ReplicaSets in Kubernetes: Core Building Blocks for Application Scaling

ReplicaSets are Kubernetes objects, like everything else in k8s. Their primary function is to ensure that a stable number of replica pods are always running the cluster in accordance with the spec in the YAML file. For this reason, they are instrumental in providing high availability for your application. Typically, you don't work directly with ReplicaSets but rather with Deployments which are a higher level method of ensuring the current state matches the desired state of the cluster.

Obtaining information

The most basic method to list the number of ReplicaSets is using kubectl.

$ kubectl get rs
NAME              DESIRED   CURRENT   READY   AGE
new-replica-set   4         4         0       5m12s

Here we can see that there are 4 desired pods in the ReplicaSet spec, and there are 4 pods running. Even if we deleted one of the pods with kubectl delete pod <name-of-pod>, the ReplicaSet will ensure another pod is running to match the number of replicas, i.e., 4.

Creating a ReplicaSet

Like most objects in k8s, the easiest way to create a ReplicaSet is using a YAML file.

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: replica-set-1
spec:
  replicas: 3
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx

Once the YAML has been defined, it's very simple to create the ReplicaSet.

$ kubectl apply -f replica-set.yaml`
replicaset.apps/replicaset-1 created

Scaling a ReplicaSet

They can be scaled up or down to fit your operational demands. The ReplicaSet controller will determined which pod(s) to delete, in case of a scale down event, or where to run a new pod in the case of a scale up event.

$ kubectl scale rs replica-set --replicas=5

The flag --replicas=5 will now scale the ReplicaSet to 5 pods. This could mean you're scaling down or up to 5 pods. The command is the same, which is useful and efficient.