Create and Manage Kubernetes ReplicaSets¶

Let's see how you can create and manage ReplicaSets.

Here is the Docker Image used in this tutorial: reyanshkharga/nginx

Step 1. Create ReplicaSet Manifest¶

First, we need to write the ReplicaSet manifest as follows:

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apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: my-replicaset
spec:
  replicas: 2
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
        tier: backend
    spec:
      containers:
      - name: nginx
        image: reyanshkharga/nginx:v1
        imagePullPolicy: Always
        ports:
          - containerPort: 80

Required fields:

• apiVersion - Which version of the Kubernetes API you're using to create this object.
• kind - What kind of object you want to create.
• metadata - Data that helps uniquely identify the object, including a name string, UID , and - optional namespace.
• spec - What state you desire for the object.

Step 2: Create ReplicaSet¶

Let's use kubectl apply to apply the manifest and create the ReplicaSet.

# Create replicaset
kubectl apply -f my-replicaset.yml

Step 3: List ReplicaSets¶

# List all replicasets
kubectl get replicasets

# List all replicasets with expanded (aka "wide") output
kubectl get replicasets -o wide

Resource types are case-insensitive and you can specify the singular, plural, or abbreviated forms.

The following commands produce the same output:

kubectl get rs 
kubectl get replicaset
kubectl get replicasets

Step 4: Describe a ReplicaSet¶

# Command template
kubectl describe rs <replicaset-name>
{OR}
kubectl describe rs/<replicaset-name>

# Actual command
kubectl describe rs my-replicaset
{OR}
kubectl describe rs/my-replicaset

Step 5: Scale a ReplicaSet¶

You can use kubectl scale command to scale a ReplicaSet up or down by changing the desired number of replicas.

# Command template
kubectl scale rs <replicaset-name> --replicas <desired-count>
{OR}
kubectl scale rs/<replicaset-name> --replicas <desired-count>

# Actual command - scale up
kubectl scale rs my-replicaset --replicas 3
{OR}
kubectl scale rs/my-replicaset --replicas 3

# Actual command - scale down
kubectl scale rs my-replicaset --replicas 1
{OR}
kubectl scale rs my-replicaset --replicas 1

Or, you can change the value of replicas field to a desired value in the manifest and then apply the manifest again. This method is desirable and recommended.

# Update the replicaset
kubectl apply -f my-replicaset.yml

Step 6: ReplicaSet in Action¶

Let's delete a pod in the ReplicaSet and notice what happens.

kubectl delete pod <pod-name>

You will notice that as soon as the pod terminates, the ReplicaSet will spawn a new pod using the template to ensure that the desired number of pods is always running.

# Watch and continuously update the status of pods in real-time
kubectl get pods -w

Step 7: Delete a ReplicaSet¶

# Command template
kubectl delete rs <replicaset-name>
{OR}
kubectl delete rs/<replicaset-name>

# Actual command
kubectl delete rs my-replicaset
{OR}
kubectl delete rs/my-replicaset