Playing with WASM on Docker

The idea of bytecode that can run anywhere dates back to the JVM inception (as far as I know). WebAssembly is the new implementation of an old idea. While WebAssembly is meant to run in the browser, Docker recently announced its capability to run WASM code without needing containers. In this post, I want to explore how it can work.


Running WebAssembly is a beta feature and requires using containerd. To enable containerd, go to the Docker Desktop dashboard, then Settings  Features in development | Beta features | Use containerd for storing and pulling images.

Enabling containerd previously broke one of my Kubernetes demos. Play with WASM to your heart’s content, but remember to roll back the configuration immediately afterward, or there’s a chance downloaded containers won’t run anymore.

I want to compare regular images with WebAssembly; hence, I require a project that can compile to both native code and WASM. For this reason, I chose to use the Rust language. I’ll have a single simple project with two Dockerfiles: one that compiles to native, the other that compiles to WASM.

Building locally

Here’s the Rust expected Hello World:

fn main() {
    println!("Hello, world!");

We can install the Webassembly target and build locally for comparison purposes:

rustup target add wasm32-wasi
cargo build --target wasm32-wasi --release

The file is relatively small:

-rwxr-xr-x  1 nico  staff   2.0M Jun  4 15:44   wasm-native.wasm

Building the basic Docker images

The Dockerfile that builds the Webassembly image is the following:

FROM rust:1.70-slim-bullseye as build                                    (1)

COPY Cargo.toml .
COPY Cargo.lock .
COPY src src

RUN rustup target add wasm32-wasi                                        (2)

RUN cargo build --target wasm32-wasi --release                           (3)

FROM scratch                                                             (4)

COPY --from=build /target/wasm32-wasi/release/wasm-native.wasm wasm.wasm (5)

ENTRYPOINT [ "/wasm.wasm" ]
1 Start from the last Rust Docker image
2 Add the WASM target
3 Build, targeting Webassembly
4 Use a multi-stage build. Start from scratch
5 Copy the Webassembly file generated in the previous stage

The reference material uses the --platform wasi/wasm32 argument when building the Docker image. It doesn’t work on my machine. It may be because I’m on an M1 Mac, or the documentation needs to be updated. In any case, I build "normally":

docker build -f Dockerfile-wasm -t docker-wasm:1.0 .

We can now run it, specifying a supported WASM runtime:

docker run --runtime=io.containerd.wasmedge.v1 docker-wasm:1.0

To compare, we can create a native image with the same code:

FROM rust:1.70-slim-bullseye as build

COPY Cargo.toml .
COPY Cargo.lock .
COPY src src

RUN RUSTFLAGS='-C target-feature=+crt-static' cargo build --release (1)

FROM scratch                                                        (2)

COPY --from=build /target/release/wasm-native native
1 Make the binary self-sufficient
2 Can start from scratch

We can now compare the images size:

docker-native      1.0      0c227194910a   7 weeks ago   7.09MB
docker-wasm        1.0      f9a88747f798   4 weeks ago   2.61MB

The Webassembly image is about one-third of the native binary package.

We cheat a bit because we add the WASM runtime…​ at runtime.

Building more complex images

Let’s see how we can add parameters to the binary and update the code accordingly:

use std::env;

fn main() {
    let args: Vec<String> = env::args().collect();
    if args.len() > 1 {
        println!("Hello, world!");
    } else {
        println!("Hello, {}!", args[1]);

Let’s rebuild the images and compare again:

REPOSITORY         TAG      IMAGE ID       CREATED          SIZE
docker-native      1.0      0c227194910a   7 weeks ago      7.09MB
docker-native      1.1      3ae029030e83   39 minutes ago   7.1MB
docker-wasm        1.0      f9a88747f798   4 weeks ago      2.61MB
docker-wasm        1.1      41e38b68f4e4   39 minutes ago   2.63MB

Executing HTTP calls?

With this, it’s easy to get carried away and start thinking big: what if we could execute HTTP calls?

I’ll use the reqwest crate since I’m familiar with it. reqwest relies on Tokio.

reqwest = { version = "0.11", features = ["json"] }
tokio = { version = "1.28", features = ["full"] }
serde = { version = "1.0", features = ["derive"] }

We can now update the code to make a request to <httpbin.org> and print the result:

async fn main() {
    match get("http://httpbin.org/get").await {
        Ok(response) => {
            let result = response.json::<GetBody>().await;
            match result {
                Ok(json) => {
                    println!("{:#?}", json);
                Err(err) => {
                    println!("{:#?}", err)
        Err (err) => {
            println!("{:#?}", err)

#[derive(Debug, Serialize, Deserialize)]
struct GetBody {
    args: HashMap<String, String>,
    headers: HashMap<String, String>,
    origin: String,
    url: String,

Compiling this code reveals WASM limitations, though:

#0 12.40 error: Only features sync,macros,io-util,rt,time are supported on wasm.
#0 12.40    --> /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.28.2/src/lib.rs:488:1
#0 12.40     |
#0 12.40 488 | compile_error!("Only features sync,macros,io-util,rt,time are supported on wasm.");
#0 12.40     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

WASM is not multi-threaded, while Tokio is by default. We can, however, configure Tokio to work in a single-thread environment. Let’s start by using the features that we need: macros for the main function and rt for the tokio runtime.

tokio = { version = "1.28", features = ["rt", "macros"] }

Now, we can limit Tokio to the unique thread:

#[tokio::main(flavor = "current_thread")]
async fn main() {}

Compiling now works. However, I encounter issues when running:

[2023-06-05 12:22:11.986] [error] instantiation failed: unknown import, Code: 0x62
[2023-06-05 12:22:11.986] [error]     When linking module: "__wbindgen_placeholder__" , function name: "__wbindgen_object_drop_ref"
[2023-06-05 12:22:11.986] [error]     At AST node: import description
[2023-06-05 12:22:11.986] [error]     At AST node: import section
[2023-06-05 12:22:11.986] [error]     At AST node: module
docker: Error response from daemon: Others("unknown import"): unknown.

The reqwest crate doesn’t work with the WASI environment. Until it does, there’s a fork aptly named reqwest_wasi. The tokio_wasi is the WASI-compatible crate for tokio. Note that the latter’s version needs to catch up. Let’s replace the crates:

reqwest_wasi = { version = "0.11", features = ["json"] }
tokio_wasi = { version = "1.25", features = ["rt", "macros"] }

With the new crates, compilation works, as well as execution. On the other side, the native image works flawlessly, with slight changes for the Dockerfile:

#docker build -f Dockerfile-native -t docker-native:1.2 .
FROM rust:1.70-slim-bullseye as build

COPY Cargo.toml .
COPY Cargo.lock .
COPY src src

RUN apt-get update && apt-get install -y pkg-config libssl-dev   (1)

RUN cargo build --release

FROM debian:bullseye-slim                                        (2)

COPY --from=build /target/release/wasm-native native

ENTRYPOINT [ "/native" ]
1 Install required libraries for SSL
2 Change to a more complete base image to avoid installing additional libraries

Here’s the final comparison:

REPOSITORY         TAG      IMAGE ID       CREATED          SIZE
docker-native      1.0      0c227194910a   7 weeks ago      7.09MB
docker-native      1.1      3ae029030e83   22 hours ago     7.1MB
docker-native      1.2      4ff64cf9de46   7 hours ago      123MB
docker-wasm        1.0      1cc78a392477   23 hours ago     2.61MB
docker-wasm        1.1      41e38b68f4e4   22 hours ago     2.63MB
docker-wasm        1.2      6026f5bd789c   18 seconds ago   5.34MB

I didn’t fiddle with the optimization of the native image. However, it would be hard to beat the WASM image, as it stands below 6MB!

There’s no chance to implement an Axum server, though.


I implemented a couple of WASM Docker images in this post, from the most straightforward Hello World to an HTTP client.

While the ecosystem has room for improvement, it’s already possible to benefit from Docker’s WASM support. The small size of WASM images is a huge pro.

The complete source code for this post can be found on Github.
Nicolas Fränkel

Nicolas Fränkel

Developer Advocate with 15+ years experience consulting for many different customers, in a wide range of contexts (such as telecoms, banking, insurances, large retail and public sector). Usually working on Java/Java EE and Spring technologies, but with focused interests like Rich Internet Applications, Testing, CI/CD and DevOps. Also double as a trainer and triples as a book author.

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