kubernetes > node affinity > soft
The Node Affinity feature we are going to see here helps the Pod(s) to prefer launching on Node(s) of choice, the associated section in the Pod manifest is spec.affinity.nodeAffinity.preferredDuringSchedulingIgnoredDuringExecution
It's good to some have understanding of Deployments to make better use of this post
We have a cluster of 3 nodes
networkandcode@k8s-master-0:~$ kubectl get no
NAME STATUS ROLES AGE VERSION
k8s-master-0 Ready master 2d v1.16.3
k8s-node-0 Ready <none> 2d v1.16.3
k8s-node-1 Ready <none> 2d v1.16.3
k8s-node-2 Ready <none> 2d v1.16.3
Let's check the existing labels on the instances
networkandcode@k8s-master-0:~$ kubectl get nodes --show-labels
NAME STATUS ROLES AGE VERSION LABELS
k8s-master-0 Ready master 2d18h v1.16.3 beta.kubernetes.io/arch=amd64,beta.kubernetes.io/os=linux,kubernetes.io/arch=amd64,kubernetes.io/hostname=k8s-master-0,kubernetes.io/os=linux,node-role.kubernetes.io/master=
k8s-node-0 Ready <none> 2d18h v1.16.3 beta.kubernetes.io/arch=amd64,beta.kubernetes.io/os=linux,kubernetes.io/arch=amd64,kubernetes.io/hostname=k8s-node-0,kubernetes.io/os=linux
k8s-node-1 Ready <none> 2d18h v1.16.3 beta.kubernetes.io/arch=amd64,beta.kubernetes.io/os=linux,kubernetes.io/arch=amd64,kubernetes.io/hostname=k8s-node-1,kubernetes.io/os=linux
k8s-node-2 Ready <none> 2d18h v1.16.3 beta.kubernetes.io/arch=amd64,beta.kubernetes.io/os=linux,kubernetes.io/arch=amd64,kubernetes.io/hostname=k8s-node-2,kubernetes.io/os=linux
Let's go with the kubernetes.io/hostname label for our exercise, as it is unique for each instance. We are now going to define a deployment with 20 replicas
networkandcode@k8s-master-0:~$ cat ex35-deploy-node-affinity-soft.yaml
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: deploy35
spec:
selector:
matchLabels:
tag: label35
replicas: 20
template:
metadata:
labels:
tag: label35
spec:
affinity:
# nodeAffinity configuration starts here
nodeAffinity:
preferredDuringSchedulingIgnoredDuringExecution:
- weight: 1
preference:
matchExpressions:
- key: kubernetes.io/hostname
operator: In
values:
- k8s-node-0
# nodeAffinity configuration ends here
containers:
- name: ctr35
image: nginx
...
The weight parameter in the manifest above is associated with the preference section, and it ranges from 1 to 100, higher number denoting higher preference, we could have a list of such preferences and associated weights, so while the Pod gets scheduled it could prefer one among the list based on the weight. However here we only have a single preference section with associated weight 1. The weight is going to be considered in some artithmetic calculation, when there are multiple matchExpressions too. Our case is simple though as we only have a single target node i.e., node-0. The Pods of this Deployment are going to prefer k8s-node-0 based on a match expression we have set.
Let's launch the deployment
networkandcode@k8s-master-0:~$ kubectl create -f ex35-deploy-node-affinity-soft.yaml
deployment.apps/deploy35 created
We see the Pods are launched on all the nodes
networkandcode@k8s-master-0:~$ kubectl get po -o wide
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
deploy35-6df945f555-6kdlx 1/1 Running 0 24m 192.168.140.65 k8s-node-2 <none> <none>
deploy35-6df945f555-8fs55 1/1 Running 0 24m 192.168.11.200 k8s-node-0 <none> <none>
deploy35-6df945f555-9ks96 1/1 Running 0 24m 192.168.11.205 k8s-node-0 <none> <none>
deploy35-6df945f555-9pzvs 1/1 Running 0 24m 192.168.11.203 k8s-node-0 <none> <none>
deploy35-6df945f555-c6ssc 1/1 Running 0 24m 192.168.109.69 k8s-node-1 <none> <none>
deploy35-6df945f555-crx2f 1/1 Running 0 24m 192.168.140.67 k8s-node-2 <none> <none>
deploy35-6df945f555-ddqjv 1/1 Running 0 24m 192.168.11.204 k8s-node-0 <none> <none>
deploy35-6df945f555-j6wcm 1/1 Running 0 24m 192.168.109.65 k8s-node-1 <none> <none>
deploy35-6df945f555-jlrbk 1/1 Running 0 24m 192.168.11.196 k8s-node-0 <none> <none>
deploy35-6df945f555-knhq9 1/1 Running 0 24m 192.168.11.198 k8s-node-0 <none> <none>
deploy35-6df945f555-n8f6r 1/1 Running 0 24m 192.168.11.199 k8s-node-0 <none> <none>
deploy35-6df945f555-pnqsv 1/1 Running 0 24m 192.168.140.66 k8s-node-2 <none> <none>
deploy35-6df945f555-qctx6 1/1 Running 0 24m 192.168.140.68 k8s-node-2 <none> <none>
deploy35-6df945f555-qp7qk 1/1 Running 0 24m 192.168.11.197 k8s-node-0 <none> <none>
deploy35-6df945f555-rq55k 1/1 Running 0 24m 192.168.11.201 k8s-node-0 <none> <none>
deploy35-6df945f555-rr8v2 1/1 Running 0 24m 192.168.11.202 k8s-node-0 <none> <none>
deploy35-6df945f555-szwvf 1/1 Running 0 24m 192.168.109.66 k8s-node-1 <none> <none>
deploy35-6df945f555-vc4sv 1/1 Running 0 24m 192.168.109.67 k8s-node-1 <none> <none>
deploy35-6df945f555-vlcj6 1/1 Running 0 24m 192.168.140.69 k8s-node-2 <none> <none>
deploy35-6df945f555-zg6wt 1/1 Running 0 24m 192.168.109.68 k8s-node-1 <none> <none>
Let's check the count of Pods launched on node-0
networkandcode@k8s-master-0:~$ kubectl get po -o wide | grep node-0 | wc
10 90 1170
So there are 10 Pods as the first output of wc, on the left refers to the no. of lines, so in this case most of the Pods, i.e., 50% of 20 Pods in the Deployment have been launched on node-0 as it was preferred, and rest of the Pods were launched on nodes 1 and 2. This is how soft preference i.e., spec.affinity.nodeAffinity.preferredDuringSchedulingIgnoredDuringExecution works.
Reference: https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.15/#preferredschedulingterm-v1-core
Cleanup
networkandcode@k8s-master-0:~$ kubectl delete deploy deploy35
deployment.apps "deploy35" deleted
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