ReplicaSet YAML Walkthrough

  • A ReplicaSet is a process that runs multiple instances of a Pod and keeps the specified number of Pods constant.
  • Its purpose is to maintain the specified number of Pod instances running in a cluster at any given time.

If you are new to the replica set you can read the replica set blog here

Let’s understand the replica set the YAML file.

apiVersion

  • This refers to the version of Kubernetes.
  • API version consists of two things group name and Version.
  • There are several versions, and several objects are introduced with each version.
  • Some common ones are v1, apps/v1, and extensions/v1beta1.
  • On the right-hand side, you can see the different types of values
  • You can check out the below command for the list of versions.
$ kubectl api-versions

kind:

  • This is the type of Kubernetes object. In this case (the example above), we’re creating a ReplicaSet.
  • You can see the other types of objects that we create commonly on the right side here.
  • You can check out the below command for the list of kinds.
$ kubectl api-resources | awk '{print $5}'

metadata:

  • The metadata houses information that describes the object briefly.
  • The information in the metadata usually contains the name you want to give the object (Replicaset in our case), the labels, and the annotation.
  • name the object is the only mandatory field and the remaining are optional.

Spec:

In the spec section, we have three fields — replicas, selector, and template

  • label selector to be used to select the pods, a number of replicas of the pod to be created.
  • In the above example, we are running 3 replicas of nginx container.
  • The selector is used to create pods using matchlabels defined in the template field.
  • The template is a pod template that is used by a selector to create a set of pods.

Now, we can use two types of selectors in replicaset (Equality/Set)

Equality based Selectors (matchLabels)

  • Equality based selectors allow filtering by label keys and values.
  • Three operators used in set based equality based selectors are = , == , !=. The first two represent equality (and are simply synonyms), while the latter represents inequality.
  • For example, if we provide the following selectors:
    app=nginx
  • Here, we’ll select all resources with key equal to app and value equal to nginx-app.

Set-Based Selectors (matchExpressions)

  • Unlike Equality based, Set-based label selectors allow filtering keys according to a set of values.
  • Three kinds of operators used in set-based selectors are in , notin, exists(only the key identifier).
  • Here, the first selector will select all resources with key equal to app and value equal to nginx-app.
  • The second example selects all resources with key equal to tier and values other than frontend.

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DevOps Engineer

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Bikram

Bikram

DevOps Engineer

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