Subscribe to receive notifications of new posts:

Introducing workerd: the Open Source Workers runtime


10 min read
Introducing workerd: the Open Source Workers runtime

Today I'm proud to introduce the first beta release of workerd, the JavaScript/Wasm runtime based on the same code that powers Cloudflare Workers. workerd is Open Source under the Apache License version 2.0.

workerd shares most of its code with the runtime that powers Cloudflare Workers, but with some changes designed to make it more portable to other environments. The name "workerd" (pronounced "worker dee") comes from the Unix tradition of naming servers with a "-d" suffix standing for "daemon". The name is not capitalized because it is a program name, which are traditionally lower-case in Unix-like environments.

What it's for

Self-hosting Workers

workerd can be used to self-host applications that you'd otherwise run on Cloudflare Workers. It is intended to be a production-ready web server for this purpose. workerd has been designed to be unopinionated about hosting environments, so that it should fit nicely into whatever server/VM/container hosting and orchestration system you prefer. It's just a web server.

Workers has always been based on standardized APIs, so that code is not locked into Cloudflare, and we work closely with other runtimes to promote compatibility. workerd provides another option to ensure that applications built on Workers can run anywhere, by leveraging the same underlying code to get exact, "bug-for-bug" compatibility.

Local development and testing

workerd is also designed to facilitate realistic local testing of Workers. Up until now, this has been achieved using Miniflare, which simulated the Workers API within a Node.js environment. Miniflare has worked well, but in a number of cases its behavior did not exactly match Workers running on Cloudflare. With the release of workerd, Miniflare and the Wrangler CLI tool will now be able to provide a more accurate simulation by leveraging the same runtime code we use in production.

Programmable proxies

workerd can act as an application host, a proxy, or both. It supports both forward and reverse proxy modes. In all cases, JavaScript code can be used to intercept and process requests and responses before forwarding them on. Traditional web servers and proxies have used bespoke configuration languages with quirks that are hard to master. Programming proxies in JavaScript instead provides more power while making the configuration easier to write and understand.

What it is

workerd is not just another way to run JavaScript and Wasm. Our runtime is uniquely designed in a number of ways.


Many non-browser JavaScript and Wasm runtimes are designed to be general-purpose: you can use them to build command-line apps, local GUI apps, servers, or anything in between. workerd is not. It specifically focuses on servers, in particular (for now, at least) HTTP servers.

This means in particular that workerd-based applications are event-driven at the top level. Applications do not open listen sockets and accept connections from them; instead, the runtime pushes events to the application. It may seem like a minor difference, but this basic change in perspective directly enables many of the features below.

Web standard APIs

Wherever possible, Workers (and workerd in particular) offers the same standard APIs found in web browsers, such as Fetch, URL, WebCrypto, and others. This means that code built on workerd is more likely to be portable to browsers as well as to other standards-based runtimes. When Workers launched five years ago, it was unusual for a non-browser to offer web APIs, but we are pleased to see that the broader JavaScript ecosystem is now converging on them.


workerd is a nanoservice runtime. What does that mean?

Microservices have become popular over the last decade as a way to split monolithic servers into smaller components that could be maintained and deployed independently. For example, a company that offers several web applications with a common user authentication flow might have a separate team that maintains the authentication logic. In a monolithic model, the authentication logic might have been offered to the application teams as a library. However, this could be frustrating for the maintainers of that logic, as making any change might require waiting for every application team to deploy an update to their respective server. By splitting the authentication logic into a separate server that all the others talk to, the authentication team is able to deploy changes on their own schedule.

However, microservices have a cost. What was previously a fast library call instead now requires communicating over a network. In addition to added overhead, this communication requires configuration and administration to ensure security and reliability. These costs become greater as the codebase is split into more and more services. Eventually, the costs outweigh the benefits.

Nanoservices are a new model that achieve the benefits of independent deployment with overhead closer to that of library calls. With workerd, many Workers can be configured to run in the same process. Each Worker runs in a separate "isolate", which gives the appearance of running independently of the others: each isolate loads separate code and has its own global scope. However, when one Worker explicitly sends a request to another Worker, the destination Worker actually runs in the same thread with zero latency. So, it performs more like a function call.

With nanoservices, teams can now break their code into many more independently-deployed pieces without worrying about the overhead.

(Some in the industry prefer to call nanoservices "functions", implying that each individual function making up an application could be its own service. I feel, however, that this puts too much emphasis on syntax rather than logical functionality. That said, it is the same concept.)

To really make nanoservices work well, we had to minimize the baseline overhead of each service. This required designing workerd very differently from most other runtimes, so that common resources could be shared between services as much as possible. First, as mentioned, we run many nanoservices within a single process, to share basic process overhead and minimize context switching costs. A second big architectural difference between workerd and other runtimes is how it handles built-in APIs. Many runtimes implement significant portions of their built-in APIs in JavaScript, which must then be loaded separately into each isolate. workerd does not; all the APIs are implemented in native code, so that all isolates may share the same copy of that code. These design choices would be difficult to retrofit into another runtime, and indeed these needs are exactly why we chose to build a custom runtime for Workers from the start.

Homogeneous deployment

In a typical microservices model, you might deploy different microservices to containers running across a cluster of machines, connected over a local network. You might manually choose how many containers to dedicate to each service, or you might configure some form of auto-scaling based on resource usage.

workerd offers an alternative model: Every machine runs every service.

workerd's nanoservices are much lighter-weight than typical containers. As a result, it's entirely reasonable to run a very large number of them – hundreds, maybe thousands – on a single server. This in turn means that you can simply deploy every service to every machine in your fleet.

Homogeneous deployment means that you don't have to worry about scaling individual services. Instead, you can simply load balance requests across the entire cluster, and scale the cluster as needed. Overall, this can greatly reduce the amount of administration work needed.

Cloudflare itself has used the homogeneous model on our network since the beginning. Every one of Cloudflare's edge servers runs our entire software stack, so any server can answer any kind of request on its own. We've found it works incredibly well. This is why services on Cloudflare – including ones that use Workers – are able to go from no traffic at all to millions of requests per second instantly without trouble.

Capability bindings: cleaner configuration and SSRF safety

workerd takes a different approach to most runtimes – indeed, to most software development platforms – in how an application accesses external resources.

Most development platforms start from assuming that the application can talk to the whole world. It is up to the application to figure out exactly what it wants to talk to, and name it in some global namespace, such as using a URL. So, an application server that wants to talk to the authentication microservice might use code like this:

// Traditional approach without capability bindings.
fetch("", {
  method: "POST",
  body: JSON.stringify(authRequest),
  headers: { "Authorization": env.AUTH_SERVICE_TOKEN }

In workerd, we do things differently. An application starts out with no ability to talk to the rest of the world, and must be configured with specific capability bindings that provide it access to specific external resources. So, an application which needs to be able to talk to the authentication service would be configured with a binding called authService, and the code would look something like this:

// Capability-based approach. Hostname doesn't matter; all
// requests to AUTH_SERVICE.fetch() go to the auth service.
env.AUTH_SERVICE.fetch("https://auth/api", {
 method: "POST",
 body: JSON.stringify(authRequest),

This may at first appear to be a trivial difference. In both cases, we have to use configuration to control access to external services. In the traditional approach, we'd provide access tokens (and probably the service's hostname) as environment variables. In the new approach, the environment goes a bit further to provide a full-fledged object. Is this just syntax sugar?

It turns out, this slight change has huge advantages:

First, we can now restrict the global fetch() function to accept only publicly-routable URLs. This makes applications totally immune to SSRF attacks! You cannot trick an application into accessing an internal service unintentionally if the code to access internal services is explicitly different. (In fact, the global fetch() is itself backed by a binding, which can be configured. workerd defaults to connecting it to the public internet, but you can also override it to permit private addresses if you want, or to route to a specific proxy service, or to be blocked entirely.)

With that done, we now have an interesting property: All internal services which an application uses must be configurable. This means:

  • You can easily see a complete list of the internal services an application talks to, without reading all the code.
  • You can always replace these services with mocks for testing purposes.
  • You can always configure an application to authenticate itself differently (e.g. client certificates) or use a different back end, without changing code.

The receiving end of a binding benefits, too. Take the authentication service example, above. The auth service may be another Worker running in workerd as a nanoservice. In this case, the auth service does not need to be bound to any actual network address. Instead, it may be made available strictly to other Workers through their bindings. In this case, the authentication service doesn't necessarily need to verify that a request received came from an allowed client – because only allowed clients are able to send requests to it in the first place.

Overall, capability bindings allow simpler code that is secure by default, more composable, easier to test, and easier to understand and maintain.

Always backwards compatible

Cloudflare Workers has a hard rule against ever breaking a live Worker running in production. This same dedication to backwards compatibility extends to workerd.

workerd shares Workers' compatibility date system to manage breaking changes. Every Worker must be configured with a "compatibility date". The runtime then ensures that the API behaves exactly as it did on that date. At your leisure, you may check the documentation to see if new breaking changes are introduced at a future date, and update your code for them. Most such changes are minor and most code won't require any changes. However, you are never obliged to update. Old dates will continue to be supported by newer versions of workerd. It is always safe to update workerd itself without updating your code.

What it's not

To avoid misleading or disappointing anyone, I need to take a moment to call out what workerd is not.

workerd is not a Secure Sandbox

It's important to note that workerd is not, on its own, a secure way to run possibly-malicious code. If you wish to run code you don't trust using workerd, you must enclose it in an additional sandboxing layer, such as a virtual machine configured for sandboxing.

workerd itself is designed such that a Worker should not be able to access any external resources to which it hasn't been granted a capability. However, a complete sandbox solution not only must be designed to restrict access, but also must account for the possibility of bugs – both in software and in hardware. workerd on its own is not sufficient to protect against hardware bugs like Spectre, nor can it adequately defend against the possibility of vulnerabilities in V8 or in workerd's own code.

The Cloudflare Workers service uses the same code found in workerd, but adds many additional layers of security on top to harden against such bugs. I described some of these in a past blog post. However, these measures are closely tied to our particular environment. For example, we rely on build automation to push V8 patches to production immediately upon becoming available; we separate customers according to risk profile; we rely on non-portable kernel features and assumptions about the host system to enforce security and resource limits. All of this is very specific to our environment, and cannot be packaged up in a reusable way.

workerd is not an independent project

workerd is the core of Cloudflare Workers, a fast-moving project developed by a dedicated team at Cloudflare. We are not throwing code over the wall to forget about, nor are we expecting volunteers to do our jobs for us. workerd's GitHub repository will be the canonical source used by Cloudflare Workers and our team will be doing much of their work directly in this repository. Just like V8 is developed primarily by the Chrome team for use in Chrome, workerd will be developed primarily by the Cloudflare Workers team for use in Cloudflare Workers.

This means we cannot promise that external contributions will sit on a level playing field with internal ones. Code reviews take time, and work that is needed for Cloudflare Workers will take priority. We also cannot promise we will accept every feature contribution. Even if the code is already written, reviews and maintenance have a cost. Within Cloudflare, we have a product management team who carefully evaluates what features we should and shouldn't offer, and plenty of ideas generated internally ultimately don't make the cut.

If you want to contribute a big new feature to workerd, your best bet is to talk to us before you write code, by raising an issue on GitHub early to get input. That way, you can find out if we're likely to accept a PR before you write it. We also might be able to give hints on how best to implement.

It's also important to note that while workerd's internal interfaces may sometimes appear clean and reusable, we cannot make any guarantee that those interfaces won't completely change on a whim. If you are trying to build on top of workerd internals, you will need to be prepared either to accept a fair amount of churn, or pin to a specific version.

workerd is not an off-the-shelf edge compute platform

As hinted above, the full Cloudflare Workers service involves a lot of technology beyond workerd itself, including additional security, deployment mechanisms, orchestration, and so much more. workerd itself is a portion of our runtime codebase, which is itself a small (albeit critical) piece of the overall Cloudflare Workers service.

We are pleased, though, that this means it is possible for us to release this code under a permissive Open Source license.

Try the Beta

As of this blog post, workerd is in beta. If you want to try it out,

We protect entire corporate networks, help customers build Internet-scale applications efficiently, accelerate any website or Internet application, ward off DDoS attacks, keep hackers at bay, and can help you on your journey to Zero Trust.

Visit from any device to get started with our free app that makes your Internet faster and safer.

To learn more about our mission to help build a better Internet, start here. If you're looking for a new career direction, check out our open positions.
Birthday WeekCloudflare WorkersDevelopersServerlessProduct NewsOpen SourceDeveloper Platform

Follow on X

Kenton Varda|@kentonvarda

Related posts