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API Aggregation: Bolting On a Second API Server

A CRD adds a new kind that the main API server itself stores in etcd. API aggregation goes further: it bolts a second API server in behind the main one, serving an API group that it stores and computes on its own terms. Our cluster has been running one example since Article 39 — metrics-server. This article examines it as an aggregated API: how the APIService registers it, where requests get proxied, and why the CPU/memory figures it returns never sit in etcd.