Preemption

spec.priority still orders pending Machines. It halts a running Machine only when both sides opt in.

On the pending Machine:

metadata:
  annotations:
    kairon.zyvor.dev/preempt: "true"
spec:
  priority: 10

On a Machine that may be halted:

metadata:
  annotations:
    kairon.zyvor.dev/preemption-policy: Halt
spec:
  priority: 0

Rules:

• Same namespace. A victim in another namespace is ignored.
• Victim priority must be strictly lower.
• Victim must already be scheduled and desired Running.
• Pause is rejected. A Paused Machine still counts in node load, so it would not free the capacity the preemptor is waiting on. Halted does, and it also drops out of MachineQuota.
• One victim per preemptor per tick. The lowest priority eligible victim wins. Ties break by name.
• A non-terminal MachineMigration or a MachineDisruptionBudget with no allowance refuses the victim.
• The controller writes kairon.zyvor.dev/preempted-by and restores powerState: Running when the preemptor is gone, Halted, Stopped, or no longer pending and unscheduled.

Annotations

Annotation	On	Set by	Meaning
kairon.zyvor.dev/preempt	pending Machine	you	"true" lets this Machine halt a lower-priority victim. Any other value is off.
kairon.zyvor.dev/preemption-policy	candidate victim	you	Halt (or Halted) allows the Machine to be halted. Anything else means it is never a victim.
kairon.zyvor.dev/preempted-by	victim	kairon-controller	namespace/name of the preemptor that halted it. A Machine with this set is not chosen as a victim again, and it is what the controller looks for when deciding to resume.
kairon.zyvor.dev/preempted-power	victim	kairon-controller	The spec.powerState the victim had before the halt (Running; an empty or missing value is read as Running). On resume the controller sets powerState back to Running and removes both preempted-* annotations. A halted Machine whose value is anything other than Running or empty is left halted.

Do not set or edit preempted-by / preempted-power yourself. To resume a victim by hand, set spec.powerState: Running and remove both annotations.

Placement is retried on the next tick, after the halt is visible to the scheduler.

See examples/preemption.yaml.

Limits

• Rejecting Pause (or any other value) for preemption-policy happens in the admission webhook. Without it, an unknown value is simply ignored: that Machine is never a victim.
• Halt is the only action. Nothing is evicted, deleted, or migrated, and a preemptor never takes a victim from another namespace.
• A resumed victim uses FluxVM's kept config, so CPU or memory hotplugged after its last create is lost (see machine-hotplug.md).
• The scheduler still decides placement. Halting a victim frees its capacity, but the preemptor can still fail placement for another reason (affinity, quota, taints) and the victim stays halted until the preemptor is deleted, halted, or stopped.

Inspect

kubectl -n NS get machines -o custom-columns=NAME:.metadata.name,PRIO:.spec.priority,POWER:.spec.powerState,BY:.metadata.annotations.kairon\.zyvor\.dev/preempted-by
journalctl -u kairon-controller | grep -E 'preempted machine|resumed preempted machine|preempt refused'

Verified on a live k3s host on 2026-10-06: a pending priority-10 Machine halted an opted-in priority-0 Machine, deleting the preemptor resumed it, and without preemption-policy: Halt nothing was halted.