40 Days Of Kubernetes (15/40)

Day 15/40

Kubernetes Node Affinity Explained

Video Link
@piyushsachdeva
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We're going to understand node affinity in kubernetes system.

(Photo from the video)

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Node affinity in Kubernetes is a set of rules used to specify preferences that affect how pods are placed on nodes. It allows you to constrain which nodes your pod is eligible to be scheduled on, based on labels on nodes and if those labels match the rules.

Types of Node Affinity:

• RequiredDuringSchedulingIgnoredDuringExecution: Pods will only be placed on nodes that match the specified rules. If no matching nodes are available, the pods won’t be scheduled.
• PreferredDuringSchedulingIgnoredDuringExecution: Specifies preferences that the scheduler will attempt to enforce but will not guarantee.

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In simple words:
RequiredDuringSchedulingIgnoredDuringExecution

• It will make sure that the pod only get scheduled when the operator matches with the label.

PreferredDuringSchedulingIgnoredDuringExecution

• It will try to matches the operator with labels, if it doesn't find, even that schedule the pod on any node.

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Demo

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apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
          - matchExpressions:
            - key: disktype
              operator: In
              values:
                - ssd
  containers:
  - name: nginx
    image: nginx

• Run the pod:

root@localhost:~# kubectl get pods --show-labels -o wide
NAME    READY   STATUS    RESTARTS   AGE   IP       NODE     NOMINATED NODE   READINESS GATES   LABELS
nginx   0/1     Pending   0          8s    <none>   <none>   <none>           <none>            <none>

• See the logs:

Events:
  Type     Reason            Age   From               Message
  ----     ------            ----  ----               -------
  Warning  FailedScheduling  82s   default-scheduler  0/3 nodes are available: 1 node(s) didn't match Pod's node affinity/selector, 1 node(s) had untolerated taint {gpu: true}, 1 node(s) had untolerated taint {node-role.kubernetes.io/control-plane: }. preemption: 0/3 nodes are available: 3 Preemption is not helpful for scheduling.

• Let's add label to a node and see what will be happened:

root@localhost:~# kubectl label node lucky-luke-worker disktype=ssd
node/lucky-luke-worker labeled
root@localhost:~# kubectl get pods --show-labels -o wide
NAME    READY   STATUS    RESTARTS   AGE     IP            NODE                NOMINATED NODE   READINESS GATES   LABELS
nginx   1/1     Running   0          6m19s   10.244.1.17   lucky-luke-worker   <none>           <none>            <none>

• Another example:

apiVersion: v1
kind: Pod
metadata:
  name: redis
spec:
  affinity:
    nodeAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 1
        preference:
          matchExpressions:
          - key: disktype
            operator: In
            values:
            - hdd
  containers:
  - name: redis
    image: redis

• Run the pod:

root@localhost:~# kubectl get pods --show-labels -o wide
NAME    READY   STATUS    RESTARTS   AGE   IP            NODE                NOMINATED NODE   READINESS GATES   LABELS
nginx   1/1     Running   0          22m   10.244.1.17   lucky-luke-worker   <none>           <none>            <none>
redis   1/1     Running   0          14s   10.244.1.18   lucky-luke-worker   <none>           <none>            <none>

• Let's delete the node label:

root@localhost:~# kubectl label node lucky-luke-worker disktype-
node/lucky-luke-worker unlabeled
root@localhost:~# kubectl get pods
NAME    READY   STATUS    RESTARTS   AGE
nginx   1/1     Running   0          29m
redis   1/1     Running   0          6m43s

As you can see, it will be affected the pods which will be create after that. So There's no change for running pods.

Also, there's an operator: Exists which means it's not important what value the key has, if key exists, it's matchs.

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Node Affinity VS Taints & Toleration:

• A taint is a label that can be applied to a node in a Kubernetes cluster, which signifies that the node is not able to accept pods that do not have a corresponding toleration.
• A toleration is a label that can be applied to a pod, which signifies that the pod is able to tolerate a node with a matching taint.
• In Kubernetes, a Node Selector is a way to specify which nodes in a cluster a particular pod should be scheduled on. It works by assigning labels to nodes and then matching those labels with the pod’s specification.
• In Kubernetes, Node Affinity is a way to specify rules that determine which nodes in a cluster a particular pod should be scheduled on. Node affinity can be used to ensure that pods are deployed on nodes with specific characteristics, such as available resources, location, or hardware capabilities.

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