Kube Scheduler

🧠 What is kube-scheduler?¶

It's the brain that assigns pods to nodes:

• It looks at all unscheduled pods.
• It analyzes each node’s available resources, taints, affinities, etc.
• It picks the best node and updates the pod spec.

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    component: kube-scheduler
    tier: control-plane
  name: kube-scheduler
  namespace: kube-system
spec:
  containers:
  - command:
    - kube-scheduler
    - --authentication-kubeconfig=/etc/kubernetes/scheduler.conf
    - --authorization-kubeconfig=/etc/kubernetes/scheduler.conf
    - --bind-address=127.0.0.1
    - --kubeconfig=/etc/kubernetes/scheduler.conf
    - --leader-elect=true
    image: registry.k8s.io/kube-scheduler:v1.33.0
    imagePullPolicy: IfNotPresent
    livenessProbe:
      failureThreshold: 8
      httpGet:
        host: 127.0.0.1
        path: /livez
        port: 10259
        scheme: HTTPS
      initialDelaySeconds: 10
      periodSeconds: 10
      timeoutSeconds: 15
    name: kube-scheduler
    readinessProbe:
      failureThreshold: 3
      httpGet:
        host: 127.0.0.1
        path: /readyz
        port: 10259
        scheme: HTTPS
      periodSeconds: 1
      timeoutSeconds: 15
    resources:
      requests:
        cpu: 100m
    startupProbe:
      failureThreshold: 24
      httpGet:
        host: 127.0.0.1
        path: /livez
        port: 10259
        scheme: HTTPS
      initialDelaySeconds: 10
      periodSeconds: 10
      timeoutSeconds: 15
    volumeMounts:
    - mountPath: /etc/kubernetes/scheduler.conf
      name: kubeconfig
      readOnly: true
  hostNetwork: true
  priority: 2000001000
  priorityClassName: system-node-critical
  securityContext:
    seccompProfile:
      type: RuntimeDefault
  volumes:
  - hostPath:
      path: /etc/kubernetes/scheduler.conf
      type: FileOrCreate
    name: kubeconfig
status: {}

🔹 COMMAND FLAGS EXPLAINED¶

Let's go line by line.

✅ --authentication-kubeconfig=/etc/kubernetes/scheduler.conf¶

🔍 Purpose: Used by the scheduler when it needs to authenticate itself to the kube-apiserver.

💡 Think of it like:

"Hey API server, I'm the legit kube-scheduler, here's my kubeconfig + cert."

📁 This kubeconfig:

• Has client cert/key to prove identity.
• Points to the API server endpoint.
• Includes cluster CA.

✅ --authorization-kubeconfig=/etc/kubernetes/scheduler.conf¶

🔐 Purpose: Used for authorization of kube-scheduler — i.e., what it’s allowed to do.

💡 Think of it like:

"What operations am I allowed to perform on the cluster objects?"

✅ Both authentication- and authorization- configs can be same file — which it is here (scheduler.conf).

✅ --bind-address=127.0.0.1¶

📌 Purpose: This tells the scheduler:

"Only listen on the local interface."

It listens on:

• https://127.0.0.1:10259 → for health checks & metrics

🔒 Security Benefit:

• Prevents access from other hosts.
• Scheduler does not need to be accessed externally.

✅ --kubeconfig=/etc/kubernetes/scheduler.conf¶

🧭 Purpose: Used by the scheduler to communicate with the kube-apiserver.

📁 Same file as above, but this is the primary config for API calls like:

• Getting pods
• Listing nodes
• Watching for new pod objects

✅ --leader-elect=true¶

🧠 Purpose: Enables leader election, important for HA (High Availability) setups.

💡 In multi-control-plane clusters:

• Multiple scheduler pods might run.
• But only one becomes the active leader.
• Others standby and take over if the leader fails.

📦 This uses Kubernetes built-in lease objects for coordination.

🔹 PROBES¶

These define how kubelet monitors the scheduler container’s health.

✅ startupProbe, livenessProbe, readinessProbe¶

📍 All hit:

• host: 127.0.0.1
• port: 10259
• scheme: HTTPS

🛡️ These endpoints are only reachable from localhost.

🔹 VOLUME MOUNTS¶

✅ Volume + Mount¶

volumeMounts:
  - mountPath: /etc/kubernetes/scheduler.conf
    name: kubeconfig

It mounts the host's kubeconfig file (scheduler.conf) into the container at the same path — used for authentication + communication with the API server.

🔹 OTHER FIELDS¶

🧠 Summary (Plain English)¶

💡 Optional Info¶

• The scheduler.conf file is auto-generated by kubeadm init and is stored at:

/etc/kubernetes/scheduler.conf

You can inspect it:

kubectl config view --kubeconfig=/etc/kubernetes/scheduler.conf