ReplicaSets

Reconciliation in Miniature

A ReplicaSet stores a desired count and a selector; its controller counts pods that satisfy both the label selector and an ownerReference pointing at the ReplicaSet, then creates or deletes until the counts agree.

selector:
  matchLabels: { app: web, tier: frontend }
template:
  metadata:
    labels: { app: web, tier: frontend, track: stable }

Extra labels on pods are harmless; omitting a required selector label means the pod falls outside the count. ownerReferences do two jobs: they stop two controllers from fighting over one pod, and they enable cascading garbage collection when the ReplicaSet itself is deleted.

Scaling and Inspection

kubectl scale replicaset/web-stable --replicas=6
kubectl get rs -l app=web \
  -o custom-columns=NAME:.metadata.name,DESIRED:.spec.replicas,READY:.status.readyReplicas
kubectl delete rs web-stable --cascade=orphan   # keep the pods

Reaching into a ReplicaSet that a Deployment owns is usually a mistake — the Deployment reconciles the count right back. Scale the Deployment instead. Standalone ReplicaSets still fit fixed pools of workers you manage without rollouts.

Note spec.minReadySeconds: a pod counts as available only after remaining Ready that long, which slows rollout accounting rather than the scheduler.

Pitfalls and Interview Angles

Pitfall 1 — immutable selectors. spec.selector cannot change after creation. If template labels drift from the selector, the ReplicaSet reports desired=N with zero matching pods and emits no useful error beyond events.

Pitfall 2 — accidental adoption. A pod whose labels match but whose ownerReference points elsewhere will not be adopted; the controller creates new pods instead, leaving duplicates that confuse capacity accounting.

Interview-ready: explain the count-match loop, why Deployments layer ReplicaSets on top — rollout history plus traffic shifting via the pod-template-hash label — and how you debug "RS exists but no pods": selector versus template labels, scheduler events, then resource requests.