Wasmtime Has a Free API: The Production-Ready WebAssembly Runtime From the Bytecode Alliance

Wasmer is flashy. Wasmtime is industrial. Backed by Mozilla, Fastly, Intel, and Microsoft — Wasmtime is the WASI reference implementation.

What Is Wasmtime?

Wasmtime is a fast, secure, production-grade WebAssembly runtime from the Bytecode Alliance. It's the reference implementation for WASI (WebAssembly System Interface).

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

# Run a Wasm module
wasmtime hello.wasm

Embed in Your Application

use wasmtime::*;

fn main() -> anyhow::Result<()> {
    let engine = Engine::default();
    let module = Module::from_file(&engine, "add.wasm")?;
    let mut store = Store::new(&engine, ());
    let instance = Instance::new(&mut store, &module, &[])?;

    let add = instance.get_typed_func::<(i32, i32), i32>(&mut store, "add")?;
    let result = add.call(&mut store, (5, 3))?;
    println!("5 + 3 = {result}");  // 8
    Ok(())
}

from wasmtime import Store, Module, Instance

store = Store()
module = Module.from_file(store.engine, "add.wasm")
instance = Instance(store, module, [])
add = instance.exports(store)["add"]
result = add(store, 5, 3)
print(f"5 + 3 = {result}")

WASI: System Interface

// Wasm module can access files, env vars, clocks — with explicit permissions
let wasi = WasiCtxBuilder::new()
    .inherit_stdout()
    .preopened_dir("./data", "data")?  // Only this directory
    .env("API_KEY", "secret")?         // Only this env var
    .build();

Component Model

# Compose Wasm components
wasm-tools compose auth.wasm -d database.wasm -o app.wasm
# Two modules linked together, sharing typed interfaces

Why Wasmtime

• Production-grade — used by Fastly, Shopify, SingleStore in production
• WASI reference — the standard others follow
• Cranelift JIT — near-native execution speed
• Fuel metering — limit computation (prevent infinite loops)
• Component Model — typed interfaces between Wasm modules
• Embeddings — Rust, C, Python, Go, .NET, Ruby

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Building Wasm-powered systems? Check out my developer tools or email [e-mail].