wasi-experimental-http

This is an experiment intended to provide a temporary workaround until the WASI networking API is stable, and is compatible with Wasmtime v0.24 by using the wasi_experiemental_http_wasmtime crate. We expect that once the WASI sockets proposal gets adopted and implemented in language toolchains, the need for this library will vanish.

Writing a module that makes an HTTP request

We use the wasi-experimental-http crate (from this repository) and the http crate to create an HTTP request from a WebAssembly module, make a host call to the runtime using the request, then get a response back:

use bytes::Bytes;
use http;
use wasi_experimental_http;

#[no_mangle]
pub extern "C" fn _start() {
    let url = "https://postman-echo.com/post".to_string();
    let req = http::request::Builder::new()
        .method(http::Method::POST)
        .uri(&url)
        .header("Content-Type", "text/plain")
        .header("abc", "def");
    let b = Bytes::from("Testing with a request body. Does this actually work?");
    let req = req.body(Some(b)).unwrap();

    let res = wasi_experimental_http::request(req).expect("cannot make request");
    let str = std::str::from_utf8(&res.body_read_all()).unwrap().to_string();
    println!("{:#?}", res.header_get("Content-Type"));
    println!("{}", str);
    println!("{:#?}", res.status_code);
}

Build the module using the wasm32-wasi target, then follow the next section to update a Wasmtime runtime with the experimental HTTP support.

Adding support to a Wasmtime runtime

The easiest way to add support is by using the Wasmtime linker:

let store = Store::default();
let mut linker = Linker::new(&store);
let wasi = Wasi::new(&store, ctx);

// link the WASI core functions
wasi.add_to_linker(&mut linker)?;

// link the experimental HTTP support
let allowed_hosts = Some(vec!["https://postman-echo.com".to_string()]);
wasi_experimental_http_wasmtime::link_http(&mut linker, allowed_hosts)?;

Then, executing the module above will send the HTTP request and write the response:

wasi_experimental_http::data_from_memory:: length: 29
wasi_experimental_http::data_from_memory:: length: 41
wasi_experimental_http::data_from_memory:: length: 4
wasi_experimental_http::data_from_memory:: length: 53
wasi_experimental_http::write_guest_memory:: written 336 bytes
wasi_experimental_http::write_guest_memory:: written 374 bytes
{
    "content-length": "374",
    "connection": "keep-alive",
    "set-cookie": "sails.Path=/; HttpOnly",
    "vary": "Accept-Encoding",
    "content-type": "application/json; charset=utf-8",
    "date": "Fri, 26 Feb 2021 18:31:03 GMT",
    "etag": "W/\"176-Ky4OTmr3Xbcl3yNah8w2XIQapGU\"",
}
{"args":{},"data":"Testing with a request body. Does this actually work?","files":{},"form":{},"headers":{"x-forwarded-proto":"https","x-forwarded-port":"443","host":"postman-echo.com","x-amzn-trace-id":"Root=1-60393e67-02d1c8033bcf4f1e74a4523e","content-length":"53","content-type":"text/plain","abc":"def","accept":"*/*"},"json":null,"url":"https://postman-echo.com/post"}
"200 OK"

The Wasmtime implementation also enables allowed hosts - an optional and configurable list of domains or hosts that guest modules are allowed to send requests to. If None or an empty vector is passed, guest modules are NOT allowed to make HTTP requests to any server. (Note that the hosts passed MUST have the protocol also specified - i.e. https://my-domain.com, or http://192.168.0.1, and if making requests to a subdomain, the subdomain MUST be in the allowed list. See the the library tests for more examples).

Note that the Wasmtime version currently supported is 0.24.

Sending HTTP requests from AssemblyScript

This repository also contains an AssemblyScript implementation for sending HTTP requests:

// @ts-ignore
import * as wasi from "as-wasi";
import {
  Method,
  RequestBuilder,
  Response,
} from "@deislabs/wasi-experimental-http";
export { alloc } from "@deislabs/wasi-experimental-http";

export function _start_(): void {
  let body = String.UTF8.encode("testing the body");
  let res = new RequestBuilder("https://postman-echo.com/post")
    .header("Content-Type", "text/plain")
    .method(Method.POST)
    .body(body)
    .send();
  wasi.Console.log(res.status.toString());
  wasi.Console.log(res.headers);
  wasi.Console.log(String.UTF8.decode(res.body));
}

Known limitations

• there is no support for streaming HTTP responses, which this means guest modules have to wait until the entire body has been written by the runtime before reading it.
• request and response bodies are Bytes.
• there are no WITX definitions, which means we have to manually keep the host call and guest implementation in sync. Adding WITX definitions could also enable support for other WASI runtimes.
• this library does not aim to add support for running HTTP servers in WebAssembly.

Code of Conduct

This project has adopted the Microsoft Open Source Code of Conduct.

For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.