How does the scheduler decide where a pod runs, and how do you influence it with affinity and taints?

Question

Accepted Answer

The scheduler works in 2 phases. First it filters, dropping any node that cannot run the pod, not enough free CPU or memory against the pod's requests, node selectors that do not match, taints that are not tolerated. Then it scores the survivors and places the pod on the best one. To influence that, there are two complementary mechanisms. Affinity attracts. nodeSelector is a plain label match, node affinity is the richer form with required and preferred rules, where a preferred rule carries a weight from 1 to 100, and pod affinity and anti affinity place a pod relative to other pods, so I can co locate two services that chat constantly or keep replicas apart. Taints and tolerations are the inverse, a taint repels every pod unless the pod carries a matching toleration, which is how you reserve nodes, say a pool of 4 GPU nodes or the control plane, for specific workloads. For resilience I use topology spread constraints or anti affinity with a maxSkew of 1 across zones, so 6 replicas land 2 per zone across 3 availability zones and losing one zone never takes the whole service down. And most pod stuck in Pending cases trace right back to this filter phase, no node satisfied the requests, the selectors, or the tolerations.

Answer

A pod is stuck in Pending. What scheduling causes do you check?

No node with enough free CPU/memory for the requests, an unsatisfied nodeSelector/affinity, a taint with no matching toleration, or no node in the required zone. kubectl describe pod shows the scheduler reason.

Answer

What is the difference between node affinity and a taint?

Affinity is the pod choosing nodes (attraction). A taint is the node repelling pods unless they tolerate it (rejection). You often combine them: taint a node pool and give the intended pods both a toleration and affinity for it.

How does the scheduler decide where a pod runs, and how do you influence it with affinity and taints?

How does the scheduler decide where a pod runs, and how do you influence it with affinity and taints?

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