stack-orchestrator/docs/k8s-deployment-enhancements.md

K8S Deployment Enhancements

Controlling pod placement

The placement of pods created as part of a stack deployment can be controlled to either avoid certain nodes, or require certain nodes.

Pod/Node Affinity

Node affinity rules applied to pods target node labels. The effect is that a pod can only be placed on a node having the specified label value. Note that other pods that do not have any node affinity rules can also be placed on those same nodes. Thus node affinity for a pod controls where that pod can be placed, but does not control where other pods are placed.

Node affinity for stack pods is specified in the deployment's spec.yml file as follows:

node-affinities:
  - label: nodetype
    value: typeb

This example denotes that the stack's pods should only be placed on nodes that have the label nodetype with value typeb.

Node Taint Toleration

K8s nodes can be given one or more "taints". These are special fields (distinct from labels) with a name (key) and optional value. When placing pods, the k8s scheduler will only assign a pod to a tainted node if the pod posesses a corresponding "toleration". This is metadata associated with the pod that specifies that the pod "tolerates" a given taint. Therefore taint toleration provides a mechanism by which only certain pods can be placed on specific nodes, and provides a complementary mechanism to node affinity.

Taint toleration for stack pods is specified in the deployment's spec.yml file as follows:

node-tolerations:
  - key: nodetype
    value: typeb

This example denotes that the stack's pods will tolerate a taint: nodetype=typeb