Mutating admission webhooks are invoked first, and can modify objects sent to the API server to enforce custom defaults. You can define two types of admission webhooks, validating admission webhook and mutating admission webhook. I encourage you to run it yourself, explore the code, deploy some pods, and experiment with the webhook! A Kubernetes admission what?įirst let’s have a look at the definition in the official docs :Īdmission webhooks are HTTP callbacks that receive admission requests and do something with them. This blog post can be consumed on its own however, the source code has been made available at and is fully runnable on your local machine using a few make commands. This illustrates how admission webhooks work and offers a lightweight solution to real problems. However, when boiled down to its core elements, the complexity fades away, and today we’ll look at how to write a Kubernetes admission webhook in Go with minimal dependencies. When I first heard about Kubernetes admission controllers a few years ago, it took me a moment to wrap my head around them, and I didn’t think that I would ever be able to write one from scratch. We had simple needs, however, and decided to write our own stateless web service that replies to POST requests with a bit of JSON. There are powerful frameworks like Kubebuilder which address the many aspects of writing Kubernetes admission controllers. Surely there must be a canonical solution to this simple problem? Well, sort of. The mutation needed to follow simple business rules, and didn’t need to keep track of any state. While adding a recent feature to our Kubernetes compute platform, we had the need to mutate newly-created pods based on annotations set by users.
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