# Monolith → ECS Fargate Zero-Downtime Cutover (July 2026)

## Before containers

- [ ] Scale-in-place ladder considered (don’t containerize a dying host)
- [ ] Dockerfile + local health endpoint
- [ ] Image in ECR; same DB connection strategy for wave 1 (lift app first)

## Parallel path

- [ ] ALB with blue (legacy) + green (Fargate `target-type=ip`) target groups
- [ ] Green healthy under smoke tests
- [ ] Weighted shift: 10% → 50% → 100% with error/p95 gates
- [ ] Keep blue **2–4 weeks** as rollback

## Do not

- Migrate DB + app in the same cutover unless rehearsed
- Kill blue after one quiet hour
- Skip security-group egress to DB/cache