Cluster Split Axis
The dimension along which the estate is divided into clusters — the single decision that determines both blast radius and operational load.
Every cluster boundary is a trade. Fewer, larger clusters mean better bin-packing, less duplicated platform infrastructure and fewer control planes to upgrade, at the cost of a larger blast radius and a more disruptive upgrade. More, smaller clusters invert all of that.
The useful discipline is to name the axis explicitly. Common ones: environment (production separate from everything else — nearly always correct), region (forced by latency and residency), compliance boundary (forced by scope reduction, as with cardholder data), and tenant or team (rarely justified, and the usual cause of sprawl).
Estates get into trouble when there is no axis, only history: clusters created per project, per experiment or per acquisition, none retired. The symptom is a platform team whose entire capacity goes into upgrading control planes. The remedy is to declare the axis, map existing clusters onto it, and consolidate the ones that do not fit — which is unglamorous and is usually the single highest return available to a platform team.