Migration Workflow

Updating code to new versions or patterns: framework upgrades, API changes, pattern migrations.

Trigger

• Framework/library major version upgrade
• Moving to new architectural pattern
• Replacing deprecated APIs
• Technology stack migration

Goal

• Successfully migrate to new version/pattern
• Maintain functionality throughout
• Minimize risk and disruption
• Document the migration for others

Prerequisites

• Clear understanding of target state
• Tests passing before migration
• Rollback plan in place
• Time allocated for verification

Decomposition Strategy

Assess → Plan → Execute → Verify

1. ASSESS: Understand scope
   - What needs to change?
   - What's the impact radius?
   - What are the breaking changes?
   - What's the migration path?

2. PLAN: Create migration strategy
   - Incremental vs. big-bang?
   - What order to migrate?
   - What can be automated?
   - What needs manual attention?

3. EXECUTE: Perform migration
   - Follow the plan
   - Handle exceptions
   - Keep tests passing
   - Commit frequently

4. VERIFY: Confirm success
   - Run full test suite
   - Manual smoke testing
   - Performance validation
   - User acceptance testing

Tools Used

Phase	Tools
Assess	view, text-search, changelogs
Plan	Document, analyze impact
Execute	edit, codemods, refactoring tools
Verify	Tests, normalize analyze, monitoring

Migration Strategies

Incremental Migration

Best for: Large codebases, high-risk changes
Approach:
1. Add compatibility layer
2. Migrate piece by piece
3. Remove compatibility layer
Benefits: Lower risk, can pause/resume
Costs: Longer duration, dual maintenance

Big-Bang Migration

Best for: Small codebases, low-risk changes
Approach:
1. Prepare all changes
2. Apply all at once
3. Fix any issues
Benefits: Faster, cleaner
Costs: Higher risk, harder to debug

Strangler Pattern

Best for: System replacements
Approach:
1. New system handles new features
2. Route some traffic to new system
3. Gradually migrate old features
4. Decommission old system
Benefits: Low risk, easy rollback
Costs: Running two systems

Common Migration Types

Framework Upgrade

1. Read migration guide
2. Update dependencies
3. Fix compile errors
4. Fix deprecation warnings
5. Update tests
6. Verify functionality

API Pattern Change

1. Create new API
2. Update callers to new API
3. Mark old API deprecated
4. Remove old API after grace period

Database Migration

1. Create migration script
2. Add new schema alongside old
3. Backfill data
4. Switch to new schema
5. Remove old schema

Failure Modes

Failure	Detection	Recovery
Incomplete migration	Old patterns still present	Search for old patterns, continue
Hidden dependency	Breaks at runtime	Add tests, fix dependency
Data loss	Data not migrated correctly	Rollback, fix migration script
Performance regression	Slower than before	Profile, optimize or rollback

Example Session

Goal: Migrate from React Router v5 to v6

Turn 1: Assess scope
  $(text-search "react-router-dom" path:package.json)
  → Currently on v5.3.0
  $(text-search "<Route" path:src)
  → 45 route definitions across 12 files

Turn 2: Read migration guide
  → Breaking changes:
    - Switch → Routes
    - Render prop → element prop
    - useHistory → useNavigate
    - Nested routes restructured

Turn 3: Plan migration
  - Incremental: Update component by component
  - Order: Leaf routes first, then containers
  - Automation: Codemod for simple patterns
  - Manual: Complex nested routes

Turn 4: Execute - update dependencies
  $(npm install react-router-dom@6)
  → Compile errors expected

Turn 5: Execute - fix Switch → Routes
  $(text-search "<Switch" path:src)
  → 8 occurrences
  $(edit src/App.tsx)
  → Replace <Switch> with <Routes>

Turn 6: Execute - fix render prop
  $(text-search "render=" path:src)
  → Convert render={...} to element={...}

Turn 7: Execute - fix useHistory
  $(text-search "useHistory" path:src)
  → Replace with useNavigate, update API calls

Turn 8: Verify
  $(npm test)
  → All tests pass
  $(npm start)
  → Manual testing: all routes work

Automation Opportunities

Codemods

• AST-based transformations
• Pattern replacement
• Mass renames

IDE Refactoring

• Find/replace with regex
• Rename symbol
• Change signature

Custom Scripts

• Data migration scripts
• Configuration updates
• Build process changes

Anti-patterns

• Boiling the ocean: Trying to migrate everything at once
• No rollback plan: Can't undo if things go wrong
• Skipping tests: Migrating without verification
• Ignoring deprecations: Leaving deprecated code "for later"
• Over-automation: Automating incorrectly is worse than manual

See Also

• Cross-Language Migration - Language-to-language porting
• Breaking API Changes - Handling upstream breaks
• Refactoring - Internal code restructuring