how to have only one Pod created for/by Cronjob

Question

I am running this Cronjob at 2 AM in the morning:

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: postgres-backup
spec:
  # Backup the database every day at 2AM
  schedule: "0 2 * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: postgres-backup
            image: postgres:10.4
            command:
            - "/bin/sh"
            - -c
            - |
              pg_dump -Fc -d postgresql://$DBUSER:$DBPASS@$DBHOST:$DBPORT/$DBNAME > /var/backups/backup_$(date +"%d-%m-%Y_%H-%M").bak;
            env:
            - name: DBHOST
              valueFrom:
                configMapKeyRef:
                  name: dev-db-config
                  key: db_host
            - name: DBPORT
              valueFrom:
                configMapKeyRef:
                  name: dev-db-config
                  key: db_port
            - name: DBNAME
              valueFrom:
                configMapKeyRef:
                  name: dev-db-config
                  key: db_name
            - name: DBUSER
              valueFrom:
                secretKeyRef:
                  name: dev-db-secret
                  key: db_username
            - name: DBPASS
              valueFrom:
                secretKeyRef:
                  name: dev-db-secret
                  key: db_password
            volumeMounts:
            - mountPath: /var/backups
              name: postgres-backup-storage
          - name: postgres-restore
            image: postgres:10.4
            volumeMounts:
            - mountPath: /var/backups
              name: postgres-backup-storage
          restartPolicy: OnFailure
          volumes:
          - name: postgres-backup-storage
            hostPath:
            # Ensure the file directory is created.
              path: /var/volumes/postgres-backups
              type: DirectoryOrCreate

The jobs are getting executed successfully, but what I don't like is that for every Job execution a new Pod is created:

Is there a way to clean previous (old) created Pods? Or maybe is there a way to rerun one an the same Pod/Job every time?

Answer 1

You can try to set ttlSecondsAfterFinished, to find out more try:

kubectl explain cronjob.spec.jobTemplate.spec.ttlSecondsAfterFinished --api-version=batch/v1beta1

ttlSecondsAfterFinished limits the lifetime of a Job that has finished execution (either Complete or Failed). If this field is set, ttlSecondsAfterFinished after the Job finishes, it is eligible to be automatically deleted. When the Job is being deleted, its lifecycle guarantees (e.g. finalizers) will be honored. If this field is unset, the Job won't be automatically deleted. If this field is set to zero, the Job becomes eligible to be deleted immediately after it finishes. This field is alpha-level and is only honored by servers that enable the TTLAfterFinished feature.

Shortly, the Job created by CronJob will be deleted after specified seconds.

---

In your case:

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: postgres-backup
spec:
  # Backup the database every day at 2AM
  schedule: "0 2 * * *"
  jobTemplate:
    spec:
      template:
        spec:
          ttlSecondsAfterFinished: 60 # after 1 minute the job and the pods will be deleted by kubernetes
...

Please note, that ttlSecondsAfterFinished will be applied iff the job was terminated successfully.

Answer 2

There are two ways to fix this

First method, with the job spec

add a .spec.activeDeadlineSeconds field of the Job to a number of seconds. The activeDeadlineSeconds applies to the duration of the job

Once a Job reaches activeDeadlineSeconds, all of its running Pods are terminated and the Job status will become type: Failed with reason: DeadlineExceeded.

Set the activeDeadlineSeconds to be much longer than the expected running time

Second method, with the pod

make the pod so that it terminates itself when the job is complete. Something like altering the command to

pg_dump -Fc -d postgresql://$DBUSER:$DBPASS@$DBHOST:$DBPORT/$DBNAME > /var/backups/backup_$(date +"%d-%m-%Y_%H-%M").bak; exit

Answer 3

If only last job and pod need to be preserved, you can use .spec.successfulJobsHistoryLimit field set to 1.

This way only last job and corresponding pod will be preserved. By default it's set to 3. Also it's possible to set this value to 0 and nothing will be saved after cronjob's execution.

Same logic has .spec.failedJobsHistoryLimit field, it has 1 by default.

See jobs history limits.

---

This is how it looks when I get events from cronjob:

$ kubectl describe cronjob test-cronjob
...
Events:
  Type    Reason            Age    From                Message
  ----    ------            ----   ----                -------
  Normal  SuccessfulCreate  2m31s  cronjob-controller  Created job test-cronjob-27304493
  Normal  SawCompletedJob   2m30s  cronjob-controller  Saw completed job: test-cronjob-27304493, status: Complete
  Normal  SuccessfulCreate  91s    cronjob-controller  Created job test-cronjob-27304494
  Normal  SawCompletedJob   90s    cronjob-controller  Saw completed job: test-cronjob-27304494, status: Complete
  Normal  SuccessfulDelete  90s    cronjob-controller  Deleted job test-cronjob-27304493
  Normal  SuccessfulCreate  31s    cronjob-controller  Created job test-cronjob-27304495
  Normal  SawCompletedJob   30s    cronjob-controller  Saw completed job: test-cronjob-27304495, status: Complete
  Normal  SuccessfulDelete  30s    cronjob-controller  Deleted job test-cronjob-27304494

Only one last job is presented:

$ kubectl get jobs
NAME                    COMPLETIONS   DURATION   AGE
test-cronjob-27304496   1/1           1s         3s

And one pod:

$ kubectl get pods
NAME                               READY   STATUS      RESTARTS   AGE
test-cronjob-27304496-r4qd8        0/1     Completed   0          38s