Kubernetes pod distribution amongst nodes

Here I leverage Anirudh’s answer adding example code.

My initial kubernetes yaml looked like this:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: say-deployment
spec:
  replicas: 6
  template:
    metadata:
      labels:
        app: say
    spec:
      containers:
      - name: say
        image: gcr.io/hazel-champion-200108/say
        ports:
        - containerPort: 8080
---
kind: Service
apiVersion: v1
metadata:
  name: say-service
spec:
  selector:
    app: say
  ports:
    - protocol: TCP
      port: 8080
  type: LoadBalancer
  externalIPs:
    - 192.168.0.112

At this point, kubernetes scheduler somehow decides that all the 6 replicas should be deployed on the same node.

Then I added requiredDuringSchedulingIgnoredDuringExecution to force the pods beeing deployed on different nodes:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: say-deployment
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: say
    spec:
      containers:
      - name: say
        image: gcr.io/hazel-champion-200108/say
        ports:
        - containerPort: 8080
      affinity:
              podAntiAffinity:
                requiredDuringSchedulingIgnoredDuringExecution:
                  - labelSelector:
                      matchExpressions:
                        - key: "app"
                          operator: In
                          values:
                          - say
                    topologyKey: "kubernetes.io/hostname"
---
kind: Service
apiVersion: v1
metadata:
  name: say-service
spec:
  selector:
    app: say
  ports:
    - protocol: TCP
      port: 8080
  type: LoadBalancer
  externalIPs:
    - 192.168.0.112

Now all the pods are run on different nodes. And since I have 3 nodes and 6 pods, other 3 pods (6 minus 3) can’t be running (pending). This is because I required it: requiredDuringSchedulingIgnoredDuringExecution.

kubectl get pods -o wide 

NAME                              READY     STATUS    RESTARTS   AGE       IP            NODE
say-deployment-8b46845d8-4zdw2   1/1       Running            0          24s       10.244.2.80   night
say-deployment-8b46845d8-699wg   0/1       Pending            0          24s       <none>        <none>
say-deployment-8b46845d8-7nvqp   1/1       Running            0          24s       10.244.1.72   gray
say-deployment-8b46845d8-bzw48   1/1       Running            0          24s       10.244.0.25   np3
say-deployment-8b46845d8-vwn8g   0/1       Pending            0          24s       <none>        <none>
say-deployment-8b46845d8-ws8lr   0/1       Pending            0          24s       <none>        <none>

Now if I loosen this requirement with preferredDuringSchedulingIgnoredDuringExecution:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: say-deployment
spec:
  replicas: 6
  template:
    metadata:
      labels:
        app: say
    spec:
      containers:
      - name: say
        image: gcr.io/hazel-champion-200108/say
        ports:
        - containerPort: 8080
      affinity:
              podAntiAffinity:
                preferredDuringSchedulingIgnoredDuringExecution:
                  - weight: 100
                    podAffinityTerm:
                      labelSelector:
                        matchExpressions:
                          - key: "app"
                            operator: In
                            values:
                            - say
                      topologyKey: "kubernetes.io/hostname"
---
kind: Service
apiVersion: v1
metadata:
  name: say-service
spec:
  selector:
    app: say
  ports:
    - protocol: TCP
      port: 8080
  type: LoadBalancer
  externalIPs:
    - 192.168.0.112

First 3 pods are deployed on 3 different nodes just like in the previous case. And the rest 3 (6 pods minus 3 nodes) are deployed on various nodes according to kubernetes internal considerations.

NAME                              READY     STATUS    RESTARTS   AGE       IP            NODE
say-deployment-57cf5fb49b-26nvl   1/1       Running   0          59s       10.244.2.81   night
say-deployment-57cf5fb49b-2wnsc   1/1       Running   0          59s       10.244.0.27   np3
say-deployment-57cf5fb49b-6v24l   1/1       Running   0          59s       10.244.1.73   gray
say-deployment-57cf5fb49b-cxkbz   1/1       Running   0          59s       10.244.0.26   np3
say-deployment-57cf5fb49b-dxpcf   1/1       Running   0          59s       10.244.1.75   gray
say-deployment-57cf5fb49b-vv98p   1/1       Running   0          59s       10.244.1.74   gray