Webassembly

There are 3 things you need to do to run a WebAssembly module with youki.

Build youki with one of the wasm feature flags enabled
Build a container image with the WebAssembly module
Run the container with youki

Build youki with `wasm-wasmedge`, `wasm-wasmer`, or `wasm-wasmtime` feature flag enabled

Run build.sh with -f wasm-wasmedge option.

./scripts/build.sh -o . -r -f wasm-wasmedge

Run build.sh with -f wasm-wasmer option.

./scripts/build.sh -o . -r -f wasm-wasmer

Run build.sh with -f wasm-wasmtime option.

./scripts/build.sh -o . -r -f wasm-wasmtime

Build a container image with the WebAssembly module

If you want to run a webassembly module with youki, your config.json has to include either run.oci.handler or module.wasm.image/variant=compat.

It also needs to specify a valid .wasm (webassembly binary) or .wat (webassembly text format) module as entrypoint for the container. If a wat module is specified it will be compiled to a wasm module by youki before it is executed. The module also needs to be available in the root filesystem of the container obviously.

"ociVersion": "1.0.2-dev",
"annotations": {
    "run.oci.handler": "wasm"
},
"process": {
    "args": [
        "hello.wasm",
        "hello",
        "world"
    ],
...
}
...

Compile a sample wasm module

A simple wasm module can be created by running

rustup target add wasm32-wasip1
cargo new wasm-module --bin
cd ./wasm-module
vi src/main.rs

fn main() {
    println!("Printing args");
    for arg in std::env::args().skip(1) {
        println!("{}", arg);
    }

    println!("Printing envs");
    for envs in std::env::vars() {
        println!("{:?}", envs);
    }  
}

Then compile the program to WASI.

cargo build --target wasm32-wasip1

Build a container image with the module

Create a Dockerfile.

vi Dockerfile

FROM scratch
COPY target/wasm32-wasip1/debug/wasm-module.wasm /
ENTRYPOINT ["/wasm-module.wasm"]

Then build a container image with module.wasm.image/variant=compat annotation.

buildah build --annotation "module.wasm.image/variant=compat" -t wasm-module .

Run the wasm module with youki and podman

Run podman with youki as runtime.

podman run --annotation "run.oci.handler=wasm" --runtime /PATH/WHERE/YOU/BUILT/youki localhost/wasm-module 1 2 3

The --annotation "run.oci.handler=wasm" flag tells youki to use its wasm handler. It is passed explicitly here because the module.wasm.image/variant=compat annotation set at build time is not always propagated to the container’s config.json by podman.

Note: The command above runs rootless, which is podman’s default for a non-root user, so sudo is not required. If your host is not set up for rootless containers (for example, missing subuid/subgid configuration) or you need functionality that requires elevated privileges, run the same command with sudo (rootful mode).

Make sure buildah and podman run in the same context (both rootless, or both with sudo). They use separate container storage per user (~/.local/share/containers for rootless, /var/lib/containers for rootful), so building the image with one and running it with the other will fail to find localhost/wasm-module.

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