13.2 Reading Kubernetes Source | Reading & Contributing to Open Source

Repository Structure

k8s.io/kubernetes is massive (~2M lines). Key directories:

Directory	What's There
`cmd/kubelet/`	The kubelet binary entry point
`cmd/kube-apiserver/`	API server entry point
`cmd/kube-controller-manager/`	All built-in controllers
`pkg/kubelet/`	Kubelet implementation
`pkg/controller/`	Controller implementations
`staging/src/k8s.io/`	Libraries published as separate modules

How kubectl Works

Trace a kubectl get pods command:

cmd/kubectl/main.go
  → pkg/cmd/cmd.go (NewDefaultKubectlCommand)
    → pkg/cmd/get/get.go (NewCmdGet)
      → Builds a REST request to the API server
        → Uses client-go's discovery + REST client
          → HTTP GET to /api/v1/namespaces/default/pods

Reading this teaches you: CLI design, API client patterns, error handling, output formatting.

The Controller Pattern in Practice

In pkg/controller/deployment/, the Deployment controller:

Watches Deployments and ReplicaSets via informers
On each event, enqueues the Deployment key to a work queue
Workers dequeue keys and call syncDeployment()
syncDeployment() compares desired replicas vs actual ReplicaSets
Creates, scales, or deletes ReplicaSets to match

This is the exact pattern from Module 11, used in production by every K8s cluster.