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Kubernetes: 7. Manual Scheduling

  • As new pod definition files are created, Kubernetes goes through them and looks for the property nodeName
  • If this property does not exist, then Kubernetes has the job of scheduling this pod
  • It then looks for the nodes that can host this pod and schedules it there
  • And the pod definition is updated with the nodeName where it is running
  • The property nodeName in the below definition file is optional.
  • If the property is specified then kubernetes automatically schedules the pod on the selected node. This is called manual scheduling.
  • Once the Kubernetes identifies the node on which to run, it creates a binding object that binds the pod with the node on which the pod will run

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

spec:
    nodeName: node01
    containers:
    - name: nginx-container
      image: nginx
    - name: backed-db
      image: redis

Manual Scheduling
  • If the scheduler is not running or there is no scheduler in kubernetes, then pods will remain in Pending state
  • To run the pods, the definition file has to be updated to add the node on which the pod can run
  • Remember that Kubernetes does not allow a nodeName to be specified once the pod is created
  • So for manual scheduling, nodeName has to be added to the yaml file before creating the pod  
  • So if the pod is already created with no scheduler, then a bind object has to be created
  • This has to be converted to a JSON format and sent to Kubernetes as a POST API request, mimicking what Kubernetes actually does. 

pod-binding-definition.yaml
apiVersion: v1
kind: Binding
metadata:
    name: myapp-pod

target:
    apiVersion: v1
    kind: Node
    name: node01

-----------------
{
"apiVersion": "v1",
"kind": "Binding",
"metadata": {
    "name": "nginx"
},
"target": {
"apiVersion": "v1",
"kind": "Node",
"name": "node01"
    }
}
----------------------

curl --header "Content-Type:application/json" --request POST --data '{ "apiVersion": "v1", "kind": "Binding", "metadata": { "name": "nginx" }, "target": { "apiVersion": "v1", "kind": "Node", "name": "node01" }}' http://$SERVER/api/v1/namespaces/default/pods/$PODNAME/binding/

How to know if scheduler is running ?
Check the pods running in the kube-system namespace
Here there is no scheduler running, so Pods will not be scheduled in this kubernetes
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