Kubernetes Scheduler: How Pods Get Placed on Nodes#

The scheduler (kube-scheduler) watches for newly created pods that have no node assignment. For each unscheduled pod, the scheduler selects the best node and writes a binding back to the API server. The kubelet on that node then starts the pod. If no node is suitable, the pod stays Pending until conditions change.

The scheduler is the reason pods run where they do. Understanding its internals is essential for diagnosing Pending pods, designing placement constraints, and managing cluster utilization.

Pod Topology Spread Constraints#

Pod anti-affinity gives you binary control: either a pod avoids another pod’s topology domain or it does not. But it does not give you even distribution. If you have 6 replicas and 3 zones, anti-affinity cannot express “put exactly 2 in each zone.” Topology spread constraints solve this by letting you specify the maximum allowed imbalance between any two topology domains.

How Topology Spread Works#

A topology spread constraint defines: