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Hello WASI HTTP!

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

The WASI HTTP API is now stable, and part of WASI Preview 2.

So without further ado...

Let's go!

First, install cargo component, version 0.9.1, which is a tool for building Wasm components implemented in Rust. (See here for information about building Wasm components from other languages too!)

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

$ cargo component build
   Compiling hello-wasi-http v0.0.0 (/home/wasm/hello-wasi-http)
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
    Creating component /home/wasm/hello-wasi-http/target/wasm32-wasi/debug/hello_wasi_http.wasm
$

This builds a Wasm component, target/wasm32-wasi/debug/hello_wasi_http.wasm.

To run it, we'll need at least Wasmtime 18.0.0. Installation instructions are on wasmtime.dev:

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

Then, in a new terminal, we can run wasmtime serve on our Wasm component:

$ wasmtime serve target/wasm32-wasi/debug/hello_wasi_http.wasm

This starts up an HTTP server on 0.0.0.0:8080 (the specific address and port can be configured with the --addr= flag).

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—but 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.

Run cargo-component new --proxy --lib to create a new project:

$ cargo component new --reactor 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 Wasmtime 18.0.0, to ensure that we're using the same version of the API that Wasmtime is built with:

https://github.com/bytecodealliance/wasmtime/tree/release-18.0.0

I then 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!();

and renaming the T type to Component, which the bindings expect.

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.