6.9. Understanding node rebooting


Review the following information to learn about rebooting a node without causing an outage for applications running on the platform by first evacuating the pods on the node.

For pods that are made highly available by the routing tier, nothing else needs to be done. For other pods needing storage, typically databases, it is critical to ensure that they can remain in operation with one pod temporarily going offline. While implementing resiliency for stateful pods is different for each application, in all cases it is important to configure the scheduler to use node anti-affinity to ensure that the pods are properly spread across available nodes.

Another challenge is how to handle nodes that are running critical infrastructure such as the router or the registry. The same node evacuation process applies, though it is important to understand certain edge cases.

6.9.1. About rebooting nodes running critical infrastructure

When rebooting nodes that host critical OpenShift Container Platform infrastructure components, such as router pods, registry pods, and monitoring pods, ensure that there are at least three nodes available to run these components.

The following scenario demonstrates how service interruptions can occur with applications running on OpenShift Container Platform when only two nodes are available:

  • Node A is marked unschedulable and all pods are evacuated.
  • The registry pod running on that node is now redeployed on node B. Node B is now running both registry pods.
  • Node B is now marked unschedulable and is evacuated.
  • The service exposing the two pod endpoints on node B loses all endpoints, for a brief period of time, until they are redeployed to node A.

When using three nodes for infrastructure components, this process does not result in a service disruption. However, due to pod scheduling, the last node that is evacuated and brought back into rotation does not have a registry pod. One of the other nodes has two registry pods. To schedule the third registry pod on the last node, use pod anti-affinity to prevent the scheduler from locating two registry pods on the same node.

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