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.

Also, check

Kubernetes Tutorials