CronJobs are a way to run a task on a time-based schedule and have been around for a long time in Linux and UNIX systems. They can be used in Kubernetes (K8S) to run recurring tasks such as backup jobs, triggering emails, report generation, or automating restarts of containers.
In this article, we will dive into how to define a CronJob, look at how to implement them in K8S manifest files with an example, and the available options.
Traditionally on Unix-based systems, CronJobs work as follows:
- The user creates a cron job by using the crontab command to edit their “crontab” file. This file contains a list of commands or scripts to be executed and the times when they should be executed.
- The cron daemon, a background process that runs continuously, reads the crontab files of all users and checks if any jobs are scheduled to run at the current time.
- If a job is scheduled to run, the cron daemon executes the command or script associated with the job.
In K8S, CronJobs are automatically managed by the cluster control plane. The cluster creates regular jobs with the pod spec from your CronJob object. CronJobs are a higher-level abstraction than standard K8S jobs that repeat the cycle periodically.
To define a Cronjob, the schedule is defined using the CronJob syntax below:
# ┌───────────── minute (0 - 59) # │ ┌───────────── hour (0 - 23) # │ │ ┌───────────── day of the month (1 - 31) # │ │ │ ┌───────────── month (1 - 12) # │ │ │ │ ┌───────────── day of the week (0 - 6) (Sun to Sat; # │ │ │ │ │ 7 is also Sunday on some systems) # │ │ │ │ │ OR sun, mon, tue, wed, thu, fri, sat # │ │ │ │ │ # * * * * *
The Cronjob schedule is specified using 5 entries separated by spaces.
For example, running a job every day at 11 pm would be defined as:
0 23 * * *
A job running every minute would look like this:
* * * * *
Check out Crontab.guru to experiment with defining CronJobs.
For CronJobs with no time zone specified, the kube-controller-manager interprets schedules relative to its local time zone. As of Kubernetes v1.25 [beta] the
CronJobTimeZone feature gate can be enabled, which enables a specific time zone to be specified should it be required. For example:
To see a CronJob in action, we first need to specify the CronJob in a manifest file.
Create the below file and name it cronjob.yaml. Note the kind is set to CronJob, and the
.spec.schedule is set to run the job every minute. The
.spec.schedule is a required field of the
.spec. It takes a Cron format string, as detailed previously.
The name you provide must be a valid DNS subdomain name no longer than 52 characters. This is because the CronJob controller will automatically append 11 characters to the Job name provided, and there is a constraint that the maximum length of a Job name is no more than 63 characters.
The example itself will output ‘Hello from the Kubernetes cluster’ to the logs.
apiVersion: batch/v1 kind: CronJob metadata: name: hello spec: schedule: "* * * * *" jobTemplate: spec: template: spec: containers: - name: hello image: busybox:1.28 imagePullPolicy: IfNotPresent command: - /bin/sh - -c - date; echo Hello from the Kubernetes cluster restartPolicy: OnFailure
Create the deployment:
kubectl create -f .\cronjob.yaml
Verify the CronJob has been created:
kubectl get cronjob hello
The LAST SCHEDULE field shows how long ago the job last ran.
The ACTIVE field shows how many jobs are in progress, 0 meaning it has either already been completed or failed.
Note that the job name is different from the pod name, which is specified in the manifest file.
You can view the running jobs in real-time using the
kubectl get jobs --watch
To view the pods that have been created to run the jobs:
kubectl get pods
If you have lots of running pods you’ll want to filter the selection to the job name in your system using the
--selector argument. Note that only the last three pods will be shown by default unless a different value has been specified by the optional field
kubectl get pods --selector=job-name=hello-27827258
To view the logs from the pod to verify the command ran successfully:
kubectl logs hello-27827258--1-rdf4s
To clean up, delete the CronJob. Deleting the CronJob removes all the jobs and pods it created and stops it from creating additional jobs:
kubectl delete cronjob hello
See how to delete Pods from a Kubernetes Node.
There are a number of optional fields that can be used to further control CronJobs.
The Starting Deadline is an optional field that specifies the deadline in seconds for starting the job if it misses its scheduled time for any reason. After the deadline, CronJob does not start the job. Jobs that do not meet their deadline in this way count as failed jobs.
If this field is not specified, the jobs have no deadline. For example, if it is set to
60, it allows a job to be created for up to 60 seconds after the actual schedule.
Note that if the
startingDeadlineSeconds is set to a value of less than 10 seconds, the CronJob may not be scheduled. This is because the CronJob controller checks things every 10 seconds.
The Concurrency policy is also an optional field that specifies how to handle concurrently running jobs. This may be useful for jobs that need to run independently to avoid unwanted results. By default, if this is not specified, concurrently running jobs are always allowed.
Allow(default): The CronJob allows concurrently running jobs.
Forbid: The CronJob does not allow concurrent runs; if it is time for a new job run and the previous job run hasn’t finished yet, the CronJob skips the new job run.
Replace: If it is time for a new job run and the previous job run hasn’t finished yet, the CronJob replaces the currently running job run with a new job run.
The Suspend field is optional that can be set to either
false . If it is set to true, then all subsequent operations are suspended. The current suspend status can be viewed using the command below:
kubectl get cronjob hello
An optional field to specify the number of successful jobs to keep in the history, which by default is 3.
Similar to the above option, the failed jobs history limit can also be specified. This defaults to 1.
If you encounter errors when setting up a CronJob, check the following:
- Syntax: CronJobs use the same syntax as traditional UNIX cron jobs, which can be complex and difficult to get right. Common errors include specifying the wrong number of fields, using incorrect wildcards, or mistyping the cron schedule. Check the syntax section of the article and copy your expression into crontab.guru to make sure the syntax is correct.
- Timezone: CronJobs run in the timezone of the Kubernetes cluster by default, which may not match the timezone of the user or application. This can lead to scheduling conflicts or unexpected behavior.
- Image: If a CronJob specifies an image that is not available, the job will fail.
- Resources: Resource limits on images that are set too high will cause jobs to fail.
- Job concurrency: If a CronJob is set to run too frequently or with too many replicas, it might lead to excessive load on the cluster and cause other jobs to fail.
- Permissions: Check the CronJob has sufficient permissions to access resources or perform actions that are defined.
To troubleshoot CronJob errors, the K8S logs will be the first port of call. You can use the
kubectl logs command to interrogate the K8S server API logs, CronJob controller logs, Pod logs, and Container logs.
kubectl logs -n <namespace> <cronjob-controller-pod-name> -c cronjob-controller
If you are using a centralized logging solution for your cluster (as is recommended), such as Elasticsearch, Fluentd, or Kibana (EFK), to collect and analyze logs from multiple nodes and containers in the cluster, you should check the logs using those tools for deeper insight.
Monitoring solutions such as Prometheus, Datadog, Grafana, or New Relic can be used to track job execution, such as the number of successful and failed jobs, job duration, and resource usage.
CronJobs are used to schedule recurring tasks in Kubernetes.
For more information on CronJobs check out the official documentation on kubernetes.io.
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