Science and technology

Use OCI containers to run WebAssembly workloads

WebAssembly (additionally known as Wasm) has gained reputation as a conveyable binary instruction format with an embeddable and remoted execution setting for shopper and server functions. Think of WebAssembly as a small, quick, environment friendly, and really safe stack-based digital machine designed to execute moveable bytecode that does not care what CPU or working system it runs on. WebAssembly was initially designed for internet browsers to be a light-weight, quick, protected, and polyglot container for features, however it’s now not restricted to the net.

On the net, WebAssembly makes use of the prevailing APIs offered by browsers. WebAssembly System Interface (WASI) was created to fill the void between WebAssembly and techniques operating exterior the browser. This permits non-browser techniques to leverage the portability of WebAssembly, making WASI a sensible choice for portability whereas distributing and isolation whereas operating the workload.

WebAssembly presents a number of benefits. Because it’s platform impartial, one single binary may be compiled and executed on quite a lot of working techniques and architectures concurrently, with a really low disk footprint and startup time. Useful security measures embrace module signing and safety knobs controllable on the run-time stage slightly than relying on the host working system’s person privilege. Sandboxed reminiscence can nonetheless be managed by current container instruments infrastructure.

In this text, I’ll stroll by means of a situation for configuring container runtimes to run Wasm workloads from light-weight container pictures.

Adoption on cloud infrastructure and blockers

WebAssembly and WASI are pretty new, so the requirements for operating Wasm workloads natively on container ecosystems haven’t been set. This article presents just one answer, however there are different viable strategies.

Some of the options embrace switching native Linux container runtimes with parts which might be Wasm suitable. For occasion, Krustlet v1.0.0-alpha1 permits customers to introduce Kubernetes nodes the place Krustlet is used as a alternative for the standard kubelet. The limitation of this strategy is that customers have to decide on between Linux container runtime and Wasm runtime.

Another answer is utilizing a base picture with Wasm runtime and manually invoking compiled binary. However, this methodology makes container pictures bloated with runtime, which isn’t essentially wanted if we invoke Wasm runtime natively at a decrease stage than container runtime.

I’ll describe how one can keep away from this by making a hybrid setup the place current Open Containers Initiative (OCI) runtimes can run each native Linux containers and WASI-compatible workloads.

Using crun in a hybrid setup of Wasm and Linux containers

Some of the issues mentioned above may be simply addressed by permitting an current OCI runtime to invoke each Linux containers and Wasm containers at a decrease stage. This avoids points like relying on container pictures to hold Wasm runtime or introducing a brand new layer to infrastructure that helps solely Wasm containers.

One container runtime that may deal with the duty: crun.

Crun is quick, has a low-memory footprint, and is a completely OCI-compliant container runtime that can be utilized as a drop-in alternative in your current container runtime. Crun was initially written to run Linux containers, however it additionally presents handlers able to operating arbitrary extensions contained in the container sandbox in a local method.

This is a casual means of changing current runtime with crun simply to showcase that crun is a whole alternative in your current OCI runtime.

$ mv /path/to/exisiting-runtime /path/to/existing-runtime.backup
$ cp /path/to/crun /path/to/existing-runtime

One such handler is crun-wasm-handler, which delegates specifically configured container pictures (a Wasm compat picture) to the components of current Wasm runtimes in a local strategy contained in the crun sandbox. This means, finish customers don’t want to keep up Wasm runtimes by themselves.

Crun has native integration with wasmedge, wasmtime, and wasmer to assist this performance out of the field. It dynamically invokes components of those runtimes as crun detects whether or not the configured picture accommodates any Wasm/WASI workload, and it does so whereas nonetheless supporting the native Linux containers.

For particulars on constructing crun with Wasm/WASI assist, see the crun repository on GitHub.

Building and operating Wasm pictures utilizing Buildah on Podman and Kubernetes

Users can create and run platform-agnostic Wasm pictures on Podman and Kubernetes utilizing crun as an OCI runtime beneath the hood. Here’s a tutorial:

Creating Wasm compat pictures utilizing Buildah

Wasm/WASI suitable pictures are particular. They include a magic annotation that helps an OCI runtime like crun classify whether or not it’s a Linux-native picture or a picture with a Wasm/WASI workload. Then it may well invoke handlers if wanted.

Creating these Wasm compat pictures is extraordinarily straightforward with any container picture construct instruments, however for this text, I’ll l exhibit utilizing Buildah.

1. Compile your .wasm module.

2. Prepare a Containerfile along with your .wasm module.

FROM scratch

COPY hey.wasm /

CMD ["/hello.wasm"]

3. Build a Wasm picture utilizing Buildah with annotation module.wasm.picture/variant=compat

$ buildah construct --annotation "module.wasm.image/variant=compat" -t mywasm-image

Once the picture is constructed and the container engine is configured to make use of crun, crun will automagically do the needful and run the offered workload by the configured Wasm handler.

Running a WASM workload with Podman

Crun is the default OCI runtime for Podman. Podman accommodates knobs and handles to make the most of most crun options, together with the crun Wasm handler. Once a Wasm compat picture is constructed, it may be utilized by Podman similar to some other container picture:

$ podman run mywasm-image:newest

Podman runs the requested Wasm compat picture mywasm-image:newest utilizing crun’s Wasm handler and returns output confirming that our workload was executed.

$ hey world from the webassembly module !!!!

Kubernetes-supported and examined container run-time interface (CRI) implementations

Here’s tips on how to configure two widespread container runtimes:


  • Configure CRI-O to make use of crun as an alternative of runc by enhancing config at /and many others/crio/crio.conf. Red Hat OpenShift documentation accommodates extra particulars about configuring CRI-O.
  • Restart CRI-O with sudo systemctl restart crio.
  • CRI-O mechanically propagates pod annotations to the container spec.


  • Containerd helps switching container runtime through a customized configuration outlined at /and many others/containerd/config.toml.
  • Configure containerd to make use of crun by ensuring the run-time binary factors to crun. More particulars can be found within the containerd documentation.
  • Configure containerd to allowlist Wasm annotations to allow them to be propagated to the OCI spec by setting pod_annotations within the configuration: pod_annotations = ["module.wasm.image/variant.*"].
  • Restart containerd with sudo systemctl begin containerd.
  • Now containerd ought to propagate Wasm pod annotations to containers.

The following is an instance of a Kubernetes pod spec that works with each CRI-O and containerd:

apiVersion: v1
: Pod
: pod-with-wasm-workload
: mynamespace
: compat
  - identify
: wasm-container
: myrepo/mywasmimage:newest

Known points and workarounds

Complex Kubernetes infrastructure accommodates pods and, in lots of circumstances, pods with sidecars. That means crun’s Wasm integration will not be helpful when a deployment accommodates sidecars and sidecar containers don’t include a Wasm entry level, equivalent to infrastructure setups with service mesh like Linkerd, Gloo, and Istio or a proxy like Envoy.

You can remedy this problem by including two sensible annotations for Wasm handlers: compat-smart and wasm-smart. These annotations function a sensible change that solely toggles Wasm runtime if it is necessary for a container. Hence whereas operating deployments with sidecars, solely containers that include legitimate Wasm workloads are executed by Wasm handlers. Regular containers are handled as traditional and delegated to the native Linux container runtime.

Thus when constructing pictures for such a use case, use annotation module.wasm.picture/variant=compat-smart as an alternative of module.wasm.picture/variant=compat.

You can discover different identified points in crun documentation on GitHub.

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