Admission Controller

Ondat implements a MutatingAdmissionWebhook Admission Controller to ensure that Pods using Ondat Volumes use the storageos-scheduler. An admission controller intercepts requests to the Kubernetes API server prior to persistence of the object, but after the request is authenticated and authorized.

The Admission Controller is responsible for mutating the PodSpec at creation time to populate the PodSpec.schedulerName field with the name of the Ondat Scheduler - storageos-scheduler.

During Pod creation, Kubernetes sends a web request to the Ondat WebHook with the Pod specification. The PodSpec is only altered to use the Ondat scheduler if the Pod uses an Ondat volume.

Web Server

The Web Server hosting the web hook is executed in the Ondat Cluster Operator. Since only HTTPS requests are allowed, the Operator generates a self-signed x509 certificate every time it starts. The Cluster Operator will also renew certificates upon expiry (certs are valid for one year).

There is no manual intervention required regarding the SSL configuration as the setup is completely transparent between Ondat and Kubernetes.

Skipping Mutation

To avoid scheduler mutation, the storageos.com/scheduler=false annotation can be added to resources that use Ondat volumes.

When using StatefulSets the annotation can be set on the spec.template.metadata.annotations field.

apiVersion: apps/v1
kind: StatefulSet
spec:
  ...
  template:
    metadata:
      annotations:
        storageos.com/scheduler: "false" # N.B. the value must be a string and not a boolean

When using Pods the annotation is set on the metadata.annotations field.

apiVersion: v1
kind: Pod
metadata:
    ...
    annotations:
        storageos.com/scheduler: "false" # N.B. the value must be a string and not a boolean
    ...

Explicit SchedulerName

It is not necessary to explicitly set the SchedulerName as the Admission Controller automatically populates the PodSpec field. Set the SchedulerName in your manifests, manually, only if you disable or can’t execute the Ondat Admission Controller.

Kubernetes allows the use of different schedulers by setting the field .spec.schedulerName: storageos-scheduler.

For instance a Pod manifest utilising the Ondat scheduler would appear as follows:

apiVersion: v1
kind: Pod
metadata:
  name: d1
spec:
  schedulerName: storageos-scheduler # --> Ondat Scheduler
                                     # No need if using Admission Controller
                                     # (enabled by default)
  containers:
    - name: debian
      image: debian:9-slim
      command: ["/bin/sleep"]
      args: [ "3600" ]
      volumeMounts:
        - mountPath: /mnt
          name: v1
  volumes:
    - name: v1
      persistentVolumeClaim:
        claimName: persistent-volume # ----> Ondat PVC

Compatibility

The Admission Controller doesn’t need to be enabled at Kubernetes cluster bootstrap time because it is a Dynamic Admission Controller. Hence, any cluster that has the MutatingAdmissionWebhook enabled is supported. Most Kubernetes cluster enable the Webhook admission controller by default.

The MutatingAdmissionWebhook is available from Kubernetes v1.13.

You can check your Kubernetes cluster compatibility by checking if the following object exists.

kubectl api-versions | grep admissionregistration.k8s.io