A History of HTML Parsing at Cloudflare: Part 1
To coincide with the launch of streaming HTML rewriting functionality for Cloudflare Workers we are open sourcing the Rust HTML rewriter ( LOL HTML) used to back the Workers HTMLRewriter API. We also thought it was about time to review the history of HTML rewriting at Cloudflare.
The first blog post will explain the basics of a streaming HTML rewriter and our particular requirements. We start around 8 years ago by describing the group of ‘ad-hoc’ parsers that were created with specific functionality such as to rewrite e-mail addresses or minify HTML. By 2016 the state machine defined in the HTML5 specification could be used to build a single spec-compliant HTML pluggable rewriter, to replace the existing collection of parsers. The source code for this rewriter is now public and available here: https://github.com/cloudflare/lazyhtml.
The second blog post will describe the next iteration of rewriter. With the launch of the edge compute platform Cloudflare Workers we came to realise that developers wanted the same HTML rewriting capabilities with a JavaScript API. The post describes the thoughts behind a low latency streaming HTML rewriter with a CSS-selector based API. We open-sourced the Rust library as it can also be used as a stand-alone HTML rewriting/parsing library.
What is a streaming HTML rewriter ?
A streaming HTML rewriter takes either a HTML string or byte stream input, parses it into tokens or any other structured intermediate representation (IR) — such as an Abstract Syntax Tree (AST). It then performs transformations on the tokens before converting back to HTML. This provides the ability to modify, extract or add to an existing HTML document as the bytes are being processed. Compare this with a standard HTML tree parser which needs to retrieve the entire file to generate a full DOM tree. The tree-based rewriter will both take longer to deliver the first processed bytes and require significantly more memory.
For example; consider you own a large site with a lot of historical content that you want to now serve over HTTPS. You will quickly run into the problem of resources (images, scripts, videos) being served over HTTP. This ‘mixed content’ opens a security hole and browsers will warn or block these resources. It can be difficult or even impossible to update every link on every page of a website. With a streaming HTML rewriter you can select the URI attribute of any HTML tag and change any HTTP links to HTTPS. We built this very feature Automatic HTTPS rewrites back in 2016 to solve mixed content issues for our customers.
The reader may already be wondering: “Isn’t this a solved problem, aren’t there many widely used open-source browsers out there with HTML parsers that can be used for this purpose?”. The reality is that writing code to run in 190+ PoPs around the world with a strict low latency requirement turns even seemingly trivial problems into complex engineering challenges.
The following blog posts will detail the journey of how starting with a simple idea of finding email addresses within an HTML page led to building an almost spec compliant HTML parser and then on to a CSS selector matching Virtual Machine. We learned a lot on this journey. I hope you find some of this as interesting as we did.
Rewriting at the edge
When rewriting content through Cloudflare we do not want to impact site performance. The balance in designing a streaming HTML rewriter is to minimise the pause in response byte flow by holding onto as little information as possible whilst retaining the ability to rewrite matching tokens.
The difference in requirements compared to an HTML parser used in a browser include:
Output latency
For browsers, the Document Object Model (DOM) is the end product of the parsing process but in our case we have to parse, rewrite and serialize back to HTML. In the case of Cloudflare’s reverse proxy any content processing on the edge server results in latency between the server and an eyeball. It is desirable to minimize the latency impact of HTML handling, which involves parsing, rewriting and serializing back to HTML. In all of these stages we want to be as fast as possible to minimize latency.
Parser throughput
Let’s assume that usually browsers rarely need to deal with HTML pages bigger than 1Mb in size and an average page load time is somewhere around 3s at best. HTML parsing is not the main bottleneck of the page loading process as the browser will be blocked on running scripts and loading other render-critical resources. We can roughly estimate that ~3Mbps is an acceptable throughput for browser’s HTML parser. At Cloudflare we have hundreds of megabytes of traffic per CPU, so we need a parser that is faster by an order of magnitude.
Find the rest of this post, as well as part 2, over on the Cloudflare blog.
Originally published at https://blog.cloudflare.com on November 28, 2019.
