Deployments

Rollouts via ReplicaSets

A Deployment never runs pods itself — it manages ReplicaSets. When the pod template changes, it creates a new ReplicaSet and shifts weight while honoring maxUnavailable and maxSurge:

strategy:
  type: RollingUpdate
  rollingUpdate:
    maxUnavailable: 0
    maxSurge: 1

maxUnavailable: 0 keeps ready capacity at or above replicas; maxSurge: 1 allows exactly one extra pod during the swap. Progress depends on readiness — pods that never pass the readiness probe stall the rollout, and progressDeadlineSeconds (default 600) marks it failed.

Old ReplicaSets are retained at zero replicas as rollout history.

Controlling a Release

kubectl set image deploy/web app=registry.example.com/web:1.5.0
kubectl rollout status deploy/web
kubectl rollout pause deploy/web
kubectl rollout resume deploy/web
kubectl rollout history deploy/web
kubectl rollout undo deploy/web --to-revision=3

Pausing freezes the Deployment's reactions so you can apply several edits — labels, env vars, images — before resuming once. Pair releases with a PodDisruptionBudget so voluntary evictions cannot empty the fleet mid-rollout.

rollout status exits non-zero on failure, which makes it the CI gate: never mark a deploy successful because the command returned without drama.

Pitfalls and Interview Angles

Pitfall 1 — readiness that does not reflect serving ability. With maxUnavailable: 0, traffic safety rests entirely on the readiness probe; a trivial probe lets the rollout replace healthy pods with broken ones.

Pitfall 2 — changes that trigger no rollout. Only pod-template edits create a new ReplicaSet — editing Deployment labels or pointing at an image tag that did not move does nothing. revisionHistoryLimit (default 10) controls how many old revisions remain available to undo.

Interview-ready: compute surge and unavailable numbers for replicas=3, explain the pod-template-hash label separating ReplicaSets, and clarify rollback scope — undo swaps the pod template back; it never restores deleted Services or ConfigMaps.