01 / The control plane coordinates

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Desired state is a promise to keep

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Learning objective and mental model verified

Trace a declared workload to running containers. The API server exposes and validates the cluster API; etcd holds cluster state. Controllers reconcile objects, the scheduler places unassigned Pods, and node kubelets coordinate with the container runtime. CNI and CSI integrations supply networking and storage interfaces.

Mechanism verified

A controller observes resources, compares actual state with the desired specification and requests changes. Different controllers handle separate responsibilities. Reconciliation is repeated, rather than a one-time deployment script.

Worked example synthesis

Suppose a quote API should have three replicas and one Pod disappears. Its ReplicaSet creates another Pod. Scheduling, image retrieval, startup and readiness still need to succeed. The application must preserve durable state outside that replaceable process.

Decision, pitfall and improvement synthesis

For an illustrative service, declare the desired count through a Deployment rather than manually creating replacement Pods. Keep spare placement capacity and test recovery delay. A controller cannot repair invalid credentials, an unavailable database or insufficient subnet addresses merely by trying again. This inference follows from the separate scheduling and execution responsibilities; it is not a recovery-time guarantee.