Deterministic updates through instance replacement

With Deterministic upgrades through versioned repositories on AL2027 in AL2027, you can use instance replacement as a deterministic and safe way to roll out updated versions of AL2027. As you progressively roll out a new version, you can revert to the previous AMI at any point while you determine the cause of an issue.

Using instance replacement rather than patching in-place makes updates more deterministic and predictable. Launching new capacity is a well-tested code path with clear A and B states. You can fully test both states in a CI/CD system before deployment starts. In-place patching passes through many intermediary states between before and after applying updates, which is harder to test for all combinations.

An OS update strategy of instance replacement with deterministic updates fits well into blue/green, wave, and phase based deployment models.