Running background tasks on nodes automatically with daemon sets

As an administrator, you can create and use daemon sets to run replicas of a pod on specific or all nodes in an OKD cluster.

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, only cluster administrators can create daemon sets.

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

Scheduled by default scheduler

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 Pending 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.

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

In addition, a node.kubernetes.io/unschedulable:NoSchedule toleration is added automatically to daemon set pods. The default scheduler ignores unschedulable Nodes when scheduling daemon set pods.

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:

  $ oc patch namespace myproject -p \
      '{"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:

   apiVersion: apps/v1
   kind: DaemonSet
   metadata:
     name: hello-daemonset
   spec:
     selector:
         matchLabels:
           name: hello-daemonset (1)
     template:
       metadata:
         labels:
           name: hello-daemonset (2)
       spec:
         nodeSelector: (3)
           role: worker
         containers:
         - image: openshift/hello-openshift
           imagePullPolicy: Always
           name: registry
           ports:
           - containerPort: 80
             protocol: TCP
           resources: {}
           terminationMessagePath: /dev/termination-log
         serviceAccount: default
         terminationGracePeriodSeconds: 10

   1	The label selector that determines which pods belong to the daemon set.
   2	The pod template’s label selector. Must match the label selector above.
   3	The node selector that determines on which nodes pod replicas should be deployed. A matching label must be present on the node.

2. Create the daemon set object:

   $ oc create -f daemonset.yaml

3. To verify that the pods were created, and that each node has a pod replica:

   1. Find the daemonset pods:

      $ oc get pods

      Example output

      hello-daemonset-cx6md   1/1       Running   0          2m
      hello-daemonset-e3md9   1/1       Running   0          2m

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

      $ oc describe pod/hello-daemonset-cx6md|grep Node

      Example output

      Node:        openshift-node01.hostname.com/10.14.20.134

      $ oc describe pod/hello-daemonset-e3md9|grep Node

      Example output

      Node:        openshift-node02.hostname.com/10.14.20.137