Cron Jobs in Kubernetes - connect to existing Pod, execute script

Question

I'm certain I'm missing something obvious. I have looked through the documentation for ScheduledJobs / CronJobs on Kubernetes, but I cannot find a way to do the following on a schedule:

1. Connect to an existing Pod
2. Execute a script
3. Disconnect

I have alternative methods of doing this, but they don't feel right.

1. Schedule a cron task for: kubectl exec -it $(kubectl get pods --selector=some-selector | head -1) /path/to/script

2. Create one deployment that has a "Cron Pod" which also houses the application, and many "Non Cron Pods" which are just the application. The Cron Pod would use a different image (one with cron tasks scheduled).

I would prefer to use the Kubernetes ScheduledJobs if possible to prevent the same Job running multiple times at once and also because it strikes me as the more appropriate way of doing it.

Is there a way to do this by ScheduledJobs / CronJobs?

http://kubernetes.io/docs/user-guide/cron-jobs/

Answer 1

As far as I'm aware there is no "official" way to do this the way you want, and that is I believe by design. Pods are supposed to be ephemeral and horizontally scalable, and Jobs are designed to exit. Having a cron job "attach" to an existing pod doesn't fit that module. The Scheduler would have no idea if the job completed.

Instead, a Job can to bring up an instance of your application specifically for running the Job and then take it down once the Job is complete. To do this you can use the same Image for the Job as for your Deployment but use a different "Entrypoint" by setting command:.

If they job needs access to data created by your application then that data will need to be persisted outside the application/Pod, you could so this a few ways but the obvious ways would be a database or a persistent volume. For example useing a database would look something like this:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: APP
spec:
  template:
    metadata:
      labels:
        name: THIS
        app: THAT
    spec:
      containers:
        - image: APP:IMAGE
          name: APP
          command:
          - app-start
          env:
            - name: DB_HOST
              value: "127.0.0.1"
            - name: DB_DATABASE
              value: "app_db"

And a job that connects to the same database, but with a different "Entrypoint" :

apiVersion: batch/v1
kind: Job
metadata:
  name: APP-JOB
spec:
  template:
    metadata:
      name: APP-JOB
      labels:
        app: THAT
    spec:
      containers:
      - image: APP:IMAGE
        name: APP-JOB
        command:
        - app-job
        env:
          - name: DB_HOST
            value: "127.0.0.1"
          - name: DB_DATABASE
            value: "app_db"

Or the persistent volume approach would look something like this:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: APP
spec:
  template:
    metadata:
      labels:
        name: THIS
        app: THAT
    spec:
      containers:
        - image: APP:IMAGE
          name: APP
          command:
          - app-start
          volumeMounts:
          - mountPath: "/var/www/html"
            name: APP-VOLUME
      volumes:
        - name:  APP-VOLUME
          persistentVolumeClaim:
            claimName: APP-CLAIM

---

apiVersion: v1
kind: PersistentVolume
metadata:
  name: APP-VOLUME
spec:
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteMany
  persistentVolumeReclaimPolicy: Retain
  nfs:
    path: /app

---

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: APP-CLAIM
spec:
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 10Gi
  selector:
    matchLabels:
      service: app

With a job like this, attaching to the same volume:

apiVersion: batch/v1
kind: Job
metadata:
  name: APP-JOB
spec:
  template:
    metadata:
      name: APP-JOB
      labels:
        app: THAT
    spec:
      containers:
      - image: APP:IMAGE
        name: APP-JOB
        command:
        - app-job
        volumeMounts:
        - mountPath: "/var/www/html"
          name: APP-VOLUME
    volumes:
      - name:  APP-VOLUME
        persistentVolumeClaim:
          claimName: APP-CLAIM