MEASURED on prod 2026-10-18: kube-system/coredns had replicas: 1. In-cluster
DNS was a single pod on a single node, so draining or rebooting that node was a
CLUSTER-WIDE in-cluster DNS outage until the pod rescheduled -- every Service
lookup, every CNPG replica finding its primary, every operator addressing the
API by name. It was invisible because 1/1 Ready looks healthy.
It is urgent rather than latent now: the scheduled reboot automation on
feat/automated-upgrades drains and reboots one node at a time, unattended,
overnight -- turning a single point of failure into a recurring one.
WHY AN HPA AND NOT replicas: 2 -- the whole decision. k3s OWNS the coredns
Deployment: AddOn/coredns, source /data/k3s/server/manifests/coredns.yaml,
managedFields managers deploy@k3s-prod-1 + k3s, objectset.rio.cattle.io/hash
label. k3s re-stages its bundled manifests on startup, so replicas: 2 written
there -- or a kubectl scale -- is reverted on the next restart or upgrade.
Rejected alternatives:
* coredns.yaml.skip + our own templated copy (the roles/kube_vip pattern).
Deterministic, but takes ownership of coredns from k3s permanently so k3s
upgrades stop updating it. Too high a price for a replica count.
* A Flux patch of the Deployment. Rejected outright: Flux and k3s's deploy
controller would both own one object and fight -- the default-deny
NetworkPolicy drift-loop shape (~2500 events/10min) this cluster already hit.
An HPA is a separate object k3s does not own, driving /scale rather than the
Deployment spec, so there is no shared-ownership fight; if k3s re-stages
replicas: 1 it is re-asserted within a reconcile. Disclosed cost: a brief
window at 1 replica after a k3s restart.
It is a FLOOR-HOLDER, not an autoscaler -- the CPU metric exists only so the
autoscaling/v2 object is valid. Measured: 5m/4m against a 100m request (~5%),
so the 80% target is never approached. scaleDown stabilization 600s because DNS
is the worst thing to flap; scaleUp immediate, since that restores redundancy.
The kustomization edit is ADDITIVE: platform/coredns/ already existed and holds
the coredns-custom split-horizon ConfigMap (R-DNS-01/03), which is untouched.
UNAPPLIED. The HPA has never run, and the claim that k3s reverts replicas on
restart is reasoned, not observed. After merge check `describe hpa coredns` for
ScalingActive=True -- an inert HPA looks identical to a working one because the
hand scale already left 2 replicas. See the handoff entry.
Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
jakman85andClaude Opus 5