KodeCloud Kubernetes ReplicaSets

Links: 111 KodeCloud Index

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ReplicaSet

• Replication Controller is the older technology that is being replaced by Replica Set.

  • The major difference between Replication Controller and Replica Set is selector

• The Replica Set has 3 major components under spec

  • selector: this decides what pods are managed by the replica set.
  • template: pod template
    • 
  • replicas

• Sample Replica Set YAML file:

  apiVersion: apps/v1
  kind: ReplicaSet
  metadata:
    name: myapp-replicaset
    labels:
      app: myapp
  spec:
    selector:
      matchLabels:
        env: production
    replicas: 3
    template:
      metadata:
        # This name will not be used for naming the pods. 
        # Name will be <replicaset-name>-<random-hash>
        # In this case it will be myapp-replicaset-<random-hash>
        name: nginx-2 
        labels:
          env: production
      spec:
       containers:
          - name: nginx
            image: nginx
  

• In the above example note that labels of the pod is same as that of the matchLabels of the replica set.

  • They must be the same for pods to be managed by this replica set.

Since Replica Set has its own naming convention for the pods we can omit the name field.

So this means in the above example yaml file name: nginx-2 can be omitted.

• The main use cases of Replica Set are:
  • Ensuring the required number of pods are always maintained
  • Scaling the application.

The main use case of selector label is that Replica Set can also manage pods that were not created as a part of the Replica Set creation.

• For example some pods were created before the creation of Replica Set but match the labels specified in the Replica Set definition file. Then Replica Set will also take these pods into consideration.
• Now suppose if you have a Replica Set with no of replicas set to 3 and have 3 pods running. Now if you try to create and a pod then with the same selector Replica Set will try to delete the new pod since the desired number of replicas is 3.

• We can use k describe replicaset/<replica-set-name> to see the selector of that Replica Set and view the events.
  • You can use rs as a short form for replicaset

How to get the apiVersion of a specific resource to be used in a yaml file?

k explain <resource-name> | grep VERSION

Hence for replica set its version is apps/v1

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For pods it is just v1

• Deleting resources: k delete <resource-type>/<resource-name>
• Kubernetes supports self-healing applications through ReplicaSets and Replication Controllers.
  • The replication controller helps in ensuring that a POD is re-created automatically when the application within the POD crashes.

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Last updated: 2023-05-03