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Improved local development with wrangler and workerd, Developer Week

Improved local development with wrangler and workerd, Developer Week

For over a year now, we’ve been working to improve the Workers local development experience. Our goal has been to improve parity between users’ local and production environments. This is important because it provides developers with a fully-controllable and easy-to-debug local testing environment, which leads to increased developer efficiency and confidence.

To start, we integrated Miniflare, a fully-local simulator for Workers, directly into Wrangler, the Workers CLI. This allowed users to develop locally with Wrangler by running wrangler dev --local. Compared to the wrangler dev default, which relied on remote resources, this represented a significant step forward in local development. As good as it was, it couldn’t leverage the actual Workers runtime, which led to some inconsistencies and behavior mismatches.

Last November, we announced the experimental version of Miniflare v3, powered by the newly open-sourced workerd runtime, the same runtime used by Cloudflare Workers. Since then, we’ve continued to improve upon that experience both in terms of accuracy with the real runtime and in cross-platform compatibility.

As a result of all this work, we are proud to announce the release of Wrangler v3 – the first version of Wrangler with local-by-default development.

A new default for Wrangler

Starting with Wrangler v3, users running wrangler dev will be leveraging Miniflare v3 to run your Worker locally. This local development environment is effectively as accurate as a production Workers environment, providing an ability for you to test every aspect of your application before deploying. It provides the same runtime and bindings, but has its own simulators for KV, R2, D1, Cache and Queues. Because you’re running everything on your machine, you won’t be billed for operations on KV namespaces or R2 buckets during development, and you can try out paid-features like Durable Objects for free.

In addition to a more accurate developer experience, you should notice performance differences. Compared to remote mode, we’re seeing a 10x reduction to startup times and 60x reduction to script reload times with the new local-first implementation. This massive reduction in reload times drastically improves developer velocity!

Remote development isn’t going anywhere. We recognise many developers still prefer to test against real data, or want to test Cloudflare services like image resizing that aren’t implemented locally yet. To run wrangler dev on Cloudflare’s network, just like previous versions, use the new --remote flag.

Deprecating Miniflare v2

For users of Miniflare, there are two important pieces of information for those updating from v2 to v3. First, if you’ve been using Miniflare’s CLI directly, you’ll need to switch to wrangler dev. Miniflare v3 no longer includes a CLI. Secondly, if you’re using Miniflare’s API directly, upgrade to miniflare@3 and follow the migration guide (link tbd).

How we built Miniflare v3

Miniflare v3 is now built using workerd, the open-source Cloudflare Workers runtime. As workerd is a server-first runtime, every configuration defines at least one socket to listen on. Each socket is configured with a service, which can be an external server, disk directory or most importantly for us, a Worker! To start a workerd server running a Worker, create a worker.capnp file as shown below, run npx workerd serve worker.capnp and visit http://localhost:8080 in your browser:

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

const helloConfig :Workerd.Config = (
 services = [
   ( name = "hello-worker", worker = .helloWorker )
 sockets = [
   ( name = "hello-socket", address = "*:8080", http = (), service = "hello-worker" )

const helloWorker :Workerd.Worker = (
 modules = [
   ( name = "worker.mjs",
     esModule =
       `export default {
       `  async fetch(request, env, ctx) {
       `    return new Response("Hello from workerd! 👋");
       `  }
 compatibilityDate = "2023-04-04",

If you’re interested in what else workerd can do, check out the module for running tests in parallel. This module spawns new operating system threads running Node.js and provides a MessageChannel interface for communicating between them. What if instead of spawning a new OS thread, we spawned a new workerd process, and used WebSockets for communication between the Node.js host process and the workerd “thread”?

We have a proof of concept using Vitest showing this approach can work in practice. Existing Vitest IDE integrations and the Vitest UI continue to work without any additional work. We aren’t quite ready to release this yet, but will be working on improving it over the next few months. Importantly, the workerd “thread” needs access to Node.js built-in modules, which we recently started rolling out support for.

Running on every platform

We want developers to have this great local testing experience, regardless of which operating system they’re using. Before open-sourcing, the Cloudflare Workers runtime was originally only designed to run on Linux. For Miniflare v3, we needed to add support for macOS and Windows too. macOS and Linux are both Unix-based, making porting between them relatively straightforward. Windows on the other hand is an entirely different beast… 😬

The workerd runtime uses KJ, an alternative C++ base library, which is already cross-platform. We’d also migrated to the Bazel build system in preparation for open-sourcing the runtime, which has good Windows support. When compiling our C++ code for Windows, we use LLVM’s MSVC-compatible compiler driver clang-cl, as opposed to using Microsoft’s Visual C++ compiler directly. This enables us to use the “same” compiler frontend on Linux, macOS, and Windows, massively reducing the effort required to compile workerd on Windows. Notably, this provides proper support for #pragma once when using symlinked virtual includes produced by Bazel, __atomic_* functions, a standards-compliant preprocessor, GNU statement expressions used by some KJ macros, and understanding of the .c++ extension by default. After switching out unix API calls for their Windows equivalents using #if _WIN32 preprocessor directives, and fixing a bunch of segmentation faults caused by execution order differences, we were finally able to get workerd running on Windows! No WSL or Docker required! 🎉

Let us know what you think!

Wrangler v3 is now generally available! Upgrade by running npm install --save-dev wrangler@3 in your project. Then run npx wrangler dev to try out the new local development experience powered by Miniflare v3 and the open-source Workers runtime. Let us know what you think in the #wrangler channel on the Cloudflare Developers Discord, and please open a GitHub issue if you hit any unexpected behavior.

Source:: CloudFlare