Cloud data migration changes cost, latency, access control and failure behaviour — not just where the data sits. Plan for query patterns that are now billed, application call patterns that now cross a network, a move made in slices rather than one cutover, validation that runs continuously during parallel running, and an agreed date to switch the source off.
Cost behaves differently and it shows up late
On-premises, a poorly written query is slow. In the cloud it is slow and billed. Scanning patterns that were free become a line item, egress between regions becomes a line item, and idle capacity that used to be a sunk cost becomes a recurring one.
Model this before committing to a design. Partitioning, file formats and storage tiers are far cheaper to decide up front than to change once several teams depend on the layout.
Latency changes what the application can assume
Applications built beside a database often make many small calls, because each was cheap. Put a network between them and the same pattern becomes the dominant cost in a page load.
Find these before the move by measuring call counts, not just query times. The fix is usually batching or a read model close to the consumer, and both are design decisions rather than tuning.
Move in slices, not in one attempt
A single cutover concentrates every risk into one window. Where the data model allows it, move a bounded slice first — one region, one product line, one class of record — and run it in parallel long enough to compare.
Dual-running costs more for a period, and it is almost always cheaper than discovering a systematic difference after the source has been decommissioned.
Validate continuously, not once
Reconciliation at the end tells you whether the final state matches. It does not tell you when a difference appeared. Comparing on a schedule during the parallel period turns a large end-of-project investigation into a small daily one.
Compare what the business recognises — totals, counts by segment, a set of known records — rather than only technical checksums. A checksum tells you the bytes match; a total tells you the meaning does.
Decide what happens to the source
Leaving the old platform running indefinitely is the most common outcome and the most expensive one. Agree the retention period, what is archived, who confirms the new system is authoritative, and the date the source is switched off — before the migration starts, while the question is still easy to answer.
