Kubernetes: Is it possible to mount volumes to a container running as a CronJob?

Question

I'm attempting to create a Kubernetes CronJob to run an application every minute.

A prerequisite is that I need to get my application code onto the container that runs within the CronJob. I figure that the best way to do so is to use a persistent volume, a pvclaim, and then defining the volume and mounting it to the container. I've done this successfully with containers running within a Pod, but it appears to be impossible within a CronJob? Here's my attempted configuration:

apiVersion: batch/v2alpha1
kind: CronJob
metadata:
  name: update_db
spec:
  volumes:
  - name: application-code
    persistentVolumeClaim:
      claimName: application-code-pv-claim
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: update-fingerprints
            image: python:3.6.2-slim
            command: ["/bin/bash"]
            args: ["-c", "python /client/test.py"]
          restartPolicy: OnFailure

The corresponding error:

error: error validating "cron-applications.yaml": error validating data: found invalid field volumes for v2alpha1.CronJobSpec; if you choose to ignore these errors, turn validation off with --validate=false

I can't find any resources that show that this is possible. So, if not possible, how does one solve the problem of getting application code into a running CronJob?

Answer 1

A CronJob uses a PodTemplate as everything else based on Pods and can use Volumes. You placed your Volume specification directly in the CronJobSpec instead of the PodSpec, use it like this:

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: update-db
spec:
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: update-fingerprints
            image: python:3.6.2-slim
            command: ["/bin/bash"]
            args: ["-c", "python /client/test.py"]
            volumeMounts:
            - name: application-code
              mountPath: /where/ever
          restartPolicy: OnFailure
          volumes:
          - name: application-code
            persistentVolumeClaim:
              claimName: application-code-pv-claim

Answer 2

For the other question in there: "how does one solve the problem of getting application code into a running CronJob?"

You build your own image that contains the code. This is how it is normally done.

FROM python:3.6.2-slim
ADD test.py /client/test.py

CMD ['python','-c','/client/test.py']

Build and push to the docker registry.

docker build -t myorg/updatefingerprints
docker push myorg/updatefingerprints

Use this image in the descriptor.

apiVersion: batch/v2alpha1
kind: CronJob
metadata:
  name: update_db
spec:
  volumes:
  - name: application-code
    persistentVolumeClaim:
      claimName: application-code-pv-claim
  schedule: "*/1 * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: update-fingerprints
            image: myorg/update-fingerprints
            imagePullPolicy: Always
          restartPolicy: OnFailure

This requires thinking quite differently about configuration management and version control.

Answer 3

now there is ephemeral. available since 1.21

https://kubernetes.io/docs/concepts/storage/ephemeral-volumes/#generic-ephemeral-volumes

kind: Pod
apiVersion: v1
metadata:
  name: my-app
spec:
  containers:
    - name: my-frontend
      image: busybox:1.28
      volumeMounts:
      - mountPath: "/scratch"
        name: scratch-volume
      command: [ "sleep", "1000000" ]
  volumes:
    - name: scratch-volume
      ephemeral:
        volumeClaimTemplate:
          spec:
            accessModes: [ "ReadWriteOnce" ]
            resources:
              requests:
                storage: 1Gi