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

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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