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Note on missing/incorrect labels and scheduler behaviour with pod anti-affinity #8973

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Jul 20, 2018
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Note on missing/incorrect labels and scheduler behaviour with pod anti-affinity #8973

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2 changes: 2 additions & 0 deletions content/en/docs/concepts/configuration/assign-pod-node.md
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Expand Up @@ -168,6 +168,8 @@ in the section [Interlude: built-in node labels](#interlude-built-in-node-labels
processing which can slow down scheduling in large clusters significantly. We do
not recommend using them in clusters larger than several hundred nodes.

**Note:** Pod anti-affinity requires nodes to be consistently labelled, i.e. every node in the cluster must have an appropriate label matching `topologyKey`. If some or all nodes are missing the speficied `topologyKey` label, it can lead to unintended behavior.
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I think this applies to both pod affinity and pod anti-affinity, although the unexpected behavior is different.

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Suggestion: start the sentence with "Inter-pod affinity requires..." or "Pod affinity and anti-affinity require..."

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I have tested it with pod affinity and there it works as expected, i.e. a missing topologyKey label does not schedule the (second) pod with affinity configured. That's why I only pointed out the "unexpected behavior" for anti-affinity.

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I think failing to schedule the second pod as you described is unexpected behavior. Suppose pod A is on Node 1, and pod B has required affinity for pod A. If Node 1 has capacity, the expected behavior is for pod B to schedule.


As with node affinity, there are currently two types of pod affinity and anti-affinity, called `requiredDuringSchedulingIgnoredDuringExecution` and
`preferredDuringSchedulingIgnoredDuringExecution` which denote "hard" vs. "soft" requirements.
See the description in the node affinity section earlier.
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