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Kubernetes: 15. Multiple Schedulers

Custom Scheduler

  • Kubernetes allows to create custom schedulers
  • There can be multiple schedulers running at a same time apart from the default scheduler or
  • A custom scheduler can replace the default kube-scheduler to become the default one
  • So a few pods that requires additional checks apart from taints and toleration, node affinity can go through the custom scheduler before getting scheduled on the node
  • Whereas the rest of the pods can go through the default kube-scheduler
Create Custom Scheduler
  • We can either download the kube-scheduler and run it as a service or alternatively create it using a static pod
  • Below here we are downloading the binaries to run it
  • The property scheduler-name is used to define the name of the scheduler, if not set then it will be defaulted to default-scheduler
  • For your custom schedulers, update this property name to set a custom name for your scheduler
  • For Static pods, the name can be updated directly in the pod-definition file
  • Use kubectl create -f <pod-definition-file> to create the custom scheduler
  • Run kubectl get pods -n kube-system to view the custom scheduler
  • Note that when there are multiple master nodes in the cluster, then there will be multiple versions of scheduler running
  • Only one version of scheduler should be active at a time to make sure pods are scheduled correctly
  • For this there is a property "leader-elect" this must be set to true so that a leader is elected during HA (multi master) set up
  • When you add the custom scheduler, add the property "lock-object-name=<scheduler-name>" so that the scheduler knows this is a custom scheduler
Configure Pod to use custom scheduler
  • To configure the pod to use the custom scheduler, add the schedulerName property in the spec section
  • When the pod is created it will use the custom scheduler for scheduling
  • If the scheduler is not configured correctly then the pod remains in PENDING state

pod-definition.yaml
apiVersion: v1
kind: Pod
metadata:
    name: myapp-pod
    labels:
        app: myapp

spec:
    containers:
    - name: nginx-container
      image: nginx
    schedulerName: my-custom-scheduler

View Events
  • To confirm that the pod picked the custom scheduler view the events and verify that custom scheduler is used
  • Alternatively look for the scheduler logs in the kube-system namespace
  • Remember that schedulers run kube-system namespace

kubectl get events
-> Shows the events run in the current namespace

kubectl logs scheduler <custom-scheduler> -n kube-system
-> Logs of the custom scheduler
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