Kubernetes PersistentVolumeClaim issues in AWS using Pod Autoscaling

Question

We have success creating the pods, services and replication controllers according to our project requirements. Now we are planning to setup persistence storage in AWS using Kubernetes. I have created the YAML file to create an EBS volume in AWS, it's working fine as expected. I am able to claim volume and successfully mount to my pod (this is for single replica only).

I am able to successfully create the file.Volume also creating but my Pods is going to pending state, volume still shows available state in aws. I am not able to see any error logs over there.

Storage file:

kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: mongo-ssd
provisioner: kubernetes.io/aws-ebs
parameters:
  type: gp2

Main file:

apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
    name: web2
spec:
  selector:
    matchLabels:
      app: mongodb
  serviceName: "mongodb"
  replicas: 2
  template:
    metadata:
      labels:
        app: mongodb
      annotations:
         pod.alpha.kubernetes.io/initialized: "true"
    spec:
      containers:
      - image: mongo
        name: mongodb
        ports:
        - name: web2
          containerPort: 27017
          hostPort: 27017
        volumeMounts:
        - mountPath: "/opt/couchbase/var"
          name: mypd1
  volumeClaimTemplates:
  - metadata:
      name: mypd1
      annotations:
        volume.alpha.kubernetes.io/storage-class: mongo-ssd
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 10Gi

Now I am planning to set up a pod Autoscaling. I have seen pod autoscaling for deployment and ReplicationContoller. May I know can we implement pod auto-scaling for Stateful set also?

Answer 1

Horizontal Pod Autoscaler can scale only Deployment, Replica Set or Replication Controller. You cannot scale Stateful Sets. (see Kubernetes Docu for more details)

The main reason is that most of the stateful applications running in Stateful Sets (such as your MongoDB) are usually not as easy to scale up / down as the stateless applications running as Deployments. Scaling up and down is usually quite complicated process for stateful apps which you do not want to do only based on the autoscaler. It usually requires some additional support logic in the application it self. And especially with scale down it could also mean risk for your data. The autoscaling is more useful for short term changes in the load. Scaling of Stateful Sets requires more long term thinking. Because of the complexity you do not want your database to be scaling up and down every minute.