Hello WASI HTTP!

This is a simple tutorial to get started with WASI HTTP using the wasmtime serve command introduced in Wasmtime 18.0. It runs an HTTP server and forwards requests to a Wasm component via the WASI HTTP API.

Other resources you may find interesting:

This tutorial, which uses wkg and registries.
The wstd crate, which provides friendlier APIs on top of the WASI APIs, and includes examples.

The WASI HTTP API is now stable, and part of WASI 0.2.

So without further ado...

Let's go!

First, install cargo-component (version 0.13.2 or later). cargo-component is a tool for building Wasm components implemented in Rust. For more information on building Wasm components from different languages, check here!

With that, build the Wasm component from source in this repository:

$ cargo component build
  Compiling hello-wasi-http v0.0.0 (/home/wasm/hello-wasi-http)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 2.01s
    Creating component target/wasm32-wasip1/debug/hello_wasi_http.wasm

This builds a Wasm component at target/wasm32-wasip1/debug/hello_wasi_http.wasm.

To run it, we'll need at least Wasmtime 18.0. Installation instructions are on wasmtime.dev. For example, on Linux or macOS, use the command below:

$ curl https://wasmtime.dev/install.sh -sSf | bash

Then, run in your terminal:

$ cargo component serve

This starts up an HTTP server that, by default, listens on 0.0.0.0:8080.

With that running, in another window, we can now make requests!

$ curl http://localhost:8080
Hello, wasi:http/proxy world!

Optimizing!

The above uses a debug build. To make a component that runs faster, build with cargo component build --release.

It's also worth making sure you have a release build of Wasmtime; if you installed it from the instructions above with wasmtime.dev, you're good.

Wasmtime has several tuning options that can improve performance in different situations—pass -O help for a list. One that's especially useful here is -O pooling-allocator.

Notes

wasmtime serve uses the proxy world, which is a specialized world just for accepting requests and producing responses. One interesting thing about the proxy world is that it doesn't have a filesystem or network API. If you add code to the example that tries to access files or network sockets, it won't be able to build, because those APIs are not available in this world. This allows proxy components to run in many different places, including specialized serverless environments which may not provide traditional filesystem and network access.

But, what if you do want to have it serve some files? One option will be to use WASI-Virt, which is a tool that can bundle a filesystem with a component.

Another option is to use a custom world. The proxy world is meant to be able to run in many different environments, but if you know your environment, and you know it has a filesystem, you could create your own world, by including both the "wasi:http/proxy" and "wasi:filesystem/types" or any other APIs you want the Wasm to be able to access. This would require a custom embedding of Wasmtime, as it wouldn't run under plain wasmtime serve, so it's a little more work to set up.

In the future, we expect to see standard worlds emerge that combine WASI HTTP with many other APIs, such as wasi-cloud-core.

If you're interested in tutorials for any of these options, please reach out and say hi!

Creating this repo

Here are my notes on how I created this repository, in case you're interested in recreating it.

To create a new project, run:

$ cargo component new --proxy --lib hello-wasi-http
    Created binary (application) `hello-wasi-http` package
    Updated manifest of package `hello-wasi-http`
    Generated source file `src/main.rs`
$ cd hello-wasi-http

Copy the wit directory from your version of Wasmtime, to ensure that we're using the same version of the API that Wasmtime is built with (e.g., for Wasmtime 18.0.0: https://github.com/bytecodealliance/wasmtime/tree/release-18.0.0).

Then, I manually trimmed the filesystem and sockets dependencies out.

In the future, we'll have wit dependencies stored in a registry, which will make this all much easier.

I derived src/lib.rs from Wasmtime's crates/test-programs/src/bin/api_proxy.rs contents on the main branch, adapted it to work with cargo component, in particular by adding:

cargo_component_bindings::generate!();

Then, I renamed the T type to Component, which the bindings expect.

Finally, add dependencies:

$ cargo component add --target --path wit/deps/clocks wasi:clocks
$ cargo component add --target --path wit/deps/io wasi:io
$ cargo component add --target --path wit/deps/random wasi:random
$ cargo component add --target --path wit/deps/cli wasi:cli
$ cargo component add --target --path wit/deps/logging wasi:logging

These don't all actually get used in this tutorial, but they're currently needed because of some of the interfaces we copied in from the Wasmtime tree reference them.

TODO: I should also make a api_proxy_streaming.rs version to show streaming.

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