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Hrittik Bhattacharjee
Hrittik Bhattacharjee

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Bite-Sized Kubernetes Part 3 - ReplicaSets

Aye Aye!☸️

In Part 2 we looked at pods in kubernetes.
Let us now venture further with Controllers and ReplicaSets!🌊

OBJECT DESCRIPTION
Pods The lowest level, single instance of an application
👉 ReplicaSets The next level, manages a set of pods
Deployments The next level, manages a set of ReplicaSets
Services The next level, manages a set of deployments
Ingress The next level, manages a set of services
Volumes The next level, manages a set of ingress
Namespaces The next level, manages a set of volumes
Cluster The highest level, manages a set of namespaces

Controllers and ReplicaSets:

  • A controller is a k8s object that is responsible for managing a set of pods.
  • Replication controller:
    • A controller that is responsible for managing a set of pods.
    • It ensures that the specified number of pods are running at any given time.
    • It is responsible for creating new pods if the number of pods falls below the specified number.
    • It is responsible for deleting pods if the number of pods exceeds the specified number.
    • It is responsible for updating the pods if the pod template is changed.
    • It helps to ensure that multiple pods are running at all times, thus guaranteeing "high availability" of the application.
    • It is also responsible for load balancing and scaling the number of pods up and down.
    • Replication controller can span across multiple nodes.
  • Replication controller is a legacy object, and is replaced by ReplicaSet
  • Creating replication controllers:

    • Example rc-definition.yml:

      apiVersion: v1
      kind: ReplicationController
      metadata:
        name: my-app-rc
        labels:
          app: my-app
          type: my-type
      spec:
        # the template is the pod template that will be used to create the pods
        # all the contents of the pod-definition.yml file can be copied here, except the apiVersion and kind
        template:
          metadata:
            name: my-pod
            labels:
              app: my-app
              type: my-type
          spec:
            containers:
            - name: my-container
              image: my-image-name:my-image-tag
        # the number of replicas that should be running at any given time
        replicas: 5
      
    • Once the .yml file is created, it can be used to create a replication controller:

      $ kubectl create -f <file name>.yml
      
    • See list of replication controllers:

      $ kubectl get replicationcontroller
      
    • See list of pods (those created by the replication contrller will have the name of the replication controller, here my-app-rc, followed by a hash suffix):

      $ kubectl get pods  
      
  • Creating ReplicaSets:

    • Example rs-definition.yml:

      apiVersion: apps/v1 # not v1
      kind: ReplicaSet
      metadata:
        name: my-app-rs
        labels:
          app: my-app
          type: my-type
      spec:
        # the template is the pod template that will be used to create the pods
        # all the contents of the pod-definition.yml file can be copied here, except the apiVersion and kind
        template:
          metadata:
            name: my-pod
            labels:
              app: my-app
              type: my-type
          spec:
            containers:
            - name: my-container
              image: my-image-name:my-image-tag
        # the number of replicas that should be running at any given time
        replicas: 5
        # selector is needed for the ReplicaSet, not needed for replication controller
        # this identifies the pods that it is managing
        # this is because ReplicaSet can also manage pods that were created manually or already created pods 
        # i.e. those that were not a part of the ReplicaSet pod creation process
        # this ReplicaSet will only manage the pods that have the labels app=my-app and type=my-type
        selector:
          matchLabels:
            app: my-app
            type: my-type
      
    • Once the .yml file is created, it can be used to create a ReplicaSet:

      $ kubectl create -f <file name>.yml
      
    • See list of ReplicaSets:

      $ kubectl get replicaset
      
    • See list of pods (those created by the ReplicaSet will have the name of the ReplicaSet, here my-app-rs, followed by a hash suffix):

      $ kubectl get pods  
      
    • Edit a ReplicaSet:

      $ kubectl edit replicaset <replicaset name>
      
      # opens up vi editor
      # press i to enter insert mode, make necessary changes
      # press 'esc' to exit insert mode
      # type ':wq!' to save and exit
      
      # then delete all the pods managed by the ReplicaSet and then the ReplicaSet will create new pods with the updated pod template
      
  • Scaling up or down:

    • We can scale up the number of pods by updating the replicas property in the .yml file and then run the kubectl replace command:

      $ kubectl replace -f <file name>.yml
      
    • Or we can use the kubectl scale command:

      $ kubectl scale --replicas=<number of replicas> -f <file name>.yml
      
    • Delete a ReplicaSet and all pods managed by it:

      $ kubectl delete replicaset <replicaset name>
      

Great going sailor, Posiedon watches over ya!🔱
Now that you understand ReplicaSets, let us move on to "Deployments" in Part 4.

Heave Ho!!!🚣🏻

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