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Kubernetes - CronJob

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Kubernetes - CronJob
- CronJobs

CronJobs

CronJob
- configure a job to run at a specific time.
- specify a Job template and a schedule
  - CronJob controller creates a new Job object from the template, according to the schedule
  - controller will continue to create Jobs according to the schedule until the CronJob object gets deleted.
When a CronJob Object is created, CronJob Controller creates a Job object controlled by the CronJob object underneath, which creates pod based on the schedule.
- Job Object is controlled by the CronJob
  - no additional labels are added
  - The number at the end of the job name is the scheduled time of the Job in Unix Epoch Time, converted to minutes.
- the underlying pod object is controlled by the Job
  - additional labels are added by the Job object
When you delete the CronJob, all the Jobs it created will also be deleted.
- When they’re deleted, the Pods are deleted as well, which causes their containers to shut down gracefully.
- kubectl delete cj --cascade=orphan: keep the Jobs and the underlying Pods,

Schedule

Format: Minute Hour Day-of-Month Month Day-of-week
Expression:
- *: matches any value
- 5: field value matches 5
- MAY: Month field value matches “MAY”
- 1-5: A range of values, includes both limits
- JAN-MAY: A range of values in Month field; between January and May (inclusive).
- 1,2,5-8: A list of numbers or ranges.
- *\/3: Every Nth value, starting with the first value.; e.g., matches January, April, July, and October
- 5/2: Every Nth value, starting with the specified value; e.g., starting in May, May, July, September, or November.
- 3-10/2: The /N pattern can also be applied to ranges. e.g., March, May, July, and September.
special values
- @hourly: to run the Job every hour (at the top of the hour)
- @daily: to run it every day at midnight
- @weekly: to run it every Sunday at midnight
- @monthly: to run it at 0:00 on the first day of each month
- @yearly/ @annually: to run it at 0:00 on January 1st of each year.
Setting the Time Zone
- By default, the CronJob controller schedules CronJobs based on the time zone used by the Controller Manager.
- timeZone field: specify the timezone

spec:
  schedule: "0 3 * * *"
  timeZone: CET # Central European Time (CET time zone)
  jobTemplate:

Suspending and resuming

Suspend
- kubectl patch cj NAME -p '{"spec":{"suspend": true}}'
resume
- kubectl patch cj NAME -p '{"spec":{"suspend": false}}'

Automatically removing finished Jobs

successfulJobsHistoryLimit:
- specify how many successful Jobs to keep.
- default: keeps 3 successful
failedJobsHistoryLimit:
- specify how many failed Jobs to keep.
- default: keeps 1 failed Job.
Pod associated with the Jobs are also preserved.

Specify Start Deadline

if the cluster’s Control Plane is overloaded or if the Controller Manager component running the CronJob controller is offline, the delay of creating Job may be longer.
- If it’s crucial that the Job shouldn’t start too far after its scheduled time, you can set a deadline in the startingDeadlineSeconds field
startingDeadlineSeconds field
- specify the limit of time when the job must be created; Otherwise, the Job won’t be created.
- a MissSchedule event will be generated

spec:
  schedule: "* * * * *"
  startingDeadlineSeconds: 30

If the startingDeadlineSeconds field isn’t set and the CronJob controller is offline for an extended period of time, undesirable behavior may occur when the controller comes back online.
- the controller will immediately create all the Jobs that should have been created while it was offline.
- However, this will only happen if the number of missing jobs is less than 100.
  - If the controller detects that more than 100 Jobs were missed, it doesn’t create any Jobs.
  - Instead, it generates a TooManyMissedTimes event.
- By setting the start deadline, you can prevent this from happening.

Job concurrency

Job concurrency
- multiple jobs of a CronJob run at a time
By default, the CronJob controller creates new Jobs regardless of how many previous Jobs are still active.
spec.concurrencyPolicy field
- specify the behavior of job concurrency
- Allow:
  - default
  - allow 2 jobs run concurrently
  - new Job will be created on schedule, regarless of whether the previous Job completed or not.
- Forbid
  - Concurrent runs are prohibited.
  - If the previous run is still active when a new run is to be scheduled, the CronJob controller records a JobAlreadyActive event and skips creating a new Job.
- Replace
  - The active Job is canceled and replaced by a new one.
  - The CronJob controller cancels the active Job by deleting the Job object.
  - The Job controller then deletes the Pods, but they’re allowed to terminate gracefully.
    - This means that two Jobs are still running at the same time, but one of them is being terminated.

Imperative Command

Command	Description
`kubectl get cronjobs`/`kubectl get cj`	Lists all CronJobs, showing the schedule and last run time.
`kubectl describe cronjob <name>`	Shows the schedule, active jobs, and the history of successful/failed runs.
`kubectl create cronjob <name> --image=<img> --schedule="/5 * * *"`	Creates a new CronJob that runs every 5 minutes.
`kubectl create job <job-name> --from=cronjob/<cj-name>`	Manual Trigger: Runs the CronJob task immediately without waiting for the schedule.
`kubectl patch cronjob <name> -p '{"spec" : {"suspend" : true}}'`	Pause: Stops the CronJob from triggering new tasks.
`kubectl patch cronjob <name> -p '{"spec" : {"suspend" : false}}'`	Resume: Allows a suspended CronJob to start running on schedule again.
`kubectl edit cronjob <name>`	Opens the CronJob YAML in your editor to quickly change the schedule or image.
`kubectl delete cronjob <name>`	Deletes the CronJob and all Jobs/Pods it previously created.

Lab: CronJob

# demo-cronjob-minute.yaml
apiVersion: batch/v1
kind: CronJob
metadata:
  name: demo-cronjob-minute
spec:
  schedule: "10 * * * *" # every 10 minutes
  jobTemplate:
    metadata:
      labels:
        app: busybox
    spec:
      template:
        metadata:
          labels:
            app: busybox
        spec:
          restartPolicy: OnFailure
          volumes:
            - name: local-storage
              hostPath:
                path: /tmp/data
                type: DirectoryOrCreate
          containers:
            - name: busybox
              image: busybox
              volumeMounts:
                - name: local-storage
                  mountPath: /data
              command:
                - "/bin/sh"
                - "-c"
              args:
                - |
                  echo "$(date) CronJob message";
                  echo "$(date) CronJob message" >> /data/message.txt;
                  sleep 30;

kubectl apply -f demo-cronjob-minute.yaml
# cronjob.batch/demo-cronjob-minute created

kubectl get cronjob -w
# NAME                  SCHEDULE       TIMEZONE   SUSPEND   ACTIVE   LAST SCHEDULE   AGE
# demo-cronjob-minute   */10 * * * *   <none>     False     0        <none>          9m20s
# demo-cronjob-minute   */10 * * * *   <none>     False     1        0s              9m36s
# demo-cronjob-minute   */10 * * * *   <none>     False     0        34s             10m
# demo-cronjob-minute   */10 * * * *   <none>     False     1        0s              19m
# demo-cronjob-minute   */10 * * * *   <none>     False     0        36s             20m

#
kubectl describe cronjob demo-cronjob-minute
# Name:                          demo-cronjob-minute
# Namespace:                     default
# Labels:                        <none>
# Annotations:                   <none>
# Schedule:                      */10 * * * *
# Concurrency Policy:            Allow
# Suspend:                       False
# Successful Job History Limit:  3
# Failed Job History Limit:      1
# Starting Deadline Seconds:     <unset>
# Selector:                      <unset>
# Parallelism:                   <unset>
# Completions:                   <unset>
# Pod Template:
#   Labels:  app=busybox
#   Containers:
#    busybox:
#     Image:      busybox
#     Port:       <none>
#     Host Port:  <none>
#     Command:
#       /bin/sh
#       -c
#     Args:
#       echo "$(date) CronJob message";
#       echo "$(date) CronJob message" >> /data/message.txt;
#       sleep 30;

#     Environment:  <none>
#     Mounts:
#       /data from local-storage (rw)
#   Volumes:
#    local-storage:
#     Type:            HostPath (bare host directory volume)
#     Path:            /tmp/data
#     HostPathType:    DirectoryOrCreate
#   Node-Selectors:    <none>
#   Tolerations:       <none>
# Last Schedule Time:  Sat, 03 Jan 2026 14:00:00 -0500
# Active Jobs:         <none>
# Events:
#   Type    Reason            Age   From                Message
#   ----    ------            ----  ----                -------
#   Normal  SuccessfulCreate  11m   cronjob-controller  Created job demo-cronjob-minute-29457770
#   Normal  SawCompletedJob   10m   cronjob-controller  Saw completed job: demo-cronjob-minute-29457770, condition: Complete
#   Normal  SuccessfulCreate  76s   cronjob-controller  Created job demo-cronjob-minute-29457780
#   Normal  SawCompletedJob   40s   cronjob-controller  Saw completed job: demo-cronjob-minute-29457780, condition: Complete`


# confirm create a job and run per 10 min
kubectl get job -w
# NAME                           STATUS    COMPLETIONS   DURATION   AGE
# demo-cronjob-minute-29457770   Running   0/1                      0s
# demo-cronjob-minute-29457770   Running   0/1           0s         0s
# demo-cronjob-minute-29457770   Running   0/1           4s         4s
# demo-cronjob-minute-29457770   Running   0/1           33s        33s
# demo-cronjob-minute-29457770   SuccessCriteriaMet   0/1           34s        34s
# demo-cronjob-minute-29457770   Complete             1/1           34s        34s
# demo-cronjob-minute-29457780   Running              0/1                      0s
# demo-cronjob-minute-29457780   Running              0/1           0s         0s
# demo-cronjob-minute-29457780   Running              0/1           4s         4s
# demo-cronjob-minute-29457780   Running              0/1           35s        35s
# demo-cronjob-minute-29457780   SuccessCriteriaMet   0/1           36s        36s
# demo-cronjob-minute-29457780   Complete             1/1           36s        36s

# confirm controled by cronjob
kubectl describe job demo-cronjob-minute-29457770
# Name:             demo-cronjob-minute-29457770
# Namespace:        default
# Selector:         batch.kubernetes.io/controller-uid=c43ac375-388f-452b-b215-204c6816bcb7
# Labels:           app=busybox
# Annotations:      batch.kubernetes.io/cronjob-scheduled-timestamp: 2026-01-03T18:50:00Z
# Controlled By:    CronJob/demo-cronjob-minute
# Parallelism:      1
# Completions:      1
# Completion Mode:  NonIndexed
# Suspend:          false
# Backoff Limit:    6
# Start Time:       Sat, 03 Jan 2026 13:50:00 -0500
# Completed At:     Sat, 03 Jan 2026 13:50:34 -0500
# Duration:         34s
# Pods Statuses:    0 Active (0 Ready) / 1 Succeeded / 0 Failed
# Pod Template:
#   Labels:  app=busybox
#            batch.kubernetes.io/controller-uid=c43ac375-388f-452b-b215-204c6816bcb7
#            batch.kubernetes.io/job-name=demo-cronjob-minute-29457770
#            controller-uid=c43ac375-388f-452b-b215-204c6816bcb7
#            job-name=demo-cronjob-minute-29457770
#   Containers:
#    busybox:
#     Image:      busybox
#     Port:       <none>
#     Host Port:  <none>
#     Command:
#       /bin/sh
#       -c
#     Args:
#       echo "$(date) CronJob message";
#       echo "$(date) CronJob message" >> /data/message.txt;
#       sleep 30;

#     Environment:  <none>
#     Mounts:
#       /data from local-storage (rw)
#   Volumes:
#    local-storage:
#     Type:          HostPath (bare host directory volume)
#     Path:          /tmp/data
#     HostPathType:  DirectoryOrCreate
#   Node-Selectors:  <none>
#   Tolerations:     <none>
# Events:
#   Type    Reason            Age   From            Message
#   ----    ------            ----  ----            -------
#   Normal  SuccessfulCreate  12m   job-controller  Created pod: demo-cronjob-minute-29457770-tfmrj
#   Normal  Completed         11m   job-controller  Job completed

# confirm create a pod per 10 min
kubectl get pod
# NAME                                 READY   STATUS      RESTARTS      AGE
# demo-cronjob-minute-29457770-tfmrj   0/1     Completed   0             13m
# demo-cronjob-minute-29457780-n7fd9   0/1     Completed   0             3m7s

# confirm: per 10 min
kubectl get pod -w
# NAME                                  READY   STATUS    RESTARTS      AGE
# demo-cronjob-minute-29457770-tfmrj   0/1     Pending   0             0s
# demo-cronjob-minute-29457770-tfmrj   0/1     Pending   0             0s
# demo-cronjob-minute-29457770-tfmrj   0/1     ContainerCreating   0             0s
# demo-cronjob-minute-29457770-tfmrj   1/1     Running             0             3s
# demo-cronjob-minute-29457770-tfmrj   0/1     Completed           0             32s
# demo-cronjob-minute-29457770-tfmrj   0/1     Completed           0             34s
# demo-cronjob-minute-29457770-tfmrj   0/1     Completed           0             34s
# demo-cronjob-minute-29457780-n7fd9   0/1     Pending             0             0s
# demo-cronjob-minute-29457780-n7fd9   0/1     Pending             0             0s
# demo-cronjob-minute-29457780-n7fd9   0/1     ContainerCreating   0             0s
# demo-cronjob-minute-29457780-n7fd9   1/1     Running             0             3s
# demo-cronjob-minute-29457780-n7fd9   0/1     Completed           0             34s
# demo-cronjob-minute-29457780-n7fd9   0/1     Completed           0             36s
# demo-cronjob-minute-29457780-n7fd9   0/1     Completed           0             36s

# cofnirm: controlled by job
kubectl describe pod demo-cronjob-minute-29457780-n7fd9
# Name:             demo-cronjob-minute-29457780-n7fd9
# Namespace:        default
# Priority:         0
# Service Account:  default
# Node:             docker-desktop/192.168.65.3
# Start Time:       Sat, 03 Jan 2026 14:00:00 -0500
# Labels:           app=busybox
#                   batch.kubernetes.io/controller-uid=b49e44de-881b-4459-998d-8daadd455727
#                   batch.kubernetes.io/job-name=demo-cronjob-minute-29457780
#                   controller-uid=b49e44de-881b-4459-998d-8daadd455727
#                   job-name=demo-cronjob-minute-29457780
# Annotations:      <none>
# Status:           Succeeded
# IP:               10.1.4.196
# IPs:
#   IP:           10.1.4.196
# Controlled By:  Job/demo-cronjob-minute-29457780
# Containers:
#   busybox:
#     Container ID:  docker://5c4f643661177a4842aa77c3ea458be22d43a93d9f555b0b35db34c29ab900db
#     Image:         busybox
#     Image ID:      docker-pullable://busybox@sha256:d80cd694d3e9467884fcb94b8ca1e20437d8a501096cdf367a5a1918a34fc2fd
#     Port:          <none>
#     Host Port:     <none>
#     Command:
#       /bin/sh
#       -c
#     Args:
#       echo "$(date) CronJob message";
#       echo "$(date) CronJob message" >> /data/message.txt;
#       sleep 30;

#     State:          Terminated
#       Reason:       Completed
#       Exit Code:    0
#       Started:      Sat, 03 Jan 2026 14:00:03 -0500
#       Finished:     Sat, 03 Jan 2026 14:00:33 -0500
#     Ready:          False
#     Restart Count:  0
#     Environment:    <none>
#     Mounts:
#       /data from local-storage (rw)
#       /var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-sd879 (ro)
# Conditions:
#   Type                        Status
#   PodReadyToStartContainers   False
#   Initialized                 True
#   Ready                       False
#   ContainersReady             False
#   PodScheduled                True
# Volumes:
#   local-storage:
#     Type:          HostPath (bare host directory volume)
#     Path:          /tmp/data
#     HostPathType:  DirectoryOrCreate
#   kube-api-access-sd879:
#     Type:                    Projected (a volume that contains injected data from multiple sources)
#     TokenExpirationSeconds:  3607
#     ConfigMapName:           kube-root-ca.crt
#     Optional:                false
#     DownwardAPI:             true
# QoS Class:                   BestEffort
# Node-Selectors:              <none>
# Tolerations:                 node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
#                              node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
# Events:
#   Type    Reason     Age    From               Message
#   ----    ------     ----   ----               -------
#   Normal  Scheduled  3m31s  default-scheduler  Successfully assigned default/demo-cronjob-minute-29457780-n7fd9 to docker-desktop
#   Normal  Pulling    3m30s  kubelet            Pulling image "busybox"
#   Normal  Pulled     3m29s  kubelet            Successfully pulled image "busybox" in 1.383s (1.383s including waiting). Image size: 2224358 bytes.
#   Normal  Created    3m28s  kubelet            Created container: busybox
#   Normal  Started    3m28s  kubelet            Started container busybox

# check log confirm write message per 10 min
kubectl exec -it observer -- cat /data/message.txt
# Sat Jan  3 18:50:02 UTC 2026 CronJob message
# Sat Jan  3 19:00:03 UTC 2026 CronJob message

kubectl exec -it observer -- rm /data/message.txt

Note_Tech

Kubernetes - CronJob

CronJobs

Schedule

3-10/2: The /N pattern can also be applied to ranges. e.g., March, May, July, and September.

Suspending and resuming

Automatically removing finished Jobs

Specify Start Deadline

Job concurrency

Imperative Command

Lab: CronJob

`3-10/2`: The /N pattern can also be applied to ranges. e.g., March, May, July, and September.