Running background tasks on nodes automatically with daemon sets

A daemon set ensures that all (or some) nodes run a copy of a pod. As nodes are added to the cluster, pods are added to the cluster. As nodes are removed from the cluster, those pods are removed through garbage collection. Deleting a daemon set will clean up the pods it created.

You can use daemon sets to create shared storage, run a logging pod on every node in your cluster, or deploy a monitoring agent on every node.

For security reasons, the cluster administrators and the project administrators can create daemon sets.

For more information on daemon sets, see the Kubernetes documentation.

A daemon set 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 daemon set pods are created and scheduled by the daemon set controller. That introduces the following issues:

• Inconsistent pod behavior: Normal pods waiting to be scheduled are created and in Pending state, but daemon set pods are not created in state. This is confusing to the user.

• Pod preemption is handled by default scheduler. When preemption is enabled, the daemon set controller will make scheduling decisions without considering pod priority and preemption.

The ScheduleDaemonSetPods feature, enabled by default in OKD, lets you to schedule daemon sets using the default scheduler instead of the daemon set controller, by adding the NodeAffinity term to the daemon set 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 daemon set pod already exists, it is replaced. The daemon set controller only performs these operations when creating or modifying daemon set pods, and no changes are made to the spec.template of the daemon set.

Creating daemonsets

When creating daemon sets, the nodeSelector field is used to indicate the nodes on which the daemon set should deploy replicas.

Prerequisites

• Before you start using daemon sets, disable the default project-wide node selector in your namespace, by setting the namespace annotation openshift.io/node-selector to an empty string:

      '{"metadata": {"annotations": {"openshift.io/node-selector": ""}}}'

• If you are creating a new project, overwrite the default node selector:

  $ oc adm new-project <name> --node-selector=""

Procedure

To create a daemon set:

1. Define the daemon set yaml file:

2. Create the daemon set object:

   $ oc create -f daemonset.yaml

3. 1. View the pods to verify the pod has been placed onto the node:

      Example output

      Node:        openshift-node01.hostname.com/10.14.20.134

      Example output

      Node:        openshift-node02.hostname.com/10.14.20.137