CronJob
A CronJob creates Jobs on a time-based schedule.
One CronJob object is like one line of a crontab (cron table) file. It runs a job periodically on a given schedule, written in Cron format.
All CronJob
schedule:times are denoted in UTC.
Cron Job Limitations
A cron job creates a job object about once per execution time of its schedule. We say “about” because there are certain circumstances where two jobs might be created, or no job might be created. We attempt to make these rare, but do not completely prevent them. Therefore, jobs should be idempotent.
If startingDeadlineSeconds is set to a large value or left unset (the default) and if concurrencyPolicy is set to Allow, the jobs will always run at least once.
For every CronJob, the CronJob controller checks how many schedules it missed in the duration from its last scheduled time until now. If there are more than 100 missed schedules, then it does not start the job and logs the error
Cannot determine if job needs to be started. Too many missed start time (> 100). Set or decrease .spec.startingDeadlineSeconds or check clock skew.
It is important to note that if the startingDeadlineSeconds field is set (not nil), the controller counts how many missed jobs occurred from the value of startingDeadlineSeconds until now rather than from the last scheduled time until now. For example, if startingDeadlineSeconds is 200, the controller counts how many missed jobs occurred in the last 200 seconds.
A CronJob is counted as missed if it has failed to be created at its scheduled time. For example, If concurrencyPolicy is set to Forbid and a CronJob was attempted to be scheduled when there was a previous schedule still running, then it would count as missed.
For example, suppose a cron job is set to start at exactly 08:30:00 and its startingDeadlineSeconds is set to 10, if the CronJob controller happens to be down from 08:29:00 to 08:32:00, the job will not start. Set a longer startingDeadlineSeconds if starting later is better than not starting at all.
The Cronjob is only responsible for creating Jobs that match its schedule, and the Job in turn is responsible for the management of the Pods it represents.
Running Automated Tasks with a CronJob
You can use CronJobs to run jobs on a time-based schedule. These automated jobs run like Cron tasks on a Linux or UNIX system.
Each line of a crontab file represents a job, and looks like this:
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │ 7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * command to execute
Cron jobs are useful for creating periodic and recurring tasks, like running backups or sending emails. Cron jobs can also schedule individual tasks for a specific time, such as if you want to schedule a job for a low activity period.
Before you begin
- You need a working Kubernetes cluster at version >= 1.8
kubectl version
Client Version: version.Info{Major:"1", Minor:"13", GitVersion:"v1.13.1", GitCommit:"eec55b9ba98609a46fee712359c7b5b365bdd920", GitTreeState:"clean", BuildDate:"2018-12-13T19:44:19Z", GoVersion:"go1.11.2", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"10+", GitVersion:"v1.10.9-gke.5", GitCommit:"d776b4deeb3655fa4b8f4e8e7e4651d00c5f4a98", GitTreeState:"clean", BuildDate:"2018-11-08T20:33:00Z", GoVersion:"go1.9.3b4", Compiler:"gc", Platform:"linux/amd64"}
Creating a Cron Job
Cron jobs require a config file. This example cron job config .spec file prints the current time and a hello message every minute:
cat << EOF > cronjob.yaml
apiVersion: batch/v1beta1
kind: CronJob
metadata:
name: hello
spec:
schedule: "*/1 * * * *"
jobTemplate:
spec:
template:
spec:
containers:
- name: hello
image: busybox
args:
- /bin/sh
- -c
- date; echo Hello from Admatic Kubernetes cluster
restartPolicy: OnFailure
EOF
kubectl create -f cronjob.yaml
cronjob.batch/hello created
Alternatively, you can use kubectl run to create a cron job without writing a full config:
kubectl run hello2 --schedule="*/2 * * * *" --restart=OnFailure --image=busybox -- /bin/sh -c "date; echo Hello 2 from Admatic Kubernetes cluster"
kubectl run --generator=cronjob/v1beta1 is DEPRECATED and will be removed in a future version. Use kubectl run --generator=run-pod/v1 or kubectl create instead.
cronjob.batch/hello2 created
After creating the cron job, get its status using this command:
kubectl get cronjobs
NAME SCHEDULE SUSPEND ACTIVE LAST SCHEDULE AGE
hello */1 * * * * False 0 21s 1m
hello2 */2 * * * * False 0 21s 25s
You should see that the cron job “hello” successfully scheduled a job at the time specified in LAST-SCHEDULE. There are currently 0 active jobs, meaning that the job has completed or failed.
Watch for the job to be created in around one minute:
kubectl get jobs --watch
NAME COMPLETIONS DURATION AGE
hello-1546624980 1/1 3s 110s
hello-1546625040 1/1 3s 50s
hello2-1546625040 1/1 2s 50s
hello-1546625100 0/1 0s
hello-1546625100 0/1 0s 0s
hello-1546625100 1/1 2s 2s
hello-1546625160 0/1 0s
hello2-1546625160 0/1 0s
hello-1546625160 0/1 0s 0s
hello2-1546625160 0/1 0s 0s
hello2-1546625160 1/1 2s 2s
hello-1546625160 1/1 2s 2s
Now, find the pods that the last scheduled job created and view the standard output of one of the pods.
pod=$(kubectl get pods --selector=job-name=hello-1546625160 --output=jsonpath={.items..metadata.name})
echo $pod
hello-1546625160-xhbld
kubectl logs $pod
Fri Jan 4 18:06:02 UTC 2019
Hello from Admatic Kubernetes cluster
kubectl get po
NAME READY STATUS RESTARTS AGE
hello-1546625160-xhbld 0/1 Completed 0 2m
hello-1546625220-wrdqx 0/1 Completed 0 1m
hello-1546625280-zd6hx 0/1 Completed 0 57s
hello2-1546625040-k7xbl 0/1 Completed 0 4m
hello2-1546625160-4p2tf 0/1 Completed 0 2m
hello2-1546625280-ssvcd 0/1 Completed 0 57s
Deleting a Cron Job
When you don’t need a cron job any more, delete it with kubectl delete cronjob:
kubectl delete cronjob hello
cronjob.batch "hello" deleted
Deleting the cron job removes all the jobs and pods it created and stops it from creating additional jobs.
kubectl get po
NAME READY STATUS RESTARTS AGE
hello2-1546625040-k7xbl 0/1 Completed 0 6m
hello2-1546625160-4p2tf 0/1 Completed 0 4m
hello2-1546625280-ssvcd 0/1 Completed 0 2m
hello2-1546625400-2x8zb 0/1 Completed 0 7s