This is a demonstration of experimental async lift/lower, future, and stream support for the WebAssembly Component Model, based on temporary forks of wit-bindgen, wasm-tools, and wasmtime.

• A clone of this repo
• Rust
• curl

Once you have the above, you can install the temporary fork of wasm-tools and download the WASI 0.1 adapter:

cargo install --locked --git https://github.com/dicej/wasm-tools --branch async wasm-tools
curl -LO https://github.com/bytecodealliance/wasmtime/releases/download/v19.0.2/wasi_snapshot_preview1.reactor.wasm

The guest-async and guest-sync components target a simple world which imports and exports an interface with a function that accepts and returns a string, and the implementation appends to that string before and after passing it on to the host. The host implementation appends its own items to the string prior to returning it, as well as sleeping asynchronously for a short time.

In the async case, that sleep causes control to pass back to the guest and on to the original host call. The host then idles until the sleep finishes, at which point the import task finishes, allowing the export task to also finish, yielding a final result back to the host.

This will build the guest-async component using the wit-bindgen and wasm-tools forks and then run it using the wasmtime fork:

cargo build --release --target wasm32-wasi --manifest-path guest-async/Cargo.toml
wasm-tools component new --adapt wasi_snapshot_preview1.reactor.wasm \
    target/wasm32-wasi/release/guest_async.wasm -o guest-async.wasm
cargo run --manifest-path host/Cargo.toml -- round-trip guest-async.wasm \
    'hello, world!' \
    'hello, world! - entered guest - entered host - exited host - exited guest'

That last command should print "success!". If it doesn't, that's bad.

Now we'll compose the guest component with itself and run the result:

wasm-tools compose guest-async.wasm -d guest-async.wasm -o composed.wasm
cargo run --manifest-path host/Cargo.toml -- round-trip composed.wasm \
    'hello, world!' \
    'hello, world! - entered guest - entered guest - entered host - exited host - exited guest - exited guest'

Again, you should see "success!".

You can also compose the guest-async and guest-sync components in any order (sync->sync, sync->async, async->sync, and async->async) and observe the same result.

host/src/main.rs contains tests covering additional scenarios besides the above, including all async/sync composition combinations. The http-echo and middleware tests demonstrate how to use futures, streams, and post-return asynchronous execution via the spawn function.

cargo test