Why it matters
Choosing the wrong migration pattern is a classic large-scale IT mistake. Rehosting everything wastes cloud value; rebuilding everything takes years and risks failure. The right pattern per workload is the goal.
The architecture
Rehost (lift-and-shift): move as-is to VMs in cloud. Fastest, cheapest migration but doesn't leverage cloud value. Good for time pressure or datacenter exit.
Replatform (lift-tinker-shift): small changes to leverage cloud managed services. Move DB to RDS, keep app on VMs. Better cloud value with less risk than refactor.
Repurchase: replace with SaaS. Retire the on-prem app entirely; buy Salesforce, Workday, etc.
How it works end to end
Refactor: substantial changes to leverage cloud-native services. Break monolith into microservices, use managed queues, adopt event-driven patterns.
Rearchitect/Rebuild: rewrite from scratch with cloud-native design. Highest cloud value but highest risk and cost.
Retain: leave on-prem. Some apps aren't cloud candidates due to latency, regulation, or unclear ROI.
Retire: turn it off. Migration is an opportunity to shed unused apps.