Kubernetes: ReplicaSets

Useful links

• https://kubernetes.io/docs/concepts/workloads/controllers/​replicationcontroller
• https://kubernetes.io/docs/concepts/workloads/controllers/​replicaset/

Concepts

• Both ReplicationController and ReplicaSet are to be used to ensure a specified number of pod replicas are up and running at any given time
• ReplicaSet is actually the successor of ReplicationController and they both serve identical purpose
• Having said that, it’s almost always recommended to use Deployment over managing ReplicationController/ReplicaSet, as the former provides a couple of more useful features on top
• ReplicaSet requires us to define a PodTemplate, which it uses to create new pod(s) when needed
• It is also necessary to specify a selector, which defines the target (pod) it is maintaining
• ReplicaSet offers a richer way to specify label selector (set-based) over ReplicationController

Notes

Declarative manifest

• Simple example of ReplicaSet yaml definition

  apiVersion: apps/v1
  kind: ReplicaSet
  metadata:
  name: nginx-rs
  labels:
    rskey1: rsvalue1
  spec:
    replicas: 3 # number of replica pods
    selector:
      matchLabels:
        podlabel: podvalue # selector
    template: # pod template (just like what we would define in pod yaml)
      metadata:
        labels:
          podlabel: podvalue # label of pod
      spec:
        containers:
          - name: nginx
            image: nginx:1.14.2
  

Imperative commands

• k get rs <replica-set-name> -o wide
• Update number of replicas
  k scale rs <replica-set-name> --replicas=<num>
• Determine ReplicaSet (owner) of a pod
  k get pods <pod-name> -o yaml | grep -A 5 owner
• Delete ReplicaSet and all its pods
  k delete rs <replica-set-name>
• Delete ReplicaSet only
  k delete rs <replica-set-name> --cascade=false