migration-helper · diff
git:20260410.d55a9ea to git:20260428.bdee323
9 added, 0 removed. Audit A to A.
---
name: migration-helper
description: "Guide safe database and code migrations with zero-downtime strategies."
autoInvoke: false
priority: medium
triggers:
- "database migration"
- "code migration"
- "zero-downtime"
---
# Skill: Migration Helper
Safe database and code migrations with zero-downtime strategies.
---
## 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
---
+ ## 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
+
+ ---
+
## 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.
---
## 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`);
}
```
---
## Code Migration
- **Feature flags:** Behind flag → gradually increase % → remove flag + old code
- **Strangler Fig:** New impl alongside old → route traffic gradually → remove old
---
## 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.
---
## Rollback Strategies
| Strategy | When |
|----------|------|
| Script rollback | Reversible data changes |
| Backup restore | Major failure |
| Feature flag off | Code changes |
| Traffic reroute | Service migration |
---