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What this PR does / why we need it:
Yet, there is still a significant difference between the Kubernetes and the Gardener architectures: Kubernetes runs a primary "agent" on every node, the kubelet, which is mainly responsible for managing pods and containers on its particular node. Gardener uses its controller-manager which is responsible for all shoot clusters on all seed clusters, and it is performing its reconciliation loops centrally from the garden cluster.
While this works well at scale for thousands of clusters today, our goal is to enable true scalability following the Kubernetes principles (beyond the capacity of a single controller-manager): We are now working on distributing the logic (or the Gardener operator) into the seed cluster and will introduce a corresponding component, adequately named the gardenlet. It will be Gardener's primary "agent" on every seed cluster and will be only responsible for shoot clusters located in its particular seed cluster.
The gardener-controller-manager will still keep its control loops for other resources of the Gardener API, however, it will no longer talk to seed/shoot clusters.
Reversing the control flow will even allow placing seed/shoot clusters behind firewalls without the necessity of direct accessibility (via VPN tunnels) anymore.
Special notes for your reviewer: