Brotli Compression with mod_pagespeed and ngx_pagespeed

The PageSpeed modules (not to be confused with the PageSpeed Insights site analysis service), are open-source webserver modules that optimize your site automatically. Namely, there is mod_pagespeed for the Apache server and ngx_pagespeed for the Nginx server. For example, PageSpeed can automatically create WebP versions for all your image resources, and conditionally only serve the format to clients that accept image/webp. I use it on this very blog, inspect a request to any JPEG image and see how on supporting browsers it gets served as WebP.

Chrome DevTools showing a request for a JPEG image that gets served as WebP

The impact of Brotli compression

When it comes to compression, Brotli really makes a difference. Brotli compression is only supported over HTTPS and is requested by clients by including br in the accept-encoding header. In practice, Chrome sends accept-encoding: gzip, deflate, br. As an example for the positive impact compared to gzip, check out a recent case study shared by Addy Osmani in which the web team of the hotel company Treebo share their Tale of Brotli Compression.

PageSpeed does not support Brotli yet

While both webservers support Brotli compression out of the box, Apache via mod_brotli and Nginx via ngx_brotli, one thing that PageSpeed is currently missing is native Brotli support, causing resources that went through any PageSpeed optimization step to not be Brotli-encoded πŸ˜”. PageSpeed is really smart about compression in general, for instance, it always automatically enables mod_deflate for compression, optionally adds an accept-encoding: gzip header to requests that lack it, and automatically gzips compressable resources as they are stored in the cache, but Brotli support is just not there yet. The good news is that it is being worked on, GitHub Issue #1148 tracks the effort.

Making Brotli work with PageSpeed

The even better news is that while we are waiting for native Brotli support in PageSpeed, we can just outsource Brotli compression to the underlying webserver. To do so, simply disable PageSpeed's HTTPCache Compression. Quoting from the documentation:

To configure cache compression, set HttpCacheCompressionLevel to values between -1 and 9, with 0 being off, -1 being gzip's default compression, and 9 being maximum compression.

πŸ“’ So to make PageSpeed work with Brotli, what you want in your pagespeed.conf file is a new line:

# Disable PageSpeed's gzip compression, so the server's
# native Brotli compression kicks in via `mod_brotli`
# or `ngx_brotli`.
ModPagespeedHttpCacheCompressionLevel 0

One thing to have an eye on is server load. Brotli is more demanding than gzip, so for your static resources, you probably want to serve pre-compressed content wherever possible, and for the odd chance of when you are Facebook, maybe disable Brotli for your dynamic resources.

Chrome DevTools Network panel showing traffic for this blog with resources served Brotli-compressed highlighted

Happy Brotli serving, and, by the way, in case you ever wondered, Brotli is a πŸ‡¨πŸ‡­ Swiss German word for a bread roll and literally means "small bread".