My internship: Brotli compression using a reduced dictionary

Brotli is a state of the art lossless compression format, supported by all major browsers. It is capable of achieving considerably better compression ratios than the ubiquitous gzip, and is rapidly gaining in popularity. Cloudflare uses the Google brotli library to dynamically compress web content whenever possible. In 2015, we took an in-depth look at how brotli works and its compression advantages.

One of the more interesting features of the brotli file format, in the context of textual web content compression, is the inclusion of a built-in static dictionary. The dictionary is quite large, and in addition to containing various strings in multiple languages, it also supports the option to apply multiple transformations to those words, increasing its versatility.

The open sourced brotli library, that implements an encoder and decoder for brotli, has 11 predefined quality levels for the encoder, with higher quality level demanding more CPU in exchange for a better compression ratio. The static dictionary feature is used to a limited extent starting with level 5, and to the full extent only at levels 10 and 11, due to the high CPU cost of this feature.

We improve on the limited dictionary use approach and add Continue reading