Replica Sets

Replica Sets are the resources that help you make sure that a specific number of pods are running at any given time. If a pod fails, the Replica Set will create a new one to replace it.

Managing pods with Replica Sets

Replica Sets use selectors, like labels, to identify the pods they should manage. When you create a Replica Set, you specify a selector. This selector tells the Replica Set which pods to manage.

Navigate to the simple-replicaset directory:

cd bootstrapping-with-kubernetes-examples/deploy/simple-replicaset

The replicaset.yaml file in this directory contains the following configuration:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: simple-replicaset
  labels:
    env: dev
    app: simple-replicaset
spec:
  replicas: 3
  selector:
    matchLabels:
      app: simple-replicaset
  template:
    metadata:
      labels:
        app: simple-replicaset
    spec:
      containers:
      - name: apiserver
        image: rutush10/simple-restapi-server-py:v0.0.1
        ports:
        - containerPort: 8000
        resources:
          requests:
            cpu: 100m
            memory: 100Mi
          limits:
            cpu: 200m
            memory: 200Mi

To create the Replica Set, run the following command:

kubectl apply -f replicaset.yaml

You can check the status of the Replica Set using the following command:

kubectl get replicaset

$ kubectl get replicaset
NAME                DESIRED   CURRENT   READY   AGE
simple-replicaset   3         3         3       6m54s

To check the pods managed by the Replica Set, run:

kubectl get pods --show-labels

$ kubectl get pods --show-labels
NAME                      READY   STATUS    RESTARTS   AGE     LABELS
simple-replicaset-bzgpf   1/1     Running   0          7m47s   app=simple-replicaset
simple-replicaset-hj9th   1/1     Running   0          7m47s   app=simple-replicaset
simple-replicaset-xm2nm   1/1     Running   0          7m47s   app=simple-replicaset

There is a file named pod.yaml in the same folder, go ahead and create a pod using the following command:

kubectl apply -f pod.yaml

$ kubectl get pods --show-labels
NAME                      READY   STATUS    RESTARTS   AGE     LABELS
simple-pod                1/1     Running   0          3s      <none>
simple-replicaset-bzgpf   1/1     Running   0          8m58s   app=simple-replicaset
simple-replicaset-hj9th   1/1     Running   0          8m58s   app=simple-replicaset
simple-replicaset-xm2nm   1/1     Running   0          8m58s   app=simple-replicaset

You can see that the pod simple-pod is not managed by the Replica Set, as it doesn't have the label app: simple-replicaset.

To see the label selectors of the replicaset working, we will add the label app=simple-replicaset to the pod. Update the metadata field in the pod.yaml file and add the label there, as shown below:

metadata:
  name: simple-pod
  labels:
    app: simple-replicaset

Now, if you run apply on the updated configuration again, kubectl will give the following output:

$ kubectl apply -f pod.yaml
pod/simple-pod configured

Check the pods again:

$ kubectl get pods --show-labels
NAME                      READY   STATUS        RESTARTS   AGE     LABELS
simple-pod                1/1     Terminating   0          36s     app=simple-replicaset
simple-replicaset-bzgpf   1/1     Running       0          9m31s   app=simple-replicaset
simple-replicaset-hj9th   1/1     Running       0          9m31s   app=simple-replicaset
simple-replicaset-xm2nm   1/1     Running       0          9m31s   app=simple-replicaset

You can see that the pod simple-pod is in Terminating state. This happens because the controller ReplicationController detected that the pod has the label app: simple-replicaset and since there are already 3 pods managed by the Replica Set, it terminated the pod simple-pod.

Similarly, go ahead and remove the label app=simple-replicaset from a pod and see what happens.

kubectl label pod simple-replicaset-bzgpf app-

And check the pods again:

$ kubectl get pods --show-labels
NAME                      READY   STATUS              RESTARTS   AGE   LABELS
simple-replicaset-bzgpf   1/1     Running             0          14m   <none>
simple-replicaset-brbmr   0/1     ContainerCreating   0          2s    app=simple-replicaset
simple-replicaset-hj9th   1/1     Running             0          14m   app=simple-replicaset
simple-replicaset-xm2nm   1/1     Running             0          14m   app=simple-replicaset

A new pod simple-replicaset-brbmr is created by the Replica Set to maintain the desired state of 3 pods. While the old pod simple-replicaset-bzgpf is still running, it is no longer managed by the Replica Set.

This is an example of how labels and selectors are used by Kubernetes to manage resources. The same concept is used by other resources like Deployments and Services, as we'll see in the upcoming sections.

Understanding the Replica Set manifest

Let's break down the replicaset.yaml manifest:

  • apiVersion: apps/v1: This tells Kubernetes to use the apps/v1 API group.
  • kind: ReplicaSet: This tells Kubernetes that we are creating a Replica Set.
  • metadata: This is the metadata for the Replica Set.
    • name: nginx-replicaset: The name of the Replica Set.
    • labels: The labels for the Replica Set. Here, we have labels env: dev and app: simple-replicaset.
  • spec: This is the specification for the Replica Set.
    • replicas: 3: This tells the Replica Set that we want 3 replicas of the pod.
    • selector: This is the selector for the Replica Set.
      • matchLabels: This tells the Replica Set to manage pods with the label app: simple-replicaset.
    • template: This is the template for the pods which will be managed by the Replica Set.
      • metadata: This is the metadata for the pod.
        • labels: The labels for the pod. Here, we have a label app: simple-replicaset. Make sure that the labels in the pod template match the labels in the selector.
      • spec: This is the specification for the pod. Similar to the one we defined in the previous section.
        • containers: This is the list of containers in the pod.
          • name: apiserver: The name of the container.
          • image: rutush10/simple-restapi-server-py:v0.0.1: The image for the container.
          • ports: The ports for the container. Here, we are exposing port 8000.

Here's the visual representation of the state of the system. The pod simple-pod (created above, before making the label changes) is not managed by the Replica Set. The Replica Set simple-replicaset manages the pods simple-replicaset-bzgpf, simple-replicaset-hj9th, and simple-replicaset-xm2nm.

You can learn more about the replica set spec here.

Cleaning up

To clean up the resources created in this section, run the following commands:

kubectl delete -f replicaset.yaml
kubectl delete -f pod.yaml

Summary

In this section, you learned about Replica Sets and how they help you manage pods. You saw how Replica Sets use labels and selectors to identify the pods they should manage. You also saw how Replica Sets maintain the desired number of pods by creating new pods or terminating existing ones. And finally, you learned how to write a Replica Set manifest and create a Replica Set using kubectl.