ReplicaSets

Keeping N identical pods alive with a selector.

A ReplicaSet has one job: keep exactly N pods matching a label selector running. Too few and it creates more; too many and it deletes some. It is the mechanism behind scaling and self-healing — though you almost never create one by hand.

apiVersion: apps/v1
kind: ReplicaSet
metadata: { name: web }
spec:
  replicas: 3
  selector:
    matchLabels: { app: web }        # the pods this RS owns
  template:
    metadata:
      labels: { app: web }           # MUST match the selector
    spec:
      containers: [{ name: app, image: nginx:1.25 }]

The selector is the contract: a ReplicaSet owns every pod matching it, even ones it did not create. The template labels must satisfy the selector: if they do not, the API server rejects the ReplicaSet at apply time with `selector` does not match template `labels` — apps/v1 validates this on every create and update, so the object is never created rather than running away. Always make the template labels a superset of the selector.