So you wanna write Kubernetes controllers?

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2025-01-22 23:00:05

Any company using Kubernetes eventually starts looking into developing their custom controllers. After all, what’s not to like about being able to provision resources with declarative configuration: Control loops are fun, and Kubebuilder makes it extremely easy to get started with writing Kubernetes controllers. Next thing you know, customers in production are relying on the buggy controller you developed without understanding how to design idiomatic APIs and building reliable controllers.

Low barrier to entry combined with good intentions and the “illusion of working implementation1” is not a recipe for success while developing production-grade controllers. I’ve seen the real-world consequences of controllers developed without adequate understanding of Kubernetes and the controller machinery at multiple large companies. We went back to the drawing board and rewritten nascent controller implementations a few times times to observe which mistakes people new to controller development make.

It takes less than 5 minutes to write a Go struct and generate a Kubernetes CustomResourceDefinition (CRD) from it thanks to controller-gen. Then it takes several months to migrate from this poorly designed API to a better v2 design while the old API is being used in productionDon’t do yourself that.