AntiAffinity can be even more useful when they are used with higher level collections such as ReplicaSets, StatefulSets, Deployments, etc. One can easily configure a set of workloads to be co-located in the same defined topology, eg., the same node.
In a three-node cluster, a web application has an in-memory cache such as redis. We want the web-servers to be co-located with the cache whenever possible.
Here is the 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.
cat <<EoF > ~/environment/redis-with-node-affinity.yaml
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
EoF
The below YAML snippet of the web-server deployment has podAntiAffinity and podAffinity configured. This informs the scheduler that all its replicas are to be co-located with pods that have selector label app=store. This will also ensure that each web-server replica does not co-locate on a single node.
cat <<EoF > ~/environment/web-with-node-affinity.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: web-server
spec:
selector:
matchLabels:
app: web-store
replicas: 3
template:
metadata:
labels:
app: web-store
spec:
affinity:
podAntiAffinity:
requiredDuringSchedulingIgnoredDuringExecution:
- labelSelector:
matchExpressions:
- key: app
operator: In
values:
- web-store
topologyKey: "kubernetes.io/hostname"
podAffinity:
requiredDuringSchedulingIgnoredDuringExecution:
- labelSelector:
matchExpressions:
- key: app
operator: In
values:
- store
topologyKey: "kubernetes.io/hostname"
containers:
- name: web-app
image: nginx:1.12-alpine
EoF
Let’s apply these Deployments:
kubectl apply -f ~/environment/redis-with-node-affinity.yaml
kubectl apply -f ~/environment/web-with-node-affinity.yaml
When we create the above two deployments, our three node cluster should look like:
node-1 - webserver-1 - cache-1
node-2 - webserver-2 - cache-2
node-3 - webserver-3 - cache-3
As you can see, all 3 web-server pod replicas are automatically co-located with the redis cache pods as expected.
# We will use --sort-by to filter by nodes name
kubectl get pods -o wide --sort-by='.spec.nodeName'