Flavio Castelli

Debugging my life

Apr 26, 2022 - 4 minute read - Comments - kubernetes WebAssembly WASI

Write kubectl plugins using WebAssembly and WASI

A long time passed since the last time I wrote something on this blog! 😅 I haven’t been idle during this time, quite the opposite… I kept myself busy experimenting with WebAssembly and Kubernetes.

You probably have already heard about WebAssembly, but there are high chances that happened in the context of Web application development. There’s however a new emerging trend that consists of using WebAssembly outside of the browser.

WebAssembly has many interesting properties that make it great for writing plugin systems or even distributing small computational units (think of FaaS).

WebAssembly is what is being used to power Kubewarden, a project I created almost two years ago at SUSE Rancher, with the help of Rafa and other awesome folks. This is where the majority of my “blogging energies” have been focused.

Now, let’s go back to the main focus of today’s blog entry: write kubectl plugins using WebAssembly.

The current state of things

As you all know, kubectl can be easily extended by writing external plugins.

These plugins are executables named kubectl-<name of the plugin> that, once put in your $PATH can be invoked via kubectl <name of the plugin>. This is the same mechanism used to write git plugins.

These plugins can be managed via a tool called Krew.

The kubectl tool is available for multiple operating systems and architectures, which means these plugins must be available for many platforms.

Can WebAssembly help here?

I think writing kubectl plugins using WebAssembly has the following advantages:

  • Portability: you don’t have to build your WebAssembly-powered plugin for all the possible operating systems and architectures the end users might want.
  • Security: each WebAssembly module is executed inside of a dedicated sandbox. They cannot see other modules or processes running on the host. They also don’t have access to the host resources (filesystem, devices, network). Think of them as containers.
  • Size: the majority of kubectl plugins are written using Go, which produces big binaries. The average size of a kubectl plugin is around 9 Mb. WebAssembly on the other hand, can produce plugins that are half the size.

Last but not least, this sounds like a fun experiment!

Introducing krew-wasm

The idea about writing kubectl plugins with WebAssembly originated during a brainstorming session I was doing with Rafa about our upcoming talk for WasmDay EU 2022. The idea kinda “infected” me, I had to hack on it ASAP!!! This is how the krew-wasm project was created.

krew-wasm takes inspiration from Krew, but it does not aim to replace it. That’s quite the opposite, it’s a complementary tool that can be used alongside with Krew.

The sole purpose of krew-wasm is to manage and execute kubectl plugins written using WebAssembly and WASI.

krew-wasm plugins are WebAssembly modules that are distributed using container registries, the same infra used to host container images.

krew-wasm can download kubectl WebAssembly plugins from a container registry and make them discoverable to kubectl. This is achieved by creating a symbolic link for each managed plugin. This symbolic link is named kubectl-<name of the plugin> but, instead of pointing to the WebAssembly module, it points to the krew-wasm executable.

Once invoked, krew-wasm determines its usage mode which could either be a “direct invocation” (when the user invokes the krew-wasm binary to manage plugins) or it could be a “wrapper invocation” done via kubectl.

When invoked in “wrapper mode”, krew-wasm takes care of loading the WebAssembly plugin and invoking it. krew-wasm works as a WebAssembly host, and takes care of setting up the WASI environment used by the plugin.

I’ll leave the technical details out of this post, but if you want you can find more on the GitHub project page.

Some examples

The POC would not be complete without some plugins to run. Guess what, you can find a one right here!

The kubectl decoder plugin dumps Kubernetes Secret objects to the standard output, decoding all the data that is base64-encoded. On top of that, when a x509 certificate is found inside of the Secret, a detailed output is shown rather then the not so helpful PEM encoded representation of the certificate.

If you want to experiment with this idea, you can write your plugins using Rust and this SDK.


This has been a nice experiment. It proves the combination of WebAssembly and WASI can be used to produce working kubectl plugins.

What’s more interesting is the fact these technologies could be used to extend other Cloud Native projects. Did someone say helm? 😜

There are however some limitations, mostly caused by the freshness of WASI. These are documented here. However, I’m pretty sure things will definitely improve over the next months. After all the WebAssembly ecosystem is moving at a fast pace!