lambda-wasmtime - Running WebAssembly on AWS Lambda

With lambda-wasmtime we have a wasmtime-powered custom AWS Lambda runtime for running WebAssembly, including futuristic stuff like WASI (WebAssembly System Interface) and WAIT (WebAssembly Interface Types).

To run in the AWS Lambda execution environment we need to build a WebAssembly module that exports a function capable of accepting and returning JSON strings. The remainder of this post demonstrates just that.

Building a WebAssembly Lambda

Make sure to have cargo-wasi installed: cargo install cargo-wasi

Setup a new project: cargo new <project_name> --lib

Craft Cargo.toml:

• specify the crate type as cdylib to make this a C-ish shared library

• include a recent wasm-bindgen (tested with 0.2.54)
  

[lib]
crate-type = ["cdylib"]

[dependencies]
wasm-bindgen = "0.2.54"

Define a handler in src/lib.rs and #[wasm_bindgen] it:

use wasm_bindgen::prelude::wasm_bindgen;

#[wasm_bindgen]
pub fn handler(event: &str, context: &str) -> String {
    // lambda-wasmtime passes the event and context as JSON
    // context looks like: { function_arn, deadline_ms, request_id, trace_id }
    event.to_string()
}

Note that you can name your handler whatever you want. The lambda-wasmtime runtime determines the actual handler name from the environment variable _HANDLER which is user-defined in AWS Lambda.

Build the .wasm binary: cargo wasi build --release

For now, when using wasm-bindgen --release mode is required to build binaries with interface types ~strings

Zipup a lambda bundle: zip -j <project>/lambda.zip <project>/target/wasm32-wasi/release/<project_name>.wasm

Deploy the lambda bundle on AWS with the lambda-wasmtime runtime layer - get its latest release from here

If your handler performs non-trivial computations you probably need to provision the lambda with extra memory. Also note that currently all of this is in MVP state and experimental.