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kube-scheduler.md

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🧠 kube-scheduler – The Decision-Maker of Kubernetes

🔹 What is kube-scheduler?

The kube-scheduler is a control plane component in Kubernetes responsible for assigning Pods to Nodes.

It watches for unscheduled Pods and selects the most suitable Node to run each Pod based on a set of rules and constraints.

🔹 Key Responsibilities

Responsibility Description
Pod Scheduling Assigns Pods to suitable Nodes based on policy and resource availability
Score & Filter Nodes Filters out Nodes that can't run the Pod, then scores remaining ones
Scheduling Decisions Writes the selected Node name into the Pod spec (spec.nodeName)
Extensibility Supports custom policies via plugins, extenders, and profiles

🔹 When Does kube-scheduler Act?

  1. A new Pod is created (e.g., via Deployment).
  2. The Pod does not yet have a spec.nodeName.
  3. kube-scheduler:
    • Filters nodes based on requirements
    • Scores nodes for best fit
    • Assigns the Pod to the best Node
  4. kube-apiserver updates the Pod spec with the Node assignment.
  5. The kubelet on the selected Node picks up the Pod and runs it.