Nugroho

just notes

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The Kube Scheduler is a Control Plane component responsible for deciding which node should run a Pod.

The scheduler makes the placement decision; the Kubelet on the selected node actually creates and runs the Pod.

Basic Flow

Pod created
    ↓
Kube API Server
    ↓
Kube Scheduler
    ↓
Selects the best node
    ↓
Kube API Server
    ↓
Kubelet on selected node
    ↓
Pod runs

How Scheduling Works

When multiple nodes are available, the scheduler evaluates each node using two main phases:

1. Filtering

The scheduler first eliminates nodes that cannot run the and doesn’t meet the pod’s resource requirements..

For example:

Pod requires: 10 CPUs

Node A → 4 CPUs  ❌
Node B → 12 CPUs ✅
Node C → 16 CPUs ✅

Nodes without sufficient resources or that don’t meet the Pod’s requirements are filtered out.

2. Ranking

After filtering, the scheduler scores the remaining nodes and selects the best one.

Node B → Score: 6
Node C → Score: 10  ← Selected

The scheduler uses priority functions to compare candidate nodes and choose the best match.

Scheduling Criteria

The scheduler can consider different requirements, including:

  • CPU and memory availability
  • Resource limits
  • Taints and tolerations
  • Node selectors
  • Affinity rules

Scheduling is also customizable, and Kubernetes supports developing custom schedulers.

Kubeadm Deployment

With kubeadm, the scheduler runs as a Pod in the kube-system namespace.

View the scheduler Pod:

kubectl get pods -n kube-system

View its manifest:

cat /etc/kubernetes/manifests/kube-scheduler.yaml

Check the running process and its options:

ps -aux | grep kube-scheduler

Example:

kube-scheduler \
  --address=127.0.0.1 \
  --kubeconfig=/etc/kubernetes/scheduler.conf \
  --leader-elect=true

Quick Summary

Component Responsibility
Kube Scheduler Decides which node should run a Pod
Filtering Removes unsuitable nodes
Ranking Scores remaining nodes
Kubelet Actually creates/runs the Pod on the selected node
API Server Coordinates communication between components

Key Takeaway

Kube Scheduler = Pod placement decision-maker.

It filters unsuitable nodes, ranks the remaining candidates, and selects the best node for the Pod.

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