ReplicaSet

What is ReplicaSet ?

Replication Controller and ReplicaSet are the same, but ReplicaSet is the next generation of Replication Controller.

Explanation

The user sends requests to a Service (like a LoadBalancer or ClusterIP).
The Service forwards the requests to the ReplicaSet.
The ReplicaSet manages a set of Pods, ensuring that the desired number of replicas are running.

Replicaset will ensure a specified number of pod replicas are running. It will maintain the desired state of the application by creating or deleting pods as needed to match the defined number of replicas. With this setup, it will ensure high availability and fault tolerance for applications.

Key features:

Self-healing: If a pod fails, it will create a new pod to replace it. Therefore, it will ensure the specified number of pods are running at all times.
Scaling: It can scale up or down the number of pods as needed.
Load Balancing: It can distribute the traffic (share the load) to the pods.

Replication Controller

rc.yaml


apiVersion: v1
kind: ReplicationController
metadata:
  name: rc
spec:
  replicas: 3
  template:
    metadata:
      name: pod-name
    spec:
      containers:
        - name: container-name
          image: web-server-image


kubectl get replicationcontroller

ReplicaSet

When you perform the editing to the replicaset, you will need to delete the previous deployed pods first, as it won’t auto update the pods.

The differences between Replication Controller and ReplicaSet is ReplicaSet needs a selector definition. The selector will be used to identify the pods that ReplicaSet will manage. Furthermore, ReplicaSet can also manage pods that were not created as part of the ReplicaSet creation, as it uses label selectors to identify and manage pods. So if you want to create a new pod, you have to specify template section.

rc.yaml


apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: replicaset-name
spec:
  replicas: 3
  selector:
    matchLabels: # matches the pod labels specified in the template section
      app: web-server
  template:
    metadata:
      name: pod-name
      labels:
        app: web-server
    spec:
      containers:
        - name: container-name
          image: web-server-image


kubectl get replicaset
kubectl scale --replicas=5 -f <file-name>
kubectl scale --replicas=5 replicaset <replicaset-name>
kubectl delete replicaset <replicaset-name>