Room 46

CUSTOM SCHEDULER

The Scheduler’s Throne

The default scheduler places pods on nodes based on resource availability. But what if you need custom logic?
Deploy a custom scheduler and configure pods to use it.

📜 Mission Briefing

Study how the default scheduler works: predicates → priorities → binding.
Deploy custom-scheduler.yaml - a simple scheduler that runs as a pod.
Create a pod that uses schedulerName: custom-scheduler.
Verify the pod is scheduled by the custom scheduler.
Check the scheduler pod logs for the password: schedlr7.

Key Commands

Command	Purpose
`kubectl get pods -n kube-system -l component=kube-scheduler`	Default scheduler
`kubectl logs -n kube-system kube-scheduler-*`	Scheduler logs
`kubectl get events --field-selector reason=Scheduled`	Scheduling events

How the Scheduler Works

# Scheduling Cycle:
# 1. Watch:    New pod with no nodeName
# 2. Filter:   Remove nodes that can't run the pod (predicates)
#              - Insufficient resources
#              - Node taints without tolerations
#              - Node affinity mismatch
# 3. Score:    Rank remaining nodes (priorities)
#              - Spread pods across nodes
#              - Prefer nodes with images cached
#              - Resource utilization balance
# 4. Bind:     Assign pod to highest-scoring node

# Scheduler extension points (framework):
# QueueSort → PreFilter → Filter → PostFilter →
# PreScore → Score → NormalizeScore →
# Reserve → Permit → PreBind → Bind → PostBind

# Pod using a custom scheduler
apiVersion: v1
kind: Pod
metadata:
  name: custom-scheduled
spec:
  schedulerName: custom-scheduler   # Use custom scheduler
  containers:
    - name: app
      image: nginx:alpine

# Simple custom scheduler deployment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: custom-scheduler
  namespace: kube-system
spec:
  replicas: 1
  selector:
    matchLabels:
      component: custom-scheduler
  template:
    metadata:
      labels:
        component: custom-scheduler
    spec:
      serviceAccountName: custom-scheduler
      containers:
        - name: scheduler
          image: registry.k8s.io/kube-scheduler:v1.29.0
          command:
            - kube-scheduler
            - --config=/etc/kubernetes/scheduler-config.yaml
          volumeMounts:
            - name: config
              mountPath: /etc/kubernetes
      volumes:
        - name: config
          configMap:
            name: scheduler-config

# Multiple schedulers can coexist
# Each pod specifies which scheduler handles it
# Default scheduler: spec.schedulerName: default-scheduler (or omit)

# Scheduler profiles (K8s 1.18+):
# Configure multiple profiles in a single scheduler binary
# Each profile can enable/disable different plugins

Hints

Hint 1

kubectl apply -f custom-scheduler.yaml
kubectl apply -f custom-scheduled-pod.yaml

Hint 2

Check scheduler logs:

kubectl logs deploy/custom-scheduler -n kube-system | grep password

Hint 3

The password is schedlr7.

🔓 Unlock Room 47

openssl enc -aes-256-cbc -d -a -pbkdf2 \
        -in ../room_47/README -out ../room_47/README.txt -pass pass:PASSWORD
cat ../room_47/README.txt