workerd (pronounced: "worker-dee") is a JavaScript / Wasm server runtime based on the same code that powers Cloudflare Workers.

You might use it:

• As an application server, to self-host applications designed for Cloudflare Workers.
• As a development tool, to develop and test such code locally.
• As a programmable HTTP proxy (forward or reverse), to efficiently intercept, modify, and route network requests.

• Server-first: Designed for servers, not CLIs nor GUIs.

• Standard-based: Built-in APIs are based on web platform standards, such as fetch().

• Nanoservices: Split your application into components that are decoupled and independently-deployable like microservices, but with performance of a local function call. When one nanoservice calls another, the callee runs in the same thread and process.

• Homogeneous deployment: Instead of deploying different microservices to different machines in your cluster, deploy all your nanoservices to every machine in the cluster, making load balancing much easier.

• Capability bindings: workerd configuration uses capabilities instead of global namespaces to connect nanoservices to each other and external resources. The result is code that is more composable -- and immune to SSRF attacks.

• Always backwards compatible: Updating workerd to a newer version will never break your JavaScript code. workerd's version number is simply a date, corresponding to the maximum "compatibility date" supported by that version. You can always configure your worker to a past date, and workerd will emulate the API as it existed on that date.

Read the blog post to learn more about these principles.

workerd tries to isolate each Worker so that it can only access the resources it is configured to access. However, workerd on its own does not contain suitable defense-in-depth against the possibility of implementation bugs. When using workerd to run possibly-malicious code, you must run it inside an appropriate secure sandbox, such as a virtual machine. The Cloudflare Workers hosting service in particular uses many additional layers of defense-in-depth.

With that said, if you discover a bug that allows malicious code to break out of workerd, please submit it to Cloudflare's bug bounty program for a reward.

In theory, workerd should work on any POSIX system that is supported by V8 and Windows.

In practice, workerd is tested on:

• Linux and macOS (x86-64 and arm64 architectures)
• Windows (x86-64 architecture)

On other platforms, you may have to do tinkering to make things work.

To build workerd, you need:

• Bazel
  • If you use Bazelisk (recommended), it will automatically download and use the right version of Bazel for building workerd.
• On Linux:

  • We use the clang/LLVM toolchain to build workerd and support version 19 and higher. Earlier versions of clang may still work, but are not officially supported.

  • Clang 19+ (e.g. package clang-19 on Debian Trixie). If clang is installed as clang-<version> please create a symlink to it in your PATH named clang, or use --action_env=CC=clang-<version> on bazel command lines to specify the compiler name.

  • libc++ 19+ (e.g. packages libc++-19-dev and libc++abi-19-dev)

  • LLD 19+ (e.g. package lld-19).

  • python3, python3-distutils, and tcl8.6

• On macOS:
  • Xcode 16.3 installation (available on macOS 15 and higher). Building with just the Xcode Command Line Tools is not being tested, but should work too.
  • Homebrew installed tcl-tk package (provides Tcl 8.6)
• On Windows:
  • Install App Installer from the Microsoft Store for the winget package manager and then run install-deps.bat from an administrator prompt to install bazelisk, LLVM, and other dependencies required to build workerd on Windows.
  • Add startup --output_user_root=C:/tmp to the .bazelrc file in your user directory.
  • When developing at the command-line, run bazel-env.bat in your shell first to select tools and Windows SDK versions before running bazel.

You may then build workerd at the command-line with:

bazel build //src/workerd/server:workerd

You can pass --config=release to compile in release mode:

bazel build //src/workerd/server:workerd --config=release

You can also build from within Visual Studio Code using the instructions in docs/vscode.md.

The compiled binary will be located at bazel-bin/src/workerd/server/workerd.

If you run a Bazel build before you've installed some dependencies (like clang or libc++), and then you install the dependencies, you must resync locally cached toolchains, or clean Bazel's cache, otherwise you might get strange errors:

bazel fetch --configure --force

If that fails, you can try:

bazel clean --expunge

The cache will now be cleaned and you can try building again.

If you have a fairly recent clang packages installed you can build a more performant release version of workerd:

bazel build --config=thin-lto //src/workerd/server:workerd

workerd is configured using a config file written in Cap'n Proto text format.

A simple "Hello World!" config file might look like:

using Workerd = import "/workerd/workerd.capnp";

const config :Workerd.Config = (
  services = [
    (name = "main", worker = .mainWorker),
  ],

  sockets = [
    # Serve HTTP on port 8080.
    ( name = "http",
      address = "*:8080",
      http = (),
      service = "main"
    ),
  ]
);

const mainWorker :Workerd.Worker = (
  serviceWorkerScript = embed "hello.js",
  compatibilityDate = "2023-02-28",
  # Learn more about compatibility dates at:
  # https://developers.cloudflare.com/workers/platform/compatibility-dates/
);

Where hello.js contains:

addEventListener("fetch", event => {
  event.respondWith(new Response("Hello World"));
});

Complete reference documentation is provided by the comments in workerd.capnp.

There is also a library of sample config files.

To serve your config, do:

workerd serve my-config.capnp

For more details about command-line usage, use workerd --help.

Prebuilt binaries are distributed via npm. Run npx workerd ... to use these. If you're running a prebuilt binary, you'll need to make sure your system has the right dependencies installed:

• On Linux:
  • glibc 2.35 or higher (already included on e.g. Ubuntu 22.04, Debian Bookworm)
• On macOS:
  • macOS 13.5 or higher
  • The Xcode command line tools, which can be installed with xcode-select --install
• x86_64 CPU with at least SSE4.2 and CLMUL ISA extensions, or arm64 CPU with CRC extension (enabled by default under armv8.1-a). These extensions are supported by all recent x86 and arm64 CPUs.

You can use Wrangler (v3.0 or greater) to develop Cloudflare Workers locally, using workerd. First, run the following command to configure Miniflare to use this build of workerd.

export MINIFLARE_WORKERD_PATH="<WORKERD_REPO_DIR>/bazel-bin/src/workerd/server/workerd"

Then, run:

wrangler dev

workerd is designed to be unopinionated about how it runs.

One good way to manage workerd in production is using systemd. Particularly useful is systemd's ability to open privileged sockets on workerd's behalf while running the service itself under an unprivileged user account. To help with this, workerd supports inheriting sockets from the parent process using the --socket-fd flag.

Here's an example system service file, assuming your config defines two sockets named http and https:

# /etc/systemd/system/workerd.service
[Unit]
Description=workerd runtime
After=local-fs.target remote-fs.target network-online.target
Requires=local-fs.target remote-fs.target workerd.socket
Wants=network-online.target

[Service]
Type=exec
ExecStart=/usr/bin/workerd serve /etc/workerd/config.capnp --socket-fd http=3 --socket-fd https=4
Sockets=workerd.socket

# If workerd crashes, restart it.
Restart=always

# Run under an unprivileged user account.
User=nobody
Group=nogroup

# Hardening measure: Do not allow workerd to run suid-root programs.
NoNewPrivileges=true

[Install]
WantedBy=multi-user.target

And corresponding sockets file:

# /etc/systemd/system/workerd.socket
[Unit]
Description=sockets for workerd
PartOf=workerd.service

[Socket]
ListenStream=0.0.0.0:80
ListenStream=0.0.0.0:443

[Install]
WantedBy=sockets.target

Once these files are in place you can enable the service -- see the systemd documentation or ask your favorite LLM for details.