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JavaScript Cold starts

The performance of applications on the web platform is becoming increasingly bottlenecked by the startup (load) time. Large amounts of JavaScript code are required to create rich web experiences that we’ve become used to. When we look at the total size of JavaScript requested on mobile devices from HTTPArchive, we see that an average page loads 350KB of JavaScript, while 10% of pages go over the 1MB threshold. The rise of more complex applications can push these numbers even higher.

While caching helps, popular websites regularly release new code, which makes cold start (first load) times particularly important. With browsers moving to separate caches for different domains to prevent cross-site leaks, the importance of cold starts is growing even for popular subresources served from CDNs, as they can no longer be safely shared.

Usually, when talking about the cold start performance, the primary factor considered is a raw download speed. However, on modern interactive pages one of the other big contributors to cold starts is JavaScript parsing time. This might seem surprising at first, but makes sense - before starting to execute the code, the engine has to first parse the fetched JavaScript, make sure Continue reading

Building fast interpreters in Rust

In the previous post we described the Firewall Rules architecture and how the different components are integrated together. We also mentioned that we created a configurable Rust library for writing and executing Wireshark®-like filters in different parts of our stack written in Go, Lua, C, C++ and JavaScript Workers.

With a mixed set of requirements of performance, memory safety, low memory use, and the capability to be part of other products that we’re working on like Spectrum, Rust stood out as the strongest option.

We have now open-sourced this library under our Github account: https://github.com/cloudflare/wirefilter. This post will dive into its design, explain why we didn’t use a parser generator and how our execution engine balances security, runtime performance and compilation cost for the generated filters.

Parsing Wireshark syntax

When building a custom Domain Specific Language (DSL), the first thing we need to be able to do is parse it. This should result in an intermediate representation (usually called an Abstract Syntax Tree) that can be inspected, traversed, analysed and, potentially, serialised.

There are different ways to perform such conversion, such as:

1. Manual char-by-char parsing using state machines, regular expression and/or native string APIs.
2. Parser combinators, which use Continue reading

Improving request debugging in Cloudflare Workers

At Cloudflare, we are constantly looking into ways to improve development experience for Workers and make it the most convenient platform for writing serverless code.

As some of you might have already noticed either from our public release notes, on cloudflareworkers.com or in your Cloudflare Workers dashboard, there recently was a small but important change in the look of the inspector.

But before we go into figuring out what it is, let's take a look at our standard example on cloudflareworkers.com:

The example worker code featured here acts as a transparent proxy, while printing requests / responses to the console.

Commonly, when debugging Workers, all you could see from the client-side devtools is the interaction between your browser and the Cloudflare Worker runtime. However, like in most other server-side runtimes, the interaction between your code and the actual origin has been hidden.

This is where console.log comes in. Although not the most convenient, printing random things out is a fairly popular debugging technique.

Unfortunately, its default output doesn't help much with debugging network interactions. If you try to expand either of request or response objects, all you can see is just a bunch of lazy accessors:

Continue reading

Writing complex macros in Rust: Reverse Polish Notation

(This is a crosspost of a tutorial originally published on my personal blog)

Among other interesting features, Rust has a powerful macro system. Unfortunately, even after reading The Book and various tutorials, when it came to trying to implement a macro which involved processing complex lists of different elements, I still struggled to understand how it should be done, and it took some time till I got to that "ding" moment and started misusing macros for everything :) (ok, not everything as in the i-am-using-macros-because-i-dont-want-to-use-functions-and-specify-types-and-lifetimes everything like I've seen some people do, but anywhere it's actually useful)

CC BY 2.0 image by Conor Lawless

So, here is my take on describing the principles behind writing such macros. It assumes you have read the Macros section from The Book and are familiar with basic macros definitions and token types.

I'll take a Reverse Polish Notation as an example for this tutorial. It's interesting because it's simple enough, you might be already familiar with it from school, and yet to implement it statically at compile time, you already need to use a recursive macros approach.

Reverse Polish Notation (also called postfix notation) uses a stack for all its operations, Continue reading

How we brought HTTPS Everywhere to the cloud (part 1)

CloudFlare's mission is to make HTTPS accessible for all our customers. It provides security for their websites, improved ranking on search engines, better performance with HTTP/2, and access to browser features such as geolocation that are being deprecated for plaintext HTTP. With Universal SSL or similar features, a simple button click can now enable encryption for a website.

Unfortunately, as described in a previous blog post, this is only half of the problem. To make sure that a page is secure and can't be controlled or eavesdropped by third-parties, browsers must ensure that not only the page itself but also all its dependencies are loaded via secure channels. Page elements that don't fulfill this requirement are called mixed content and can either result in the entire page being reported as insecure or even completely blocked, thus breaking the page for the end user.

What can we do about it?

When we conceived the Automatic HTTPS Rewrites project, we aimed to automatically reduce the amount of mixed content on customers' web pages without breaking their websites and without any delay noticeable by end users while receiving a page that is being rewritten on the fly.

A naive way Continue reading