2: Convert internal methods to async**
   - Method by method
   - Keep public API unchanged
   - Each change is isolated and testable

3. **Phase 3: Expose async capability**
   - Users can opt-in to async usage
   - Sync usage still works
   - Zero breaking changes

4. **Phase 4: Optimize**
   - Remove sync-only paths (optional)
   - Document async-first approach
   - Maintain backward compatibility

**Key advantage**: Each step is deployable and reversible.

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## 🎯 Migration Strategies

### Strategy 1: Bottom-Up
- Start with lowest-level utilities
- Move up the stack
- Public API last
- **Risk**: Low (internal changes only)

### Strategy 2: Top-Down
- Start with public API
- Add async support gradually
- Internal sync code offloaded to threads
- **Risk**: Medium (public changes early)

### Strategy 3: Critical Path
- Identify performance bottlenecks
- Convert hot paths first
- Leave cold paths sync
- **Risk**: Low (focused changes)

### Strategy 4: Feature Branches
- New features async-first
- Legacy features stay sync
- Gradual codebase evolution
- **Risk**: Very low (isolated)

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## 🎯 When to Use

**Ideal for**:
- Large legacy codebases
- Public libraries with stable APIs
- Teams transitioning to async
- Risk-averse organizations
- Continuous delivery environments

**Perfect when**:
- Breaking changes are unacceptable
- Need to ship features during migration
- Team has mixed async expertise
- Testing async thoroughly is challenging
- Want data on async benefits before full commit

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## ⚠️ Considerations

### Migration Planning

**Timeline expectations**:
- Small project (< 10K LOC): Weeks
- Medium project (10-100K LOC): Months
- Large project (> 100K LOC): Quarters to years

**Team training**:
- Async fundamentals
- Event loop concepts
- Testing async code
- Debugging async issues

**Technical debt**:
- SmartAsync adds small overhead
- May remove decorator later if all-async
- Document migration progress

### Monitoring

**Track metrics**:
- Percentage of async methods
- Performance improvements
- Error rates during migration
- User adoption of async mode

**Rollback plan**:
- Each phase independently reversible
- Feature flags for async paths
- Monitoring and alerting

### When NOT to Use

**Avoid if**:
- Small codebase (full rewrite faster)
- No async expertise available
- No performance benefit expected
- Dependencies don't support async

**Better alternatives**:
- Full rewrite for small projects
- Separate async-only version (major version bump)
- Stay sync-only if no async benefit

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## 🔗 Related Scenarios

- **04: Unified Library API** - End goal of migration
- **02: Async App → Sync Legacy** - Calling old sync code during migration
- **03: Testing** - Testing during migration

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## 📚 Case Studies

Projects that could benefit:
- **Django ORM** - Async support being added gradually
- **SQLAlchemy** - Added async support over multiple versions
- **Celery** - Gradual async worker support
- **Flask** - Adding async support while maintaining sync compatibility

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## 🎯 Success Criteria

Migration is successful when:
- No user-facing breaking changes
- Performance improved in async mode
- Sync mode still works (backward compatibility)
- Team confident with async patterns
- Production stable throughout migration
- Can remove SmartAsync if desired (fully async)

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**Next Steps**:
- See [04: Unified Library API](04-unified-library-api.md) for API design
- Check [03: Testing](03-testing-async-code.md) for testing strategies