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ReplicaSets

Goals​

  1. Understand Kubernetes ReplicaSets:
    • Learn what a ReplicaSet is and its role in the Kubernetes architecture.
  2. Advantages of Using ReplicaSets vs. Simple Pods
    • Figure out why you should be almost always be using a ReplicaSet in production instead of a simple Pod.
  3. Basic Example(s):
    • Provide (a) basic example(s) of ReplicaSet resources.
    • Show how to define a ReplicaSet using YAML.
  4. ReplicaSet Spec Explanation:
    • Explain the components of the ReplicaSet specification using the example(s) above.
  5. Exercise:
    • Hands-on activity to create a simple ReplicaSet.

Understanding Kubernetes ReplicaSets​

A ReplicaSet in Kubernetes is a controller that ensures a specified number of pod replicas are running at any given time. If a pod goes down, the ReplicaSet will automatically create a new one to maintain the desired number of replicas.

Advantages of Using ReplicaSets vs. Simple Pods​

  1. Automatic Scaling:

    • ReplicaSets automatically create new Pods to replace any that fail, ensuring the desired number of Pods are always running.

  2. Declarative Management:

    • You can define the desired state (e.g., number of replicas) in a YAML file, and Kubernetes maintains that state automatically.

  3. Rolling Updates:

    • ReplicaSets, when used with Deployments, allow you to update your application with zero downtime by gradually replacing old Pods with new ones.

  4. Consistency and Reliability:

    • ReplicaSets ensure that your application consistently has the necessary resources to handle its workload by maintaining a fixed number of replicas.

  5. Label Selection:

    • ReplicaSets use labels to manage Pods, allowing you to adjust which Pods are managed by the ReplicaSet through label changes.

  6. Simplified Management:

    • Instead of managing individual Pods, ReplicaSets provide a single point of control for a group of Pods, making operations like scaling and updates easier.

Basic ReplicaSet Example​

Simple ReplicaSet​

Here is a basic example of a Kubernetes ReplicaSet specification in YAML:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: example-replicaset
spec:
  replicas: 3
  selector:
    matchLabels:
      app: example-app
  template:
    metadata:
      labels:
        app: example-app
    spec:
      containers:
      - name: nginx
        image: nginx:1.21.6
        ports:
        - containerPort: 80

This ReplicaSet ensures that three replicas of the nginx container are running at all times.

Save this to a file replicaset.yaml and create the resource using kubectl apply -f replicaset.yaml:

> kubectl apply -f replicaset.yaml
replicaset.apps/example-replicaset created

ReplicaSet Spec Explanation​

  1. apiVersion: apps/v1 (API group and version)
  2. kind: ReplicaSet (type of the resource)
  3. metadata: ObjectMeta
  4. spec: ReplicaSetSpec
    • replicas: The desired number of pod replicas.
    • selector: The label selector used to identify the pods managed by the ReplicaSet.
    • template: The pod template that defines the pods to be created.

Exercises​

Exercise 1: Create a Simple ReplicaSet​

Objective: Deploy a ReplicaSet that ensures a specific number of pods are running.

Task:

  • Define a ReplicaSet that runs three replicas of an nginx container.
  • Verify that the desired number of replicas are running.
  • Test scaling by increasing or decreasing the number of replicas in the ReplicaSet definition.
tip

If you want to scale the ReplicaSet, you can edit the replicas field in the YAML file and reapply it or use the following command:

kubectl scale replicaset example-replicaset --replicas=5