Rust

Cloudflare Workers provides support for Rust via the workers-rs crate ↗, which makes Runtime APIs and bindings to developer platform products, such as Workers KV, R2, and Queues, available directly from your Rust code.

By following this guide, you will learn how to build a Worker entirely in the Rust programming language.

Prerequisites

Before starting this guide, make sure you have:

• A recent version of Rust ↗
• npm ↗
• The Rust wasm32-unknown-unknown toolchain:

rustup target add wasm32-unknown-unknown

• And cargo-generate sub-command by running:

cargo install cargo-generate

1. Create a new project with Wrangler

Open a terminal window, and run the following command to generate a Worker project template in Rust:

cargo generate cloudflare/workers-rs

Your project will be created in a new directory that you named, in which you will find the following files and folders:

• Cargo.toml - The standard project configuration file for Rust's Cargo ↗ package manager. The template pre-populates some best-practice settings for building for Wasm on Workers.
• wrangler.toml - Wrangler configuration, pre-populated with a custom build command to invoke worker-build (Refer to Wrangler Bundling).
• src - Rust source directory, pre-populated with Hello World Worker.

2. Develop locally

After you have created your first Worker, run the wrangler dev command to start a local server for developing your Worker. This will allow you to test your Worker in development.

npx wrangler dev

If you have not used Wrangler before, it will try to open your web browser to login with your Cloudflare account.

Note

If you have issues with this step or you do not have access to a browser interface, refer to the wrangler login documentation for more information.

Go to http://localhost:8787 ↗ to review your Worker running. Any changes you make to your code will trigger a rebuild, and reloading the page will show you the up-to-date output of your Worker.

3. Write your Worker code

With your new project generated, write your Worker code. Find the entrypoint to your Worker in src/lib.rs:

use worker::*;

#[event(fetch)]
async fn main(req: Request, env: Env, ctx: Context) -> Result<Response> {
    Response::ok("Hello, World!")
}

Note

There is some counterintuitive behavior going on here:

1. workers-rs provides an event macro which expects a handler function signature identical to those seen in JavaScript Workers.
2. async is not generally supported by Wasm, but you are able to use async in a workers-rs project (refer to async).

Related runtime APIs

workers-rs provides a runtime API which closely matches Worker's JavaScript API, and enables integration with Worker's platform features. For detailed documentation of the API, refer to docs.rs/worker ↗.

event macro

This macro allows you to define entrypoints to your Worker. The event macro supports the following events:

• fetch - Invoked by an incoming HTTP request.
• scheduled - Invoked by Cron Triggers.
• queue - Invoked by incoming message batches from Queues (Requires queue feature in Cargo.toml, refer to the workers-rs GitHub repository and queues feature flag ↗).
• start - Invoked when the Worker is first launched (such as, to install panic hooks).

fetch parameters

The fetch handler provides three arguments which match the JavaScript API:

1. Request ↗

An object representing the incoming request. This includes methods for accessing headers, method, path, Cloudflare properties, and body (with support for asynchronous streaming and JSON deserialization with Serde ↗).

2. Env ↗

Provides access to Worker bindings.

• Secret ↗ - Secret value configured in Cloudflare dashboard or using wrangler secret put.
• Var ↗ - Environment variable defined in wrangler.toml.
• KvStore ↗ - Workers KV namespace binding.
• ObjectNamespace ↗ - Durable Object binding.
• Fetcher ↗ - Service binding to another Worker.
• Bucket ↗ - R2 Bucket binding.

3. Context ↗

Provides access to waitUntil (deferred asynchronous tasks) and passThroughOnException (fail open) functionality.

Response ↗

The fetch handler expects a Response ↗ return type, which includes support for streaming responses to the client asynchronously. This is also the return type of any subrequests made from your Worker. There are methods for accessing status code and headers, as well as streaming the body asynchronously or deserializing from JSON using Serde ↗.

Router

Implements convenient routing API ↗ to serve multiple paths from one Worker. Refer to the Router example in the worker-rs GitHub repository ↗.

4. Deploy your Worker project

With your project configured, you can now deploy your Worker, to a *.workers.dev subdomain, or a Custom Domain, if you have one configured. If you have not configured any subdomain or domain, Wrangler will prompt you during the deployment process to set one up.

npx wrangler deploy

Preview your Worker at <YOUR_WORKER>.<YOUR_SUBDOMAIN>.workers.dev.

Note

When pushing to your *.workers.dev subdomain for the first time, you may see 523 errors while DNS is propagating. These errors should resolve themselves after a minute or so.

After completing these steps, you will have a basic Rust-based Worker deployed. From here, you can add crate dependencies and write code in Rust to implement your Worker application. If you would like to know more about the inner workings of how Rust compiled to Wasm is supported by Workers, the next section outlines the libraries and tools involved.

How this deployment works

Wasm Workers are invoked from a JavaScript entrypoint script which is created automatically for you when using workers-rs.

JavaScript Plumbing (wasm-bindgen)

To access platform features such as bindings, Wasm Workers must be able to access methods from the JavaScript runtime API.

This interoperability is achieved using wasm-bindgen ↗, which provides the glue code needed to import runtime APIs to, and export event handlers from, the Wasm module. wasm-bindgen also provides js-sys ↗, which implements types for interacting with JavaScript objects. In practice, this is an implementation detail, as workers-rs's API handles conversion to and from JavaScript objects, and interaction with imported JavaScript runtime APIs for you.

Note

If you are using wasm-bindgen without workers-rs / worker-build, then you will need to patch the JavaScript that it emits. This is because when you import a wasm file in Workers, you get a WebAssembly.Module instead of a WebAssembly.Instance for performance and security reasons.

To patch the JavaScript that wasm-bindgen emits:

1. Run wasm-pack build --target bundler as you normally would.
2. Patch the JavaScript file that it produces (the following code block assumes the file is called mywasmlib.js):

import * as imports from "./mywasmlib_bg.js";

// switch between both syntax for node and for workerd
import wkmod from "./mywasmlib_bg.wasm";
import * as nodemod from "./mywasmlib_bg.wasm";
if (typeof process !== "undefined" && process.release.name === "node") {
  imports.__wbg_set_wasm(nodemod);
} else {
  const instance = new WebAssembly.Instance(wkmod, {
    "./mywasmlib_bg.js": imports,
  });
  imports.__wbg_set_wasm(instance.exports);
}

export * from "./mywasmlib_bg.js";

3. In your Worker entrypoint, import the function and use it directly:

import { myFunction } from "path/to/mylib.js";

Async (wasm-bindgen-futures)

wasm-bindgen-futures ↗ (part of the wasm-bindgen project) provides interoperability between Rust Futures and JavaScript Promises. workers-rs invokes the entire event handler function using spawn_local, meaning that you can program using async Rust, which is turned into a single JavaScript Promise and run on the JavaScript event loop. Calls to imported JavaScript runtime APIs are automatically converted to Rust Futures that can be invoked from async Rust functions.

Bundling (worker-build)

To run the resulting Wasm binary on Workers, workers-rs includes a build tool called worker-build ↗ which:

1. Creates a JavaScript entrypoint script that properly invokes the module using wasm-bindgen's JavaScript API.
2. Invokes web-pack to minify and bundle the JavaScript code.
3. Outputs a directory structure that Wrangler can use to bundle and deploy the final Worker.

worker-build is invoked by default in the template project using a custom build command specified in wrangler.toml.

Binary Size (wasm-opt)

Unoptimized Rust Wasm binaries can be large and may exceed Worker bundle size limits or experience long startup times. The template project pre-configures several useful size optimizations in your Cargo.toml file:

wrangler.toml

[profile.release]
lto = true
strip = true
codegen-units = 1

wrangler.json

{
  "profile": {
    "release": {
      "lto": true,
      "strip": true,
      "codegen-units": 1
    }
  }
}

Finally, worker-bundle automatically invokes wasm-opt ↗ to further optimize binary size before upload.

Related resources

• Rust Wasm Book ↗

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