Most cloud migrations that cause an outage don’t fail because the new infrastructure was wrong. They fail because the cutover itself, the moment DNS or a load balancer starts sending real traffic to the new environment, was treated as a single big-bang event instead of a series of small, reversible steps.
Run Both Environments in Parallel First
The safer pattern is to mirror writes to both the old and new databases for a period, and validate that the new environment produces identical results under real production traffic before it ever serves a real user. This costs more up front than a straight lift-and-shift, and it is the difference between a migration nobody notices and one that pages your on-call team at 2am.
Shift Traffic Gradually
Traffic should move in stages, not all at once:
- Route a small percentage of real users to the new environment first
- Watch error rates and latency at that percentage before increasing it
- Keep a tested rollback path ready, not designed as an afterthought once something has already gone wrong
- Increase the percentage only once the previous step has held steady under real load
Observability Has to Exist Before Cutover
If your new environment’s logs, metrics and traces aren’t wired together and actually being watched before cutover, you find out about problems from customer complaints instead of a dashboard, which is the worst possible way to learn about them.
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