A Cron Job 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.
Note: All CronJobschedule:
times are based the timezone of the master where the job is initiated.
For instructions on creating and working with cron jobs, and for an example of a spec file for a cron job, see Running automated tasks with cron jobs.
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 CronJob is set to schedule a new Job every one minute beginning at 08:30:00
, and its
startingDeadlineSeconds
field is not set. The default for this field is 100
seconds. If the CronJob controller happens to
be down from 08:29:00
to 10:21:00
, the job will not start as the number of missed jobs which missed their schedule is greater than 100.
To illustrate this concept further, suppose a CronJob is set to schedule a new Job every one minute beginning at 08:30:00
, and its
startingDeadlineSeconds
is set to 200 seconds. If the CronJob controller happens to
be down for the same period as the previous example (08:29:00
to 10:21:00
,) the Job will still start at 10:22:00. This happens as the controller now checks how many missed schedules happened in the last 200 seconds (ie, 3 missed schedules), rather than from the last scheduled time until now.
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.
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