feat(arc): let a sized profile's runner pods overflow onto burst nodes - #26
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A scale-set profile can now set burst: the label a cluster autoscaler's burst nodes carry, tolerations for their taint, and how many runners those nodes hold at most. The runner pods then trade the eligibility nodeSelector for a required node affinity accepting either the eligibility label or the burst label, with a preferred term that keeps them on the fixed nodes while those have room. The listener keeps the eligibility nodeSelector. burst.max_runners is added to the ceiling sizing derives, uniform or measured, so ARC creates more runner pods than the fixed nodes hold and the extra ones pend until the autoscaler adds a node. A profile without burst renders exactly as before.
Mearman
marked this pull request as ready for review
September 30, 2026 08:30
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Adds an optional
burstkey to a scale-set profile, for fleets whose fixed nodes can overflow onto nodes a cluster autoscaler launches on demand (cloud instances that join the cluster labelled and tainted, and carry no eligibility label).With
burstset, the runner pods drop the eligibility nodeSelector for a required node affinity that accepts either the eligibility label orburst.node_label_key(any value, or one ofnode_label_values), plus a preferred term at the highest weight for nodes without the burst label, so the scheduler keeps using the fixed pool while it has room.burst.tolerationsgoes on the pod template. The listener keeps the eligibility nodeSelector so it never lands on a node that can be removed.burst.max_runnersis added to the ceiling sizing derives (uniform in the filter, measured in the overlay at install time), so ARC creates more pods than the fixed nodes hold and the surplus pends until the autoscaler adds a node. It needs sizing and rejectsmax_runnersandautoscaleon the same profile, since an explicit ceiling is final and the usage-driven autoscaler has no notion of nodes that do not exist yet.A profile without
burstrenders exactly as before. The placement logic is a newarc_runner_placementfilter with unit tests, andtests/unit/test_scale_set_overlay.pyrenders the real overlay through ansible-playbook for both paths.