What is a ReplicaSet?
A ReplicaSet's purpose is to maintain a stable set of replica Pods running at any given time. A ReplicaSet ensures that a specified number of pod replicas are running at any given time. We often don't need to create a ReplicaSet directly. Instead we use Deployment to manage a ReplicaSet. Let's first take a look at how should we create a ReplicaSet.
We will create a ReplicaSet to ensure there is always 3 instances of a nginx container.
Here .spec.selector is a label selector and it is used to identify potential Pods to acquire by the ReplicaSet. In our case it will look for the Pods which have the label tier: frontend.
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: nginx-replicaset
labels:
tier: frontend
spec:
replicas: 2
selector:
matchLabels:
tier: frontend
template:
metadata:
labels:
tier: frontend
spec:
containers:
- name: my-nginx
image: nginx
Save this manifest into replicaset.yaml and apply it.
$ kubectl apply -f replicaset.yaml
replicaset.apps/nginx-replicaset created
$ kubectl get replicaset
NAME DESIRED CURRENT READY AGE
nginx-replicaset 2 2 2 19s
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-replicaset-7db6b 1/1 Running 0 19s
nginx-replicaset-ncz56 1/1 Running 0 19s
We can see that our ReplicaSet has been deployed and there are 2 PODs just as we specified.
To the Deployments!
Now we have seen what is a ReplicaSet and how to create it, it is time to integrate it into a Deployment.
A Deployment provides declarative updates for Pods ReplicaSets.
Let's create a Deployment manifest. Instead of creating the manifest from scratch, we can use kubectl to create a template file for us. With the --dry-run flag, we tell kubectl to not create the resource. This way, we can easily modify the attributes later.
$ kubectl create deployment nginx-deployment --image=nginx --dry-run=client -o yaml > deployment.yaml
$ kubectl get deploy
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-deployment 1/1 1 1 4m9s
$ kubectl get po
NAME READY STATUS RESTARTS AGE
nginx-deployment-5969c7f455-j6685 1/1 Running 0 3m39s
After creting the deployment, we can see that only 1 Pod is created. This is because we didn't specify a value for replicas option. We can scale our deployment by editing the yaml file or we can use the imperative way.
$ kubectl scale deploy nginx-deployment --replicas=2
deployment.apps/nginx-deployment scaled
$ kubectl get deploy
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-deployment 2/2 2 2 12m
$ kubectl get po
NAME READY STATUS RESTARTS AGE
nginx-deployment-5969c7f455-9nrs7 1/1 Running 0 71s
nginx-deployment-5969c7f455-j6685 1/1 Running 0 6m20ss
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