Non co-locating PODs scheduling

Introduction

Laymen explanation

Do you want to ensure that multiple POD of same service should not be scheduled on same node. You are concerned about node failure? K8s have approach to enforce this as a POD scheduling policy

Technical explanation

Inter-pod affinity and anti-affinity allow you to constrain which nodes your pod is eligible to be scheduled based on labels on pods that are already running on the node rather than based on labels on nodes. The rules are of the form “this pod should (or, in the case of anti-affinity, should not) run in an X if that X is already running one or more pods that meet rule Y”.

Use-cases

Interpod Affinity and AntiAffinity can be even more useful when they are used with higher level collections such as ReplicaSets, StatefulSets, Deployments, etc. One can easily configure that a set of workloads should be co-located in the same defined topology, eg., the same node.

Example YAML where PODs are not co-located in same node

Below yaml snippet of a simple redis deployment with three replicas and selector label app=store. The deployment has PodAntiAffinity configured to ensure the scheduler does not co-locate replicas on a single node

PODs are not co-located in same node

apiVersion: apps/v1

kind: Deployment

metadata:

name: redis-cache

spec:

selector:

matchLabels:

app: store

replicas: 3

template:

metadata:

labels:

app: store

spec:

affinity:

podAntiAffinity:

requiredDuringSchedulingIgnoredDuringExecution:

- labelSelector:

matchExpressions:

- key: app

operator: In

values:

- store

topologyKey: "kubernetes.io/hostname"

containers:

- name: redis-server

image: redis:3.2-alpine

In below output, notice that only one POD is created since there is only one worker node and master node is not tainted.

Output in single worker node cluster

root@ubuntu-232:~/deepak/test# kubectl get deployments | grep redis

redis-cache 3 3 3 1 2m

root@ubuntu-232:~/deepak/test# kubectl get nodes

NAME STATUS ROLES AGE VERSION

ubuntu-231 Ready <none> 242d v1.10.0

ubuntu-232 Ready master 242d v1.10.0

Reference

https://kubernetes.io/docs/concepts/configuration/assign-pod-node/