kube-scheduler

• Responsible for finding the right node for each container to start on
• The kube-scheduler does not deploy the container, but only takes the decision. The deploy itself is performed by the kubelet
• Decision making
• Filter Nodes: filter nodes which do not match the criteria
• Rank Nodes: assign a score for the node (from 0 to 10)

Scheduling Queue

• It's a queue of pending pods
• Represent a queue of pods waiting to be assigned to a node

Scheduling Rules

• no.spec.taints: Mark a node with a taint. Only pods with tolerance to this taint can be scheduled to this node
• po.spec.nodeName: bypasses kube-scheduler completely and selects a node manually
• po.spec.NodeSelector: Schedule a pod to a node based on different criteria
• po.spec.affinity.nodeAffinity: A simple node affinity. Schedule based only on a label
• po.spec.tolerations: Tolerante of a pod to a node taint
• po.spec.topologySpreadConstraints: spread the replicas across AZs

Setup

From scratch

• kube-scheduler.service must be configured manually if running a kubernetes cluster from scratch

# Download kube-scheduler binary
wget "https://storage.googleapis.com/kubernetes-release/release/v1.13.0/bin/linux/amd64/kube-scheduler"

ExecStart=/usr/local/bin/kube-scheduler \\
  --scheduler-name=default-scheduler \\ # leave as is for a single scheduler setup
  --config=/etc/kubernetes/config/kube-scheduler.yaml \\
  --leader-elect=true \\ # for multiple masters (multiple replicas of scheduler)
  --v=2

• The options can be viewed at /etc/systemd/system/kube-scheduler.service
• Or see the running options px -aux | grep kube-scheduler

Via kubeadm

• kubeadm install the service automatically as a pod kube-scheduler inside of the kube-system namespace

• The pod is deployed on the master node

• The options can be viewed at /etc/kubernetes/manifests/kube-scheduler.yaml

Multiple Schedulers

• Multiple scheduler services can run in the cluster
• Modify the po.spec.schedulerName property in order to use any other scheduler other than the default one

ExecStart=/usr/local/bin/kube-scheduler \\
  --scheduler-name=my-custom-scheduler \\
  --config=/etc/kubernetes/config/kube-scheduler.yaml \\
  --leader-elect=true \\
  --lock-object-name=my-custom-scheduler \\
  --v=2