How does Kubernetes control replication?

Question

I was curious about how Kubernetes controls replication. I my config yaml file specifies I want three pods, each with an Nginx server, for instance (from here -- https://kubernetes.io/docs/concepts/workloads/controllers/replicationcontroller/#how-a-replicationcontroller-works)

apiVersion: v1
kind: ReplicationController
metadata:
  name: nginx
spec:
  replicas: 3
  selector:
    app: nginx
  template:
    metadata:
      name: nginx
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
        ports:
        - containerPort: 80

How does Kubernetes know when to shut down pods and when to spin up more? For example, for high traffic loads, I'd like to spin up another pod, but I'm not sure how to configure that in the YAML file so I was wondering if Kubernetes has some behind-the-scenes magic that does that for you.

Answer 1

Kubernetes does no magic here - from your configuration, it does simply not know nor does it change the number of replicas. The concept you are looking for is called an Autoscaler. It uses metrics from your cluster (need to be enabled/installed as well) and can then decide, if Pods must be scaled up or down and will in effect change the number of replicas in the deployment or replication controller. (Please use a deployment, not replication controller, the later does not support rolling updates of your applications.) You can read more about the autoscaler here: https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale-walkthrough/

Answer 2

You can use HorizontalPodAutoscaler along with deployment as below. This will autoscale your pod declaratively based on target CPU utilization.

apiVersion: apps/v1beta1
kind: Deployment
metadata:
  name: $DEPLOY_NAME
spec:
  replicas: 2
  template:
    metadata:
      labels:
        app: $DEPLOY_NAME
    spec:
      containers:
      - name: $DEPLOY_NAME
        image: $DEPLOY_IMAGE
        imagePullPolicy: Always
        resources:
          requests:
            cpu: "0.2"
            memory: 256Mi            
          limits:
            cpu: "1"
            memory: 1024Mi
---
apiVersion: v1
kind: Service
metadata:
  name: $DEPLOY_NAME
spec:
  selector:
    app: $DEPLOY_NAME
  ports:
      - port: 8080
  type: ClusterIP
---
apiVersion: autoscaling/v1
kind: HorizontalPodAutoscaler
metadata:
  name: $DEPLOY_NAME
  namespace: $K8S_NAMESPACE
spec:
  scaleTargetRef:
    apiVersion: apps/v1beta1
    kind: Deployment
    name: $DEPLOY_NAME
  minReplicas: 2
  maxReplicas: 6
  targetCPUUtilizationPercentage: 60