DEV Community

Cover image for Kubernetes ReplicaSet
Dilanka Rathnasiri
Dilanka Rathnasiri

Posted on

Kubernetes ReplicaSet

#kubernetes #containers #devops

cover image: Photo by frank mckenna on Unsplash

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
Enter fullscreen mode Exit fullscreen mode

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

kubectl create -f my-replicaset.yaml
Enter fullscreen mode Exit fullscreen mode

or

kubectl apply -f my-replicaset.yaml
Enter fullscreen mode Exit fullscreen mode

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
Enter fullscreen mode Exit fullscreen mode

Deleting a ReplicaSet

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

kubectl delete -f my-replicaset.yaml
Enter fullscreen mode Exit fullscreen mode

or

kubectl delete rs my-replicaset
Enter fullscreen mode Exit fullscreen mode

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

Top comments (0)