Two Versions at Once: Progressive Release and Change Compatibility
The rollback button worked. The migration had already dropped the column the old version reads, so the old version would not start.
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Blue-Green Deployment
Switching traffic back is fast. It does not un-write what the new version already wrote, which is the part worth planning for.
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Expand and Contract
Add the new shape, move the callers, then remove the old one. The middle phase is the long one, and for external clients it is measured in quarters.
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Feature Flag
A release toggle that outlives its two weeks has stopped being a delivery tool. It is now a permanent branch in your code.