CKA Workloads and Scheduling Practice Question
A CronJob is configured to run every hour. You notice that the job did not run at the scheduled time. What is the most likely reason?
⚠ Common exam trap
Many exam-takers assume a CronJob always runs at its scheduled time, overlooking how the 'Forbid' concurrency policy can skip runs when a previous job is still active, especially when the job duration exceeds the schedule interval.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The concurrency policy is set to 'Forbid' and a previous job was still running
When a CronJob's concurrency policy is set to 'Forbid', it prevents a new job from starting if the previous job is still running. If the previous job took longer than the scheduled interval (e.g., more than one hour), the next scheduled run will be skipped, causing the job not to run at the expected time. This is a common scenario where a long-running job overlaps with the next scheduled time, and the 'Forbid' policy enforces that only one job instance runs at a time.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The concurrency policy is set to 'Forbid' and a previous job was still running
Why this is correct
When a CronJob's `concurrencyPolicy` is set to `Forbid`, the CronJob controller ensures that only one instance of the job runs at any given time. If the scheduled time for a new job arrives, but a previous job created by the same CronJob is still active (running or pending), the controller will simply skip the new scheduled run. This prevents resource contention or duplicate processing by ensuring strict sequential execution, directly explaining why a job might not run as scheduled.
- ✗
The concurrency policy is set to 'Allow'
Why it's wrong here
If the `concurrencyPolicy` is set to `Allow`, the CronJob controller permits multiple job instances to run simultaneously, even if previous jobs are still active. In this scenario, when a new scheduled time arrives, the controller would create and start a new Job object regardless of any currently running instances. Therefore, this setting would not cause a scheduled job run to be skipped, making it an incorrect reason for the observed behavior.
- ✗
The previous job run succeeded and the CronJob is configured to not rerun after success
Why it's wrong here
Kubernetes CronJobs are designed to execute Jobs on a recurring schedule, similar to a traditional Unix cron utility. Their execution is solely determined by the `schedule` field and the controller's internal timing mechanisms, not by the success or failure status of previous runs. There is no built-in configuration option or policy within CronJobs that would prevent a subsequent run based on the success of a prior job, making this statement fundamentally incorrect regarding CronJob functionality.
- ✗
The concurrency policy is set to 'Replace'
Why it's wrong here
When the `concurrencyPolicy` is configured as `Replace`, if a new job is scheduled while a previous job from the same CronJob is still running, the controller will actively terminate the existing, running job. Immediately after terminating the old job, it will then create and start the new scheduled job. This policy ensures that only the latest scheduled job runs, but it would not result in skipping a scheduled run altogether; instead, it forces a replacement.
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