deployments

A ReplicaSet keeps a count. A Deployment manages a sequence of ReplicaSets, and that is the entire difference: it is what turns a change into a rollout you can pause and undo.

The gating instinct is right and the cardinality is wrong: init containers make every new replica race the same migration. Here is the once-per-release shape, plus the rollback rule that shapes all DDL review.

The reflex is rollback and it is usually right. Shared state, changed credentials and unavailable old artifacts can make an undo incomplete or unsafe. Verify those dependencies before choosing the recovery.

An unchanged pipeline can fail because of environment state, external inputs or a latent artifact defect. Failures that start on an expiry boundary narrow the field to two suspects fast.