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scheduler.md

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scheduler

  • scheduler is responsible for deciding which pod goes to which node - it doesnt place the [[pods]] on the [[node]], that job is for [[kubelet]]
  • kube scheduler watches for new workloads/pods and assigns them to a [[node]] based on several scheduling factors. these include - are there enough resources required for the pod - is the [[node]] healthy / capacity availability - are the network ports for pods available - [[node affinity]] and anti affinity rules - [[taints]] and [[tolerations]]
  • scheduler assigns node to a pod by filltering out based on - resource requirements, this can include cpu/memory needed by the application running inside application - ranking of the pod based on the priority of 1 to 10
  • /etc/kubernetes/manifests/kube-scheduler.yml
  • a cluster can multiple schedulers same time
    • which scheduler to use can be instructed when a pod is created
    • when a new scheduler is created change the parameter --scheduler-name when launching it in the cluster
  • a custom scheduler if needed can be created as k8s is highly extensible.
    • this scheduler can be packages as default one if needed

manual scheduling

  • if no scheduler is present in the cluster, the state of the pods will be pending

nodeName

  • a nodeName object in the [[yaml]] definition for [[pods]] can be used to avoid the pending state and manually schedule to a [[node]]
  • it is the simplest form of node selection constraint
  • it is binding object that refers to name of the [[node]] the [[pods]] must be scheduled to
  • by default it's set as empty, scheduler will auto pick a [[node]] and add this in config that stored by the kubernetes
  • if the pod is already created the nodeName object can't be added by changing the [[yaml]] definition
  • to change this, send a POST request of binding object to [[pods]] binding api - curl --header 'Content-Type: application/json' --request POST --data '{"apiVersion": "v1", "kind":"Binding", "metadata":{"name":"nginx"},"spec":{"containers":[{"name":"nginx","image":"nginx"}],"nodeName":"kube-01"}}' https://$SERVER/api/v1/namespaces/default/pods/$PODNAME/binding

systemd service

default scheduler

ExecStart=/usr/local/bin/kube-scheduler \\
   --config=/etc/kubernetes/manifests/kube-scheduler.yml
   --scheduler-name=default-scheduler

custom scheduler

ExecStart=/usr/local/bin/custom-k8s-scheduler-binary \\
   --config=/etc/kubernetes/manifests/kube-custom-scheduler.yml
   --scheduler-name=kube-custom-scheduler

kubeadm

manifest

apiVersion: v1
kind: Pod
metadata:
   name: kube-scheduler
   namespace: kube-system
spec:
   containers:
   - command:
      - kube-scheduler
      - --address=127.0.0.1
      - --kubeconfig=/etc/kubernetes/scheduler.conf
      - --leader-elect=true
      - --scheduler-name=kube-custom-scheduler
      - --lock-object-name=kube-custom-scheduler # used with leader-elect
      image: k8s.gcr.io/kube-scheduler-amd64:v1.11.3
      name: kube-scheduler