Developer Guide

This guide explains how to set up your environment for developing Krustlet.


To build krustlet, you will need

  • The latest stable version of Rust
  • The latest version of just
  • openssl (Or use the rustls-tls feature)
  • git

Due to this being a relatively bleeding edge project, we don’t have any guarantees around MSRV (minimum supported Rust version). Currently, building requires rust 1.53.0+

If you want to test krustlet, you will also require

  • A Kubernetes cluster
  • The latest version of kubectl

If you want to compile your own WebAssembly modules and upload them to a registry, you’ll need wasm-to-oci.

If you want to build the Docker image, you’ll need Docker.


We use just to build our programs, but you can use cargo if you want:

$ just build

Building without openssl

If you are on a system that doesn’t have OpenSSL (or has the incorrect version), you have the option to build Krustlet using the Rustls project (Rust native TLS implementation):

$ just build --no-default-features --features rustls-tls

The same flags can be passed to just run if you want to just run the project instead.


The underlying dependencies for Rustls do not support certs with IP SANs (subject alternate names). Because of this, the serving certs requested during bootstrap will not work for local development options like minikube or KinD as they do not have an FQDN

Building on WSL (Windows Subsystem for Linux)

You can build Krustlet on WSL but will need a few prerequisites that aren’t included in the Ubuntu distro in the Microsoft Store:

$ sudo apt install build-essential libssl-dev pkg-config

NOTE: We’ve had mixed success developing Krustlet on WSL. It has been successfully run on WSL2 using the WSL2-enabled Docker Kubernetes or Azure Kubernetes. If you’re on WSL1 you may be better off running in a full Linux VM under Hyper-V.

Building on Windows

As of version 0.4, we have support for building on Windows. For convenience sake, there is a windows version of the justfile called justfile-windows. This justfile uses PowerShell and has the proper flags set for Windows builds. To use it, you’ll have to specify the justfile using the --justfile flag like so:

$ just --justfile justfile-windows build

It has all the same targets as the normal justfile, however, the test target runs a little differently than the normal target due to how we use feature flags. This means there will be some spurious warning output from clippy, but the tests will run.

NOTE: Windows builds use the rustls library, which means there are some things to be aware of. See the caveats section for more details


The default included runtime with Krustlet is wasi.

The wasi runtime uses a project called wasmtime. wasmtime is a standalone JIT-style host runtime for WebAssembly modules. It is focused primarily on standards compliance with the WASM specification as it relates to WASI. If your WebAssembly module complies with the WebAssembly specification, wasmtime can run it.

Before startup, this command will delete any nodes in your Kubernetes cluster named with your hostname, so make sure you’re running this in a test environment.

If you want to interact with the kubelet (for things like kubectl logs and kubectl exec), you’ll likely need to set a specific KRUSTLET_NODE_IP that krustlet will be available at. Otherwise, calls to the kubelet will result in errors. This may differ from machine to machine. For example, with Minikube on a Mac, you’ll have an interface called bridge0 which the cluster can talk to. So your node IP should be that IP address.

To set the node IP, run:

$ export KRUSTLET_NODE_IP=<the ip address>


Krustlet contains both integration and unit tests. For convenience, there are just targets for running one or the other.

For unit tests:

$ just test

For the integration tests, start a WASI node in a separate terminal before running the tests.

In terminal 1:

$ just run-wasi

And in terminal 2:

$ just test-e2e

You can run the integration tests without creating additional terminals or manually running the kubelets by running:

$ just test-e2e-standalone


  • Bootstraps and approves certificates if necessary
  • Runs the WASI kubelet in the background
  • Runs the integration tests
  • Terminates the kubelets when the integration tests complete
  • Reports test failures, and saves the kubelet logs if any tests failed

You will still need to set KRUSTLET_NODE_IP because the tester doesn’t know what kind of Kubernetes cluster you’re using and so doesn’t know how to infer a node IP.

WARNING: The standalone integration tester has not been, er, tested on Windows. Hashtag irony.

Integration test debris

There are some failure modes - for example image pull timeout - where the integration tests are not able to complete cleanup of their resources. Specifically you can sometimes get pods stuck in Terminating, which prevents namespace cleanup and causes the next test run to break.

You can forcibly clean up such debris by running cargo run --bin podsmiter. You may need to wait a couple of minutes after pod deletion for the namespaces to be collected.

Creating your own Kubelets with Krustlet

If you want to create your own Kubelet based on Krustlet, all you need to do is implement a Provider.

See src/ and its corresponding provider implementation in crates/wasi-provider to get started.