Rehost
also called Lift and Shift
Moving a workload to new infrastructure with no change to the application — fastest, cheapest to execute, and the source of the bill that discredits migrations.
Definition
Rehosting moves a workload as-is: the same operating system, the same application, the same configuration, running on a new platform. No code changes.
When it is the right choice
- A data centre exit with a deadline. Rehosting is the only strategy fast enough when a lease expires.
- Applications that will be retired or replaced within a couple of years. Investing in modernisation would be wasted.
- Vendor software you cannot modify.
- As a first step, deliberately, with replatforming planned afterwards. Getting out of the data centre is a legitimate goal on its own.
Why the bill surprises people
On-premises, over-provisioning is a sunk cost — the hardware is bought and idle capacity is free. In the cloud it is recurring. Moving a VM sized for a 2019 peak means paying hourly, forever, for capacity that was already paid for once.
Compounded by data transfer charges for flows that were previously free — cross-availability-zone traffic on every request from a chatty application that never had to care — and by storage provisioned at peak in premium tiers by default.
A bill 2 to 3 times the data-centre cost is common, and it discredits the migration politically even when the strategic reasoning was sound.
The mitigation, planned in advance
Commit to a replatforming phase, funded, immediately after the move: right-size against two weeks of real utilisation, schedule non-production shutdown, add private endpoints so service traffic bypasses NAT charges, and adopt managed services for the components dominating operational time.
Right-sizing plus non-production scheduling alone routinely halves the bill within weeks, and both are configuration changes.
Move data-adjacent workloads together. Splitting a chatty application from its database across the boundary produces latency and transfer charges that can kill a migration.
Failure scenarios
- No replatforming phase, so the temporary bill becomes permanent.
- Application and database split during a phased move.
- No cost allocation tags from day one, making later attribution impossible.
- The source environment never decommissioned, so both are paid for indefinitely.
- Rehosting something that should have been retired. Inventory first — 10–20% of a portfolio typically retires on inspection.
Interview question
"When is lift-and-shift the right migration strategy, and what must be planned alongside it?"