Anatomy of `kubectl apply` - Inside the Kubernetes Control Plane
Software Engineer Interview Prep PodcastShow Notes
When I run kubectl apply, the request is sent to the Kubernetes API Server, which acts as the entry point to the cluster. The API Server processes the request through several stages: Authentication – validates the client (certificates, tokens, etc.)
Authorization – checks permissions using RBAC
Admission Controllers Mutating (e.g., inject defaults, sidecars)
Validating (ensure request is compliant)
Once validated, the object is persisted. The API Server stores the Deployment object in etcd, which is the cluster’s consistent key-value store. At this point, the desired state is recorded—but nothing is running yet. The Kubernetes Controller Manager detects the new Deployment via the API Server’s watch mechanism. Deployment Controller creates a ReplicaSet
ReplicaSet Controller creates the required Pods
This is all driven by control loops comparing: Desired state (in etcd)
Current state (actual cluster)
The Pods are created without a node assigned. The kube-scheduler: Filters nodes (resource constraints, taints, node selectors)
Scores remaining nodes (resource availability, affinity rules)
Assigns the best node
Once scheduled, the kubelet on the node pulls the image and starts the container. "The important thing is Kubernetes is entirely declarative and event-driven. Nothing is executed immediately—instead, components continuously reconcile actual state toward desired state."
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