WASI Preview 3 (WASIp3) is the next major milestone release of the WebAssembly System Interface, based on the new Component Model async ABI. Unlike earlier releases, WASIp3 allows components to make concurrent, asynchronous calls to each other and to the host runtime, enabling idiomatic, cross-language code reuse for I/O-bound applications.

In this presentation, we'll demonstrate how to use WASIp3 to compose a simple web server written in Python with middleware written in Rust. We'll start by defining a custom inter-component interface using the WebAssembly Interface Type (WIT) language, making use of the new WASIp3 `future` and `stream` types, as well as the new `async` keyword for functions. Next, we'll show how to write idiomatic Rust and Python code based on that interface and then compose the resulting components together using the WebAssembly Compositions (WAC) tool. Finally, we'll run the composed component in a WASIp3-compatible host runtime.