zero-downtime-migration-planner

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.