WebAssembly on the Server-side

Run high performance Rust code safely inside nodejs

There are great use cases for WebAssembly on the server-side, especially for AI, blockchain, and big data applications. In this tutorial, I will show you how to incorporate WebAssembly functions, written in Rust, into nodejs applications on the server. We can therefore provide WebAssembly functions as a microservice (FaaS).

In this tutorial, we use the Second State Virtual Machine (SSVM) , an open source WebAssembly runtime optimized for server-side applications, together with nodejs. The SSVM provides not only a WebAssembly runtime in nodejs, but also a compiler toolchain for Rust and JavaScript.

While nodejs comes with a default WebAssmebly runtime inside its V8 JavaScript engine, V8 is not designed to handle the performance and complex integration requirements of server-side applications. Compared with V8, the server-optimized SSVM is more performant, integrates better with JavaScript, provides access to external enterprise resources, and supports finely grained metering.

The demo application is structured as follows.

  • The host application is a nodejs web application written in JavaScript. It makes WebAssembly function calls.

  • The WebAssembly bytecode program is written in Rust. It runs inside the SSVM, and is called from the nodejs web application.

The source code of the tutorial is here.


The ssvmup npm module installs the Second State Virtual Machine (SSVM) into nodejs as a native addon, and provides the necessary compiler tools. Follow the steps below to install Rust and the ssvmup tool.

# Install Rust
$ sudo apt-get update
$ sudo apt-get -y upgrade
$ curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
$ source $HOME/.cargo/env
# Install wasm-pack tools
$ curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh

WebAssembly program in Rust

In this example, our Rust program appends the input string after “hello”. Let’s create a new cargo project. Since this program is intended to be called from a host application, not to run as a stand-alone executable, we will create a hello project.

$ cargo new --lib hello
$ cd hello

Edit the Cargo.toml file to add a [lib] section. It tells the compiler where to find the source code for the library and how to generate the bytecode output. We also need to add a dependency of wasm-bindgen here. It is the utility wasm-pack uses to generate the JavaScript binding for the Rust WebAssembly program.

name = "hello_lib"
path = "src/lib.rs"
crate-type =["cdylib"]
wasm-bindgen = "0.2.50"

Below is the content of the Rust program src/lib.rs. You can actually define multiple external functions in this library file, and all of them will be available to the host JaveScript app via WebAssembly.

use wasm_bindgen::prelude::*;
pub fn say(s: String) -> String {
let r = String::from("hello ");
return r + &s;

Next, you can compile the Rust source code into WebAssembly bytecode and generate the accompanying JavaScript module for the nodejs host environment.

$ ssvmup build

The result are the following three files. the .wasm file is the WebAssembly bytecode program, and the .js files are for the JavaScript module.


The nodejs host application

Next, let’s create a node folder for the nodejs web application. Copy over the generated JavaScript module files.

$ mkdir node
$ cp pkg/hello_lib_bg.wasm node/
$ cp pkg/hello_lib_bg.js node/
$ cp pkg/hello_lib.js node/

With the generated hello_lib.js module, it is very easy to write JavaScript to call WebAssembly functions. Below is the node application app.js. It simply imports the say() function from the generated module. The node application takes the name parameter from incoming an HTTP GET request, and responds with “hello name”.

const { say } = require('./hello_lib.js');
const http = require('http');
const url = require('url');
const hostname = '';
const port = 8080;
const server = http.createServer((req, res) => {
const queryObject = url.parse(req.url,true).query;
res.statusCode = 200;
res.setHeader('Content-Type', 'text/plain');
server.listen(port, hostname, () => {
console.log(`Server running at http://${hostname}:${port}/`);

Start the nodejs application server as follows.

$ node app.js
Server running at

Then, you can test it.

$ curl
hello Wasm

What’s next?

Rust developers rejoice! Web services can now offload computationally intensive, unsafe, and novel hardware access tasks to WebAssembly. More examples and use cases to come. Stay tuned!