Pods controllers

Pods controllers#

Deployment is the most popular way to run an application. This page discusses the components of deployment and common management patterns.

ReplicaSet#

A set of pods is deployed in parallel. K8s aims to maintain the specified number of replicas as specified in the replica set configuration.

The following cell creates a replica set.

kubectl create -f - << EOF
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: php-redis
        image: us-docker.pkg.dev/google-samples/containers/gke/gb-frontend:v5
EOF

replicaset.apps/frontend created

The result is 3 pods (as specified in the spec.replicas). They all belong to the frontent replica set, that is clear from their naming.

kubectl get pods

NAME             READY   STATUS    RESTARTS   AGE
frontend-4rpzj   1/1     Running   0          7s
frontend-6bnqz   1/1     Running   0          7s
frontend-b7v94   1/1     Running   0          7s

You can use the command kubectl get rs to list replica sets.

kubectl get rs

NAME       DESIRED   CURRENT   READY   AGE
frontend   3         3         3       8s

Use the command kubectl delete rs frontend to delete the replica set.

kubectl delete rs frontend

replicaset.apps "frontend" deleted from default namespace

Deployment#

Deployment is the most popular way to run an application. It runs and manages a replica set, which is a set of pods that run in parallel. A deployment is generally tied to a single ReplicaSet, but it implements a rolling update mechanism. If the ReplicaSet’s configuration must be changed, the deployment retains the ReplicaSet’s old configration until it ensures that the newly deployed updates are healthy.

The following cell uses an imperative approach to create a deployment.

kubectl create deployment nginx-deployment --image=nginx:1.22.1 --replicas=3

deployment.apps/nginx-deployment created

You can view a list of available deployments by entering the command kubectl get deployment.

kubectl get deployment 

NAME               READY   UP-TO-DATE   AVAILABLE   AGE
nginx-deployment   3/3     3            3           102s

As was specified by the replicas=3, three identical pods have been deployed.

kubectl get pods

NAME                                READY   STATUS    RESTARTS   AGE
nginx-deployment-799db84976-fcbjb   1/1     Running   0          112s
nginx-deployment-799db84976-fwmng   1/1     Running   0          112s
nginx-deployment-799db84976-hs492   1/1     Running   0          112s

kubectl delete deployment nginx-deployment

deployment.apps "nginx-deployment" deleted from default namespace

DaemonSet#

The DaemonSet is a controller that runs a single instance of the same pod on each node in the cluster.

The following cell runs the daemon set on the new cluster consisting of three nodes.

minikube start --profile daemonset-example --nodes 3 &> /dev/null
kubectl create -f - << EOF
apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd-agent
  namespace: default
  labels:
    k8s-app: fluentd-agent
spec:
  selector:
    matchLabels:
      k8s-app: fluentd-agent
  template:
    metadata:
      labels:
        k8s-app: fluentd-agent
    spec:
      containers:
      - name: fluentd
        image: quay.io/fluentd_elasticsearch/fluentd:v4.5.2
EOF

daemonset.apps/fluentd-agent created

The following cell shows the pods in the cluster.

kubectl get pods -o wide

NAME                  READY   STATUS              RESTARTS   AGE   IP       NODE                    NOMINATED NODE   READINESS GATES
fluentd-agent-f7hl4   0/1     ContainerCreating   0          0s    <none>   daemonset-example-m02   <none>           <none>
fluentd-agent-wqkhj   0/1     ContainerCreating   0          0s    <none>   daemonset-example       <none>           <none>
fluentd-agent-ztwl4   0/1     ContainerCreating   0          0s    <none>   daemonset-example-m03   <none>           <none>

The NODE column of the output is important here - each pod is on uniqe node, and the DaemonSet garantees that.

minikube delete --profile daemonset-example &> /dev/null
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