Kubernetes Scheduler, Node Afinity, NodeSelector

 

What is Kubernetes Scheduling?

• The Kubernetes Scheduler is a core component of Kubernetes: After a user or a controller creates a Pod, the Kubernetes Scheduler, monitoring the Object Store for unassigned Pods, will assign the Pod to a Node. Then, the Kubelet, monitoring the Object Store for assigned Pods, will execute the Pod.

what is the scheduler for?

The Kubernetes scheduler is in charge of scheduling pods onto nodes. Basically it works like this:

1. You create a pod
2. The scheduler notices that the new pod you created doesn’t have a node assigned to it
3. The scheduler assigns a node to the pod

It’s not responsible for actually running the pod – that’s the kubelet’s job. So it basically just needs to make sure every pod has a node assigned to it. Easy, right?

What is node affinity ?

• In simple words this allows you to tell Kubernetes to schedule pods only to specific subsets of nodes.
• The initial node affinity mechanism in early versions of Kubernetes was the nodeSelector field in the pod specification. The node had to include all the labels specified in that field to be eligible to become the target for the pod.

nodeSelector Example

First is to give the node a label

kubectl label nodes node1 mynode=worker-1




Next is to create a pod and specify the node via the label

Lets create a pod 

$ vi pod.yml

apiVersion: v1 #version of the API to use

kind: Pod #What kind of object we're deploying

metadata: #information about our object we're deploying

  name: nginx #Name of the pod

  labels: #A tag on the pod created

    env: test

spec: #specifications for our object

  containers:

  - name: nginx  #the name of the container within the pod

    image: nginx #which container image should be pulled

    imagePullPolicy: IfNotPresent #image pull policy

  nodeSelector: #Nodeselector condition

    mynode: worker-1 # label on the node where pod is going to deploy

Then 

kubectl apply -f pod.yml




This will create a pod and will deploy it on the node with label mynode=worker-1







Viewing Your Pods

kubectl get pods --output=wide

[node1 Scheduler101]$ kubectl describe po nginx
Name:               nginx
Namespace:          default
Priority:           0
PriorityClassName:  <none>
Node:               node2/192.168.0.17
Start Time:         Mon, 30 Dec 2019 16:40:53 +0000
Labels:             env=test
Annotations:        kubectl.kubernetes.io/last-applied-configuration:
                      {"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"labels":{"env":"test"},"name":"nginx","namespace":"default"},"spec":{"contai...
Status:             Pending
IP:
Containers:
  nginx:
    Container ID:
    Image:          nginx
    Image ID:
    Port:           <none>
    Host Port:      <none>
    State:          Waiting
      Reason:       ContainerCreating
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-qpgxq (ro)
Conditions:
  Type              Status
  Initialized       True
  Ready             False
  ContainersReady   False
  PodScheduled      True
Volumes:
  default-token-qpgxq:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-qpgxq
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  mynode=worker-1
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  7s    default-scheduler  Successfully assigned default/nginx to node2
  Normal  Pulling    3s    kubelet, node2     Pulling image "nginx"

Deleting the Pod

kubectl delete -f pod.yml
pod "nginx" deleted

Node affinity

• Node affinity is conceptually similar to nodeSelector – it allows you to constrain which nodes your pod is eligible to be scheduled on, based on labels on the node.

• There are currently two types of node affinity.

1. requiredDuringSchedulingIgnoredDuringExecution (Preferred during scheduling, ignored during execution; we are also known as “hard” requirements)
2. preferredDuringSchedulingIgnoredDuringExecution (Required during scheduling, ignored during execution; we are also known as “soft” requirements)

Hands On: First Label the Nodes(any two Nodes u have my ex node2, node3)

kubectl label nodes node2 mynode=worker-1
kubectl label nodes node3 mynode=worker-3

Then create ur pod

$ vi pod.yml

apiVersion: v1

kind: Pod

metadata:

  name: with-node-affinity

spec:

  affinity:

    nodeAffinity:

      requiredDuringSchedulingIgnoredDuringExecution:

        nodeSelectorTerms:

        - matchExpressions:

          - key: mynode

            operator: In

            values:

            - worker-1

      preferredDuringSchedulingIgnoredDuringExecution:

      - weight: 1

        preference:

          matchExpressions:

          - key: mynode

            operator: In

            values:

            - worker-3

  containers:

  - name: nginx

    image: nginx

Then create 

kubectl apply -f pod.yml




Viewing Your Pods

kubectl get pods --output=wide
NAME                 READY   STATUS    RESTARTS   AGE     IP          NODE          NOMINATED NODE   READINESS GATES
with-node-affinity   1/1     Running   0          9m46s   10.44.0.1   kube-slave1   <none>           <none>

[node1 Scheduler101]$ kubectl describe po
Name:               with-node-affinity
Namespace:          default
Priority:           0
PriorityClassName:  <none>
Node:               node3/192.168.0.16
Start Time:         Mon, 30 Dec 2019 19:28:33 +0000
Labels:             <none>
Annotations:        kubectl.kubernetes.io/last-applied-configuration:
                      {"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"name":"with-node-affinity","namespace":"default"},"spec":{"affinity":{"nodeA...
Status:             Pending
IP:
Containers:
  nginx:
    Container ID:
    Image:          nginx
    Image ID:
    Port:           <none>
    Host Port:      <none>
    State:          Waiting
      Reason:       ContainerCreating
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-qpgxq (ro)
Conditions:
  Type              Status
  Initialized       True
  Ready             False
  ContainersReady   False
  PodScheduled      True
Volumes:
  default-token-qpgxq:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-qpgxq
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  26s   default-scheduler  Successfully assigned default/with-node-affinity to node3
  Normal  Pulling    22s   kubelet, node3     Pulling image "nginx"
  Normal  Pulled     20s   kubelet, node3     Successfully pulled image "nginx"
  Normal  Created    2s    kubelet, node3     Created container nginx
  Normal  Started    0s    kubelet, node3     Started container nginx

Step Cleanup

Finally you can clean up the resources you created in your cluster:

kubectl delete -f pod.yml