Running tasks in pods using jobs - Using jobs and daemon sets | Nodes

Understanding jobs and cron jobs
Creating jobs
Creating cron jobs

A job executes a task in your Red Hat OpenShift Service on AWS cluster.

A job tracks the overall progress of a task and updates its status with information about active, succeeded, and failed pods. Deleting a job will clean up any pod replicas it created. Jobs are part of the Kubernetes API, which can be managed with oc commands like other object types.

Sample Job specification

apiVersion: batch/v1
kind: Job
metadata:
  name: pi
spec:
  parallelism: 1    (1)
  completions: 1    (2)
  activeDeadlineSeconds: 1800 (3)
  backoffLimit: 6   (4)
  template:         (5)
    metadata:
      name: pi
    spec:
      containers:
      - name: pi
        image: perl
        command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(2000)"]
      restartPolicy: OnFailure    (6)
#...

1	The pod replicas a job should run in parallel.
2	Successful pod completions are needed to mark a job completed.
3	The maximum duration the job can run.
4	The number of retries for a job.
5	The template for the pod the controller creates.
6	The restart policy of the pod.

Additional resources

Jobs in the Kubernetes documentation

Understanding jobs and cron jobs

A job tracks the overall progress of a task and updates its status with information about active, succeeded, and failed pods. Deleting a job cleans up any pods it created. Jobs are part of the Kubernetes API, which can be managed with oc commands like other object types.

There are two possible resource types that allow creating run-once objects in Red Hat OpenShift Service on AWS:

Job

A regular job is a run-once object that creates a task and ensures the job finishes.

There are three main types of task suitable to run as a job:

Non-parallel jobs:
- A job that starts only one pod, unless the pod fails.
- The job is complete as soon as its pod terminates successfully.
Parallel jobs with a fixed completion count:
- a job that starts multiple pods.
- The job represents the overall task and is complete when there is one successful pod for each value in the range 1 to the completions value.
Parallel jobs with a work queue:
- A job with multiple parallel worker processes in a given pod.
- Red Hat OpenShift Service on AWS coordinates pods to determine what each should work on or use an external queue service.
- Each pod is independently capable of determining whether or not all peer pods are complete and that the entire job is done.
- When any pod from the job terminates with success, no new pods are created.
- When at least one pod has terminated with success and all pods are terminated, the job is successfully completed.
- When any pod has exited with success, no other pod should be doing any work for this task or writing any output. Pods should all be in the process of exiting.
  
  For more information about how to make use of the different types of job, see Job Patterns in the Kubernetes documentation.

Cron job

A job can be scheduled to run multiple times, using a cron job.

A cron job builds on a regular job by allowing you to specify how the job should be run. Cron jobs are part of the Kubernetes API, which can be managed with oc commands like other object types.

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. A cron job creates a Job object based on the timezone configured on the control plane node that runs the cronjob controller.

A cron job creates a Job object approximately once per execution time of its schedule, but there are circumstances in which it fails to create a job or two jobs might be created. Therefore, jobs must be idempotent and you must configure history limits.

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Understanding how to create jobs

Both resource types require a job configuration that consists of the following key parts:

A pod template, which describes the pod that Red Hat OpenShift Service on AWS creates.
The parallelism parameter, which specifies how many pods running in parallel at any point in time should execute a job.
- For non-parallel jobs, leave unset. When unset, defaults to 1.
The completions parameter, specifying how many successful pod completions are needed to finish a job.
- For non-parallel jobs, leave unset. When unset, defaults to 1.
- For parallel jobs with a fixed completion count, specify a value.
- For parallel jobs with a work queue, leave unset. When unset defaults to the parallelism value.

Understanding how to set a maximum duration for jobs

When defining a job, you can define its maximum duration by setting the activeDeadlineSeconds field. It is specified in seconds and is not set by default. When not set, there is no maximum duration enforced.

The maximum duration is counted from the time when a first pod gets scheduled in the system, and defines how long a job can be active. It tracks overall time of an execution. After reaching the specified timeout, the job is terminated by Red Hat OpenShift Service on AWS.

Understanding how to set a job back off policy for pod failure

A job can be considered failed, after a set amount of retries due to a logical error in configuration or other similar reasons. Failed pods associated with the job are recreated by the controller with an exponential back off delay (10s, 20s, 40s …) capped at six minutes. The limit is reset if no new failed pods appear between controller checks.

Use the spec.backoffLimit parameter to set the number of retries for a job.

Understanding how to configure a cron job to remove artifacts

Cron jobs can leave behind artifact resources such as jobs or pods. As a user it is important to configure history limits so that old jobs and their pods are properly cleaned. There are two fields within cron job’s spec responsible for that:

.spec.successfulJobsHistoryLimit. The number of successful finished jobs to retain (defaults to 3).
.spec.failedJobsHistoryLimit. The number of failed finished jobs to retain (defaults to 1).

Known limitations

The job specification restart policy only applies to the pods, and not the job controller. However, the job controller is hard-coded to keep retrying jobs to completion.

As such, restartPolicy: Never or --restart=Never results in the same behavior as restartPolicy: OnFailure or --restart=OnFailure. That is, when a job fails it is restarted automatically until it succeeds (or is manually discarded). The policy only sets which subsystem performs the restart.

With the Never policy, the job controller performs the restart. With each attempt, the job controller increments the number of failures in the job status and create new pods. This means that with each failed attempt, the number of pods increases.

With the OnFailure policy, kubelet performs the restart. Each attempt does not increment the number of failures in the job status. In addition, kubelet will retry failed jobs starting pods on the same nodes.

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Creating jobs

You create a job in Red Hat OpenShift Service on AWS by creating a job object.

Procedure

To create a job:

Create a YAML file similar to the following:

apiVersion: batch/v1
kind: Job
metadata:
  name: pi
spec:
  parallelism: 1    (1)
  completions: 1    (2)
  activeDeadlineSeconds: 1800 (3)
  backoffLimit: 6   (4)
  template:         (5)
    metadata:
      name: pi
    spec:
      containers:
      - name: pi
        image: perl
        command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(2000)"]
      restartPolicy: OnFailure    (6)
#...

1	Optional: Specify how many pod replicas a job should run in parallel; defaults to `1`. For non-parallel jobs, leave unset. When unset, defaults to `1`.
2	Optional: Specify how many successful pod completions are needed to mark a job completed. For non-parallel jobs, leave unset. When unset, defaults to `1`. For parallel jobs with a fixed completion count, specify the number of completions. For parallel jobs with a work queue, leave unset. When unset defaults to the `parallelism` value.
3	Optional: Specify the maximum duration the job can run.
4	Optional: Specify the number of retries for a job. This field defaults to six.
5	Specify the template for the pod the controller creates.
6	Specify the restart policy of the pod: `Never`. Do not restart the job. `OnFailure`. Restart the job only if it fails. `Always`. Always restart the job. For details on how Red Hat OpenShift Service on AWS uses restart policy with failed containers, see the Example States in the Kubernetes documentation.

Create the job:
```
$ oc create -f <file-name>.yaml
```

You can also create and launch a job from a single command using oc create job. The following command creates and launches a job similar to the one specified in the previous example:

$ oc create job pi --image=perl -- perl -Mbignum=bpi -wle 'print bpi(2000)'

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Creating cron jobs

You create a cron job in Red Hat OpenShift Service on AWS by creating a job object.

Procedure

To create a cron job:

Create a YAML file similar to the following:

apiVersion: batch/v1
kind: CronJob
metadata:
  name: pi
spec:
  schedule: "*/1 * * * *"          (1)
  concurrencyPolicy: "Replace"     (2)
  startingDeadlineSeconds: 200     (3)
  suspend: true                    (4)
  successfulJobsHistoryLimit: 3    (5)
  failedJobsHistoryLimit: 1        (6)
  jobtemplate:                     (7)
    spec:
      template:
        metadata:
          labels:                  (8)
            parent: "cronjobpi"
        spec:
          containers:
          - name: pi
            image: perl
            command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(2000)"]
          restartPolicy: OnFailure (9)

1 Schedule for the job specified in cron format. In this example, the job will run every minute.

An optional concurrency policy, specifying how to treat concurrent jobs within a cron job. Only one of the following concurrent policies may be specified. If not specified, this defaults to allowing concurrent executions.

Allow allows cron jobs to run concurrently.
Forbid forbids concurrent runs, skipping the next run if the previous has not finished yet.
Replace cancels the currently running job and replaces it with a new one.

3 An optional deadline (in seconds) for starting the job if it misses its scheduled time for any reason. Missed jobs executions will be counted as failed ones. If not specified, there is no deadline.

4 An optional flag allowing the suspension of a cron job. If set to true, all subsequent executions will be suspended.

5 The number of successful finished jobs to retain (defaults to 3).

6 The number of failed finished jobs to retain (defaults to 1).

7 Job template. This is similar to the job example.

8 Sets a label for jobs spawned by this cron job.

The restart policy of the pod. This does not apply to the job controller.

The .spec.successfulJobsHistoryLimit and .spec.failedJobsHistoryLimit fields are optional. These fields specify how many completed and failed jobs should be kept. By default, they are set to 3 and 1 respectively. Setting a limit to 0 corresponds to keeping none of the corresponding kind of jobs after they finish.

Create the cron job:
```
$ oc create -f <file-name>.yaml
```

You can also create and launch a cron job from a single command using oc create cronjob. The following command creates and launches a cron job similar to the one specified in the previous example:

$ oc create cronjob pi --image=perl --schedule='*/1 * * * *' -- perl -Mbignum=bpi -wle 'print bpi(2000)'

With oc create cronjob, the --schedule option accepts schedules in cron format.