Chapter 3. Using Jobs and DaemonSets

A DaemonSet ensures that all eligible nodes run a copy of a Pod. Normally, the node that a Pod runs on is selected by the Kubernetes scheduler. However, previously daemonSet pods are created and scheduled by the DaemonSet controller. That introduces the following issues:

ScheduleDaemonSetPods is enabled by default in OpenShift Container Platform which lets you to schedule DaemonSets using the default scheduler instead of the DaemonSet controller, by adding the NodeAffinity term to the DaemonSet pods, instead of the .spec.nodeName term. The default scheduler is then used to bind the pod to the target host. If node affinity of the DaemonSet pod already exists, it is replaced. The DaemonSet controller only performs these operations when creating or modifying DaemonSet pods, and no changes are made to the spec.template of the DaemonSet.

nodeAffinity: requiredDuringSchedulingIgnoredDuringExecution: nodeSelectorTerms: - matchFields: - key: metadata.name operator: In values: - target-host-name

In addition, node.kubernetes.io/unschedulable:NoSchedule toleration is added automatically to DaemonSet Pods. The default scheduler ignores unschedulable Nodes when scheduling DaemonSet Pods.

When creating daemonsets, the nodeSelector field is used to indicate the nodes on which the daemonset should deploy replicas.

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 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 OpenShift Container Platform:

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

For more information about how to make use of the different types of Job, see Job Patterns in the Kubernetes documentation.

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

CronJobs are useful for creating periodic and recurring tasks, like running backups or sending emails. CronJobs can also schedule individual tasks for a specific time, such as if you want to schedule a Job for a low activity period.

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

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.

You create a job in OpenShift Container Platform by creating a job object.

For details on how OpenShift Container Platform uses restart policy with failed containers, see the Example States in the Kubernetes documentation.

$ oc run pi --image=perl --replicas=1  --restart=OnFailure \
    --command -- perl -Mbignum=bpi -wle 'print bpi(2000)'

You create a CronJob in OpenShift Container Platform by creating a job object.

$ oc run pi --image=perl --schedule='*/1 * * * *' \
    --restart=OnFailure --labels parent="cronjobpi" \
    --command -- perl -Mbignum=bpi -wle 'print bpi(2000)'