• ReplicaSet monitors and maintains the number of replicas of a given pod. It will automatically spawn a new pod if the pod goes down.
• It is needed even if we only have a single pod, because if that pod dies, replica set will spawn a new pod.
• It spans the entire cluster to spawn pods on any node.

Simple Config for a Replicaset

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apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: httpd-frontend
  labels:
    name: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      name: frontend
  template:
    metadata:
      labels:
        name: frontend
    spec:
      containers:
      - name: httpd
        image: httpd:2.4-alpine

• template → metadata and spec from the config file for the pod (required to spawn new pods if any of them goes down)
• replicas → how many replicas to maintain
• It has an additional required field selector which allows the replica set to select pods that match specific labels. This way the replicaset can manage pods that were not created by it.

Scaling the number of replicas

• Recommended: edit the config file and re-apply - k apply -f config.yaml
  • Scaling changes can be easily tracked using Git
• Not recommended: using kubectl - k scale replicaset my-replicaset --replicas=2
  • This will not update the config file, so changes are hard to track