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!🌊

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!!!🚣🏻