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In this article, we'll talk about Kubernetes ReplicaSet. Being familiar with the earlier articles in this series is helpful for a better understanding.

What Is a ReplicaSet?

• A ReplicaSet is a Kubernetes object that ensures a specified number of pods of a given template are running at any given time
• It guarantees the availability of a specified number of identical pods
• Most of the time, we don't directly manage ReplicaSets
• Instead, we use Deployments for managing ReplicaSet

Creating a ReplicaSet With a Manifest File

• To understand how ReplicaSets work, let’s start with the manifest file
• Here’s an example of a ReplicaSet manifest file:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: my-replicaset
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
        - name: my-container
          image: nginx
          ports:
            - containerPort: 80

Then we can create this ReplicaSet using kubectl with one of the following commands

kubectl create -f my-replicaset.yaml

or

kubectl apply -f my-replicaset.yaml

The above manifest file creates ReplicaSet my-replicaset

Key Fields in the ReplicaSet Manifest File

• .spec.replicas:
  • Number of pods managed by the ReplicaSet
  • In the above ReplicaSet, we have 3 pods
  • If we don't specify a value, then the default value is 1
• .spec.template:
  • The pod template used to create the ReplicaSet’s pods
• .spec.template.metadata.labels:
  • labels of the pods created from the pod template
  • These labels must match .spec.selector.matchLabels labels
  • In the above ReplicaSet, the labels of the pods are app: my-app
• .spec.selector.matchLabels:
  • Replicaset identifies the pods required to be managed by the ReplicaSet from these labels
  • ReplicaSet will manage pods with matching labels (even if not created by the ReplicaSet)
  • e.g., if 2 pods with the label app: my-app already exist, the ReplicaSet will create only 1 more pod but manage all 3 pods
  • ReplicaSets can also use advanced label selectors like matchExpressions instead of matchLabels

How a ReplicaSet Links to Its Pods

• A ReplicaSet links to its pods via the metadata.ownerReferences field in the pod's metadata.
• Field metadata.ownerReferences specifies the owner of the object
• This field specifies the ReplicaSet my-replicaset as the owner
• Here's an example output YAML of a pod managed by my-replicaset

apiVersion: v1
kind: Pod
metadata:
  generateName: my-replicaset-
  labels:
    app: my-app
  name: my-replicaset-dxzzg
  namespace: default
  ownerReferences:
    - apiVersion: apps/v1
      blockOwnerDeletion: true
      controller: true
      kind: ReplicaSet
      name: my-replicaset
      uid: a93e48a6-9ec6-46e6-b92f-c61ccd9a2d40
  resourceVersion: "472"
  uid: 6cf20e31-a73a-4846-ad8d-a4b41e7711ac
spec:
  containers:
    - image: nginx
      imagePullPolicy: Always
      name: my-container
      ports:
        - containerPort: 80
          protocol: TCP

Deleting a ReplicaSet

We can delete the above ReplicaSet using kubectl with one of the following commands

kubectl delete -f my-replicaset.yaml

or

kubectl delete rs my-replicaset

Replicaset vs. Replication Controller

• ReplicaSet is the next generation of Replication Controller
• Kubernetes documentation recommends using ReplicaSets instead of Replications Controllers

Summary

In this article, we discussed the Kubernetes ReplicaSet. We looked at what ReplicaSet is, how it works, and how to create and delete a ReplicaSet.

References

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