How to migrate to AWS without downtime: a step-by-step plan

Short answer: inventory everything, codify the target environment in Terraform, replicate data continuously, then shift traffic gradually with a tested rollback. Done right, users never notice.

Phase 1: Inventory and assess

List every application, database, cron job, DNS record and third-party dependency. For each one, decide whether to rehost (move as-is), replatform (e.g. move MySQL to RDS) or refactor (containerise and redesign).

Phase 2: Design the landing zone

Set up AWS Organizations with separate accounts for production, staging and shared services. Define VPCs, subnets, IAM roles and guardrails before anything moves.

Phase 3: Codify everything

Write the target infrastructure in Terraform. This makes environments reproducible, reviewable and easy to roll back.

module "app" {
  source        = "./modules/ecs-service"
  name          = "api"
  desired_count = 3
  cpu           = 1024
  memory        = 2048
}

Phase 4: Replicate data

Use AWS DMS or native replication to keep the new database in sync with the old one. Test restores. Measure replication lag under real load.

Phase 5: Shift traffic gradually

Lower DNS TTLs a week in advance. Use weighted routing to send 5%, then 25%, then 100% of traffic to AWS, watching error rates and latency at each step. Keep the old environment hot until you are confident.

Phase 6: Optimise

After migration, right-size instances, buy Savings Plans for steady workloads and move cold data to cheaper storage classes. Our clients typically save 25–45% in the first three months.

Common mistakes

• Migrating without monitoring in place first.
• Forgetting scheduled jobs and email sending.
• Lifting oversized servers 1:1 into the cloud.

Planning a migration? Our Cloud Audit gives you a full plan and cost forecast in two weeks.