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Week 9 - client-go Internals and a Bare Controller

9.1 Conceptual Core

  • client-go is the Kubernetes Go client library-typed clients, informers, work queues, leader election, the lot.
  • Building a controller "from scratch" in client-go is verbose but instructive-every other framework hides these primitives.
  • The pattern (the informer + workqueue pattern):
  • Create a SharedInformerFactory for the resources you watch.
  • For each kind, register OnAdd/OnUpdate/OnDelete handlers that compute a key (namespace/name) and Add it to a RateLimitingQueue.
  • Start N workers that pull keys from the queue and run reconcile(key).
  • reconcile: list-from-cache (never call apiserver in the hot path), compute diff, apply changes, requeue on error with backoff.

9.2 Mechanical Detail

  • The informer's resync period: re-deliver every cached object every N (default 10 minutes). Belt-and-suspenders against missed events.
  • Indexers (cache.Indexer): O(1) lookup by namespace, by label, by custom key. Free with the informer.
  • Listers (<group>/<version>/<resource>/lister.go in generated client code): typed accessors over the indexer.
  • Leader election (tools/leaderelection): only one replica of the controller acts; others stand by. Uses a Lease resource as the lock.
  • Generated clients: for built-in types, client-go ships them. For your own CRDs, generate with controller-gen or kubebuilder (week 10).

9.3 Lab-"Controller From Scratch"

Build a controller that watches ConfigMaps with the label mirror=true and copies them into every namespace whose name matches a configurable prefix. - Use client-go informers + workqueue directly. - Add leader election. - Idempotent: same input twice produces same result. - Handle deletions: when the source is deleted, delete all mirrors. - Run as a Deployment in the cluster.

9.4 Hardening Drill

  • Define a minimum RBAC: only get/list/watch on configmaps and namespaces, plus create/update/delete on configmaps (constrained by namespace prefix? Use admission webhooks or namespace selectors).

9.5 Operations Slice

  • Expose controller_runtime_* - style metrics: queue depth, work duration, error rate. Add a/healthzand/readyz. Run with/livez` probe.

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