Statefulset's replica scheduling questions

Question

Is there anyway I can tell Kuberbetes how to schedule the replicas in the statefulset? For example, I have nodes divided into 3 different availability zones (AZ). I have labeled these nodes accordingly. Now I want K8s to put 1 replica in each AZ based on node label. Thanks

Answer 1

Pods will always try to be scheduled across different nodes, to achieve what you are looking for you can try to use DaemonSet, which will allow only one of these kind of pods in each node.

Also, you can use anti affinity based on the already scheduled pods in that node.

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

Answer 2

The feature you are looking for is called Pod Anti-Affinity and can be specified as such:

apiVersion: apps/v1
kind: StatefulSet
[..]
spec:
  template:
    spec:
      affinity:
        nodeAffinity: {}
        podAntiAffinity:
          preferredDuringSchedulingIgnoredDuringExecution:
          - podAffinityTerm:
              labelSelector:
                matchLabels:
                  app: myapp
              topologyKey: az
            weight: 100
[..]

Since Kubernetes 1.18, there is also Pod Topology Spread Constraints, which is a nicer way to specify these anti-affinity rules.