Kubebuilder: Controllers, CRDs, and Operators

How to build custom controllers, the easy way.

To begin, clone (quickstart or kube builder book): https://book.kubebuilder.io/

• Inspired by https://github.com/sethp-nr/guestbook-workshop
• See also: https://maelvls.dev/learning-kubernetes-controllers/
• See also: Kubernetes community document on controllers

For the impatient

The substantive part of this tutorial is in these two files:

1. DEPLOYMENT-CONTROLLERRUNTIME.md
2. REPLICAS-UPDATE-DEPLOYMENT.md

Runbooks

Not a tutorial based on theory. Just some basic developer workflow runbooks.

Runbook (startup)

• After running Kubebuilder init and startup basics for project,
• including a make install, make run, and probably an apply,
• follow the Developer Workflow loop below.

Developer workflow for custom controllers

1. Install custom types and generate code make install.
2. Run controller (make run).
3. Make changes to code while running controller.
4. in new terminal, apply changes to cluster and note changes. kubectl apply -f config/samples.

Note that the apply will cause the Reconciler to "reconcile." So applying a change to the cluster will, for example, log any print/log statements in the controller's reconcile() function.

Sample Controller workflows

Workflows for developing custom CRDs and controllers (custom operators).

Log statement:

1. Make change in guestbook controller's Reconcile code (add fmt.Println("henlo")).
2. Run controller (make run).
3. See the controller print "henlo" in the make run terminal window/tmux window.

Add labels imperatively and apply to cluster:

1. Run controller (make run).
2. Add labels to ./config/samples/webapp_v1_guestbook.yaml under metadata

metadata:
name: guestbook-sample
labels:
  is-awesome: totes
  is-bad: nopes

3. apply to cluster: apply -f ./config/samples.
4. Use CLI label selector. k get guestbook -l is-awesome=totes. You'll see guestbook crd.

Get guestbook CRD, its labels, its namespace ...

1. Use Reconciler function in Controller to Get object properties from NamespacedName struct.
2. Run controller (make run).
3. Note changes

Create a basic Deployment

• Use this document DEPLOYMENT-CONTROLLERRUNTIME.md
• Topics covered: controller runtime pkg, client pkg, appsv1 pkg, etc
• You'll learn how to navigate the Kubernetes API at pkg.go.dev

Specify a desired state (scaled replicas), and update Deployment

• Use this document REPLICAS-UPDATE-DEPLOYMENT.md
• Topics covered: types, schemas, more details on Deployments

Create a basic Service ...