What is ReplicaSet ?

After reading this post you will be understanding the high level of Kubernetes Replicaset, its advantages of this, and basic kubectl commands related to replicaset

If you have not yet checked the previous parts of this series, please go ahead and check this 👉Link

A ReplicaSet is a process that runs multiple instances of a Pod and keeps the specified number of Pods constant. Its purpose is to maintain the specified number of Pod instances running in a cluster at any given time to prevent users from losing access to their application when a Pod fails or is inaccessible.

ReplicaSet helps bring up a new instance of a Pod when the existing one fails, scale it up when the running instances are not up to the specified number, and scale down or delete Pods if another instance with the same label is created. A Replicaset ensures that a specified number of pod replicas are running continuously and helps with load-balancing in case of an increase in resource usage.

• It uses labels to select pods that it should be managing.
• A pod must be labeled with a matching label to the ReplicaSet selector, and it must not be already owned by another controller so that the ReplicaSet can acquire it.
• ReplicaSets can be deleted with or without deleting their dependent pods.
• You can easily control the number of replicas (pods) the ReplicaSet should maintain through the command line or by directly editing the ReplicaSet configuration on the fly.
• Usage of the application we can scale up and down the number of replicas on the fly
• ReplicaSet is the new method of use to setup replication. Technically it has replaced ReplicationController which was used in old technology.

Overview of replicasets

👌Replicaset you can short to rs

Deploying a Sample ReplicationSet in Kubernetes

apiVersion: apps/v1  
kind: ReplicaSet        
metadata:
  name: nginx-proxy
  labels:
    app: nginx-proxy
    tier: frontend 
spec:
  # modify replicas according to your case
  replicas: 3  
  selector:
    matchLabels:
      tier: frontend   
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx

Creating the replica set

kubectl create -f replica.yaml
replicaset.apps/nginx-proxy created

You can then get the current ReplicaSets deployed:

kubectl get replicaset

You can also check on the state of the ReplicaSet:

root@Kubernet-Master:~# kubectl describe replicaset nginx-proxy
Name:         nginx-proxy
Namespace:    default
Selector:     tier=frontend
Labels:       app=nginx-proxy
              tier=frontend
Annotations:  <none>
Replicas:     3 current / 3 desired
Pods Status:  3 Running / 0 Waiting / 0 Succeeded / 0 Failed
Pod Template:
  Labels:  tier=frontend
  Containers:
   nginx:
    Image:        nginx
    Port:         <none>
    Host Port:    <none>
    Environment:  <none>
    Mounts:       <none>
  Volumes:        <none>
Events:
  Type    Reason            Age    From                   Message
  ----    ------            ----   ----                   -------
  Normal  SuccessfulCreate  2m41s  replicaset-controller  Created pod: nginx-proxy-qcmdj
  Normal  SuccessfulCreate  2m41s  replicaset-controller  Created pod: nginx-proxy-5qfzp
  Normal  SuccessfulCreate  2m41s  replicaset-controller  Created pod: nginx-proxy-pgbfk

Scale Up the ReplicaSet

 kubectl scale --replicas=5 -f replica.yaml

output

NAME          DESIRED   CURRENT   READY   AGE
nginx-proxy   5         5         5       102m

Scale down the Replicaset

kubectl scale --replicas=3 -f replica.yaml

output

NAME          DESIRED   CURRENT   READY   AGE
nginx-proxy   3         3         3       103m

Is Our ReplicaSet the Owner of Those Pods?

kubectl get pods nginx-proxy-5qfzp -o yaml | grep -A 5 owner

Output is showing this pod is part of nginx-proxy replicaset

root@Kubernet-Master:~# kubectl get pods nginx-proxy-5qfzp -o yaml | grep -A 5 owner
  ownerReferences:
  - apiVersion: apps/v1
    blockOwnerDeletion: true
    controller: true
    kind: ReplicaSet
    name: nginx-proxy
root@Kubernet-Master:~#

Removing a Pod From a ReplicaSet

kubectl edit pods nginx-proxy-pgbfk

output

Change the pod label to be role

Deleting Replicaset

kubectl delete rs nginx-proxy (replicaset name)
or 
kubectl delete -f replica.yaml

Hope you have got an idea about Kubernetes Replicaset and how we can implement them in our product environments

Happy Learning 📚

Thank you!