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Merge pull request #30492 from mburke5678/issue-28020
Update wording in Reboot Nodes topic for clarity
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modules/nodes-nodes-rebooting-infrastructure.adoc

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// * nodes/nodes-nodes-rebooting.adoc
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[id="nodes-nodes-rebooting-infrastructure_{context}"]
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= Understanding infrastructure node rebooting
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= About rebooting nodes running critical infrastructure
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Infrastructure nodes are nodes that are labeled to run pieces of the
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{product-title} environment. Currently, the easiest way to manage node reboots
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is to ensure that there are at least three nodes available to run
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infrastructure. The nodes to run the infrastructure are called *master* nodes.
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When rebooting nodes that host critical {product-title} infrastructure components, such as router pods, registry pods, and monitoring pods, ensure that there are at least three nodes available to run these components.
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The scenario below demonstrates a common mistake that can lead
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to service interruptions for the applications running on {product-title} when
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only two nodes are available.
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The following scenario demonstrates how service interruptions can occur with applications running on {product-title} when only two nodes are available:
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- Node A is marked unschedulable and all pods are evacuated.
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- The registry pod running on that node is now redeployed on node B. This means
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node B is now running both registry pods.
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- The registry pod running on that node is now redeployed on node B. Node B is now running both registry pods.
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- Node B is now marked unschedulable and is evacuated.
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- The service exposing the two pod endpoints on node B, for a brief period of
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time, loses all endpoints until they are redeployed to node A.
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- 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.
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The same process using three master nodes for infrastructure does not result in a service
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disruption. However, due to pod scheduling, the last node that is evacuated and
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brought back in to rotation is left running zero registries. The other two nodes
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will run two and one registries respectively. The best solution is to rely on
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pod anti-affinity.
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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.

nodes/nodes/nodes-nodes-rebooting.adoc

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include::modules/nodes-nodes-rebooting-infrastructure.adoc[leveloffset=+1]
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.Additional information
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* For more information on pod anti-affinity, see xref:../../nodes/scheduling/nodes-scheduler-pod-affinity.adoc#nodes-scheduler-pod-affinity[Placing pods relative to other pods using affinity and anti-affinity rules].
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include::modules/nodes-nodes-rebooting-affinity.adoc[leveloffset=+1]
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include::modules/nodes-nodes-rebooting-router.adoc[leveloffset=+1]

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