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

  Steps

git clone https://github.com/collabnix/dockerlabs
cd dockerlabs/kubernetes/workshop/Scheduler101/
kubectl label nodes node2 mynode=worker-1
kubectl apply -f pod-nginx.yaml

• We have label on the node with node name,in this case i have given node2 as mynode=worker-1 label.

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"
[node1 Scheduler101]$

• You can check in above output Node-Selectors: mynode=worker-1

Deleting the Pod

kubectl delete -f pod-nginx.yaml
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 (Required during scheduling, ignored during execution; also known as “hard” requirements)
  2. preferredDuringSchedulingIgnoredDuringExecution (Preferred during scheduling, ignored during execution; also known as “soft” requirements)

Steps

git clone https://github.com/collabnix/dockerlabs
cd dockerlabs/kubernetes/workshop/Scheduler101/
kubectl label nodes node2 mynode=worker-1
kubectl label nodes node3 mynode=worker-3
kubectl apply -f pod-with-node-affinity.yaml

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-with-node-affinity.yaml

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