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A Practical Guide to Kubernetes

Creating ReplicaSets

Understand how to create and manage ReplicaSets in Kubernetes to maintain the desired number of Pod replicas. This lesson explains key components like apiVersion, selector, and template, enabling you to build scalable and fault-tolerant applications by monitoring and controlling Pod instances effectively.

We'll cover the following...

Definition

Let’s look at a ReplicaSet named go-demo-2.yml based on the Pod we created in the previous chapter:

YAML
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: go-demo-2
spec:
  replicas: 2
  selector:
    matchLabels:
      type: backend
      service: go-demo-2
  template:
    metadata:
      labels:
        type: backend
        service: go-demo-2
        db: mongo
        language: go
    spec:
      containers:
      - name: db
        image: mongo:3.3
      - name: api
        image: vfarcic/go-demo-2
        env:
        - name: DB
          value: localhost
        livenessProbe:
          httpGet:
            path: /demo/hello
            port: 8080

Note: The apiVersion, kind, and metadata fields are mandatory with all Kubernetes objects. ReplicaSet is no exception because it’s also a Kubernetes object.

  • Line 1: We specify that the apiVersion is apps/v1.

  • Lines 2–3: The kind is ReplicaSet and metadata has the name key set to go-demo-2. We could have extended ReplicaSet metadata with labels. However, we’ve skipped that part since the labels only provide extra information. They do not affect the behavior of the ReplicaSet.

Since we’ve already explored the above three fields, we won’t dwell on them a lot. However, we should note that there is a fourth section called spec and it is mandatory. Let’s look at it in more detail in the line-by-line explanation below:

  • Line 5–6: The first field we define in the spec section is replicas. It sets the desired number of replicas of the Pod. In this case, the ...