Kubernetes - Extension Points

• Kubernetes scheduler uses a set of plugins to determine the scheduling of pods.
• These plugins are categorized into several stages know as extension points:

1. QueueSort:
   • Provides the logic to sort pods in the scheduling queue.
   • Example: PrioritySort
2. PreFilter:
   • These plugins are used to pre-process or check information about the pod or the cluster before filtering.
   • Example: NodeResourcesFit, PodTopologySpread, InterPodAffinity
3. Filter:
   • These plugins are used to filter out nodes that cannot run the Pod.
   • Examples: NodeUnschedulable, TaintToleration, NodeAffinity, NodeResourcesFit, VolumeBinding
4. PostFilter:
   • These plugins are called after Filter phase, but only when no feasible nodes were found for the pod.
   • Example: DefaultPreemption
5. PreScore:
   • These plugins are used to perform any work required before scoring.
   • Example: InterPodAffinity, PodTopologySpread, TaintToleration
6. Score:
   • These plugins provide a score to each node that has passed the filtering phase to prioritize them for scheduling.
   • Examples: NodeAffinity, InterPodAffinity, NodeResourcesBalancedAllocation, NodeResourcesLeastAllocated, VolumeBinding, PodTopologySpread
7. Reserve:
   • This is an informational extension point that happens once the scheduler has assumed a pod will be scheduled to a node, useful to trigger pre-emptive actions.
   • Example: VolumeBinding
8. Permit:
   • These plugins can prevent or delay the binding of a Pod.
   • Examples: VolumeBinding, PodTopologySpread
9. PreBind:
   • These plugins perform any work required before a pod is bound.
   • Example: VolumeBinding
10. Bind:
    • These plugins bind a pod to a Node. Bind plugins are called in order and once one has done the binding, the remaining plugins are skipped.
    • Example: DefaultBinder
11. PostBind:
    • This is an informational extension point that is called after a pod has been bound.
    • No default plugins.
12. Unreserve:
    • This is an informational extension point that is called if a pod was reserved and then rejected in a later phase.
    • Example: VolumeBinding