# Migration Helper

> Guide safe database and code migrations with zero-downtime strategies.

- Skill: `nguyenthienthanh/migration-helper` (Agent Skill)
- Install (CLI): `npx skillmds@latest add nguyenthienthanh/migration-helper`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nguyenthienthanh/migration-helper/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: nguyenthienthanh (https://skillmd.com/u/nguyenthienthanh)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nguyenthienthanh/migration-helper

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> **AI-consumed reference.** Optimized for Claude to read during execution.
> Human-readable explanation: see [docs/architecture/HIERARCHICAL_PLANNING.md](../../../docs/architecture/HIERARCHICAL_PLANNING.md)
> or [docs/getting-started/](../../../docs/getting-started/) depending on topic.


# Skill: Migration Helper

Safe database and code migrations with zero-downtime strategies.

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## Safety Rules

1. Always backup before migration
2. Test on staging first
3. Make migrations reversible
4. Deploy in small batches
5. Monitor during migration

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## Related Rules

- `rules/core/simplicity-over-complexity.md` — smallest migration that closes the gap; no "while we're here" schema expansions
- `rules/core/verification.md` — verify each migration step before advancing
- `rules/workflow/immutable-workflow.md` — migrations are append-only records
- `rules/workflow/dual-llm-review.md` — destructive migrations (DROP COLUMN/TABLE) trigger dual-LLM review

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## Schema Changes

| Change | Safe Approach |
|--------|---------------|
| Add column | Add nullable → backfill → NOT NULL |
| Remove column | Stop using → deploy → drop |
| Rename column | Add new → copy → deploy → drop old |
| Add index | CREATE INDEX CONCURRENTLY |
| Change type | Add new col → migrate → drop old |

### Zero-Downtime (6 phases)

Add nullable → dual-write → backfill → switch reads → remove old writes → drop old column.

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## Migration Script Pattern

```typescript
export async function up(db: Database) {
  await db.query(`ALTER TABLE users ADD COLUMN status VARCHAR(20) DEFAULT 'active'`);
  // Backfill in batches of 1000
  // Add NOT NULL constraint
}
export async function down(db: Database) {
  await db.query(`ALTER TABLE users DROP COLUMN status`);
}
```

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## Code Migration

- **Feature flags:** Behind flag → gradually increase % → remove flag + old code
- **Strangler Fig:** New impl alongside old → route traffic gradually → remove old

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## Checklist

**Before:** Backup, tested on staging, rollback plan, monitoring alerts.
**During:** Batch processing, monitor performance, verify integrity.
**After:** Validation queries, smoke tests, monitor 24-48h, document.

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## Rollback Strategies

| Strategy | When |
|----------|------|
| Script rollback | Reversible data changes |
| Backup restore | Major failure |
| Feature flag off | Code changes |
| Traffic reroute | Service migration |

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