--- name: zero-downtime-migration-planner description: Plans blue-green rollouts, canary deployments, and expand-and-contract schema migrations with zero downtime. ---

## Purpose

Formulate zero-downtime deployment and database migration blueprints (Expand/Contract pattern, Parallel Run, Blue-Green, Canary) allowing continuous availability during breaking changes.

## When to Use

- Renaming or splitting database columns in high-traffic tables. - Replacing a core microservice or authentication provider without maintenance windows. - Executing major database engine upgrades with replication cutovers.

## What to Analyze

1. **Expand & Contract Phases**: - *Phase 1 (Expand)*: Add new nullable column/table alongside old one. - *Phase 2 (Dual Write)*: App writes to both old and new columns, reads from old. - *Phase 3 (Backfill)*: Background job migrates historical data. - *Phase 4 (Read Switch)*: App switches reads to new column. - *Phase 5 (Contract)*: Stop writing to old column and drop it. 2. **Backward Compatibility**: Ensure running v1 and v2 app versions can simultaneously operate on the database. 3. **Traffic Splitting & Canary Gates**: 1% $ ightarrow$ 5% $ ightarrow$ 25% $ ightarrow$ 100% traffic shifting with automated rollback triggers. 4. **Locking & DDL Safety**: Avoiding exclusive table locks (`ACCESS EXCLUSIVE`) during migration runs.

## Output Format

- **Multi-Phase Deployment Roadmap**: Numbered phases with release gates. - **Database DDL / Migration Scripts**: Safe non-blocking migration code. - **Rollback Procedure per Phase**: How to abort at any phase without data loss.

## Avoid

- Renaming a database column directly in a single deployment (causes instant 500 errors). - Running large table updates in a single blocking transaction.