Replication Controller & Replica Sets

✅ Replication Controller

Replication Controller is one of the key features of Kubernetes, which is responsible for managing the pod lifecycle. It is responsible for making sure that the specified number of pod replicas are running at any point of time. It is used in time when one wants to make sure that the specified number of pod or at least one pod is running. It has the capability to bring up or down the specified no of pod.

It is a best practice to use the replication controller to manage the pod life cycle rather than creating a pod again and again.

apiVersion: v1
kind: ReplicationController --------------------------> 1
metadata:
   name: Tomcat-ReplicationController --------------------------> 2
spec:
   replicas: 3 ------------------------> 3
   template:
      metadata:
         name: Tomcat-ReplicationController
      labels:
         app: App
         component: neo4j
      spec:
         containers:
         - name: Tomcat- -----------------------> 4
         image: tomcat: 8.0
         ports:
            - containerPort: 7474 ------------------------> 5

Setup Details

• Kind: ReplicationController → In the above code, we have defined the kind of replication controller which tells the kubectl that the yaml file is going to be used for creating the replication controller.

• name: Tomcat-ReplicationController → This helps in identifying the name with which the replication controller will be created. If we run the kubctl, get rc < Tomcat-ReplicationController > it will show the replication controller details.

• replicas: 3 → This helps the replication controller to understand that it needs to maintain three replicas of a pod at any point of time in the pod lifecycle.

• name: Tomcat → In the spec section, we have defined the name as tomcat which will tell the replication controller that the container present inside the pods is tomcat.

• containerPort: 7474 → It helps in making sure that all the nodes in the cluster where the pod is running the container inside the pod will be exposed on the same port 7474.

✅ Replica Sets

Replica Set ensures how many replicas of pod should be running. It can be considered as a replacement of replication controller. The key difference between the replica set and the replication controller is, the replication controller only supports equality-based selector whereas the replica set supports set-based selector.

apiVersion: extensions/v1beta1 --------------------->1
kind: ReplicaSet --------------------------> 2
metadata:
   name: Tomcat-ReplicaSet
spec:
   replicas: 3
   selector:
      matchLables:
         tier: Backend ------------------> 3
      matchExpression:
{ key: tier, operation: In, values: [Backend]} --------------> 4
template:
   metadata:
      lables:
         app: Tomcat-ReplicaSet
         tier: Backend
      labels:
         app: App
         component: neo4j
   spec:
      containers:
      - name: Tomcat
      image: tomcat: 8.0
      ports:
      - containerPort: 7474

Setup Details

• apiVersion: extensions/v1beta1 → In the above code, the API version is the advanced beta version of Kubernetes which supports the concept of replica set.

• kind: ReplicaSet → We have defined the kind as the replica set which helps kubectl to understand that the file is used to create a replica set.

• tier: Backend → We have defined the label tier as backend which creates a matching selector.

• {key: tier, operation: In, values: [Backend]} → This will help matchExpression to understand the matching condition we have defined and in the operation which is used by matchlabel to find details.