Building a WebAssembly-powered serverless platform

The article builds a bare-bones serverless platform around WebAssembly. It uses Wasmtime inside a Rust app, with Actix Web handling HTTP routing to .wasm modules. The first version returns integers from a run export, then the platform is extended to use WASI so modules can write strings to stdout and receive parameters through environment variables.

Reading notes

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• WebAssembly is presented as more useful outside the browser, as a standalone runtime used in projects such as Kubernetes, Istio, and Red Panda.
• Wasmtime is chosen because of its WASI support, and Rust is used because Wasmtime itself is written in Rust.
• A simple module exports run and returns 42, and the Rust host loads the module, finds the export, and executes it.
• The serverless platform uses Actix Web to listen on 127.0.0.1:8080 and map the request path to a module name.
• Returning strings requires shared memory or another interface, because WebAssembly only passes integers directly.
• WASI is used as the language-agnostic way to handle output, with stdout serving as the response channel.
• The host builds a WASI context with stdout wired to a buffer, instantiates the module through a Wasmtime linker, and calls _start.
• The compiled module imports WASI functions such as fd_write, proc_exit, environ_sizes_get, and environ_get, which the linker resolves.
• For input parameters, the example uses query-string values as environment variables passed into the WASI context.
• A Rust Sudoku solver reads a puzzle environment variable, solves the grid, and prints the result line by line.
• The final platform is described as multi-language, lightweight, secure, and fast, with the sample Hello World function taking 19 ms end to end and Wasmtime accounting for 6 ms.