Available-Dictionary: : =:
There's almost no added complexity since zstd already handles separate compression dictionaries quite well.
Brotli has a default dictionary with bits of HTML and scripts. This is built in into the decompressor, and not sent with the files.
The decompression dictionaries aren't magic. They're basically a prefix for decompressed files, so that a first occurrence of some pattern can be referenced from the dictionary instead of built from scratch. This helps only with the first occurrences of data near the start of the file, and for all the later repetitions the dictionary becomes irrelevant.
The dictionary needs to be downloaded too, and you're not going to have dictionaries all the way down, so you pay the cost of decompressing the data without a dictionary whether it's a dictionary + dictionary-using-file, or just the full file itself.
show significant gain of using dictionary over compressed w/o dictionary.
It seems like instead of sites reducing bloat, they will just shift the bloat to your hard-drive. Some of the examples said dictionary of 1MB which doesn't seem big, but could add up if everyone is doing this.
Analyze the most common responses of a website on their platform, build an efficient dictionary from that data, and then automatically inject a link to that site-specific dictionary so future responses are optimally compressed and save on bandwidth. All transparent to the customers and end users.
However, I'm sceptical about usefulness of multi-page shared dictionaries (where you construct one for a site or group of pages). They're a gamble that can backfire.
The extra dictionary needs to be downloaded, so it starts as an extra overhead. It's not enough for it to just match something. It has to beat regular (per-page) compression to be better than nothing, and it must be useful enough to repay its own cost before it even starts being a net positive. This basically means everything in the dictionary must be useful to a user, and has to be used more than once, otherwise it's just an unnecessary upfront slowdown.
Standard (per-page) compression is already very good at removing simple repetitive patterns, and Brotli even comes with a default built-in dictionary of random HTML-like fragments. This further narrows down usefulness of the shared dictionaries, because generic page-like content is enough to be an advantage. They need to contain more specific content to beat standard compression, but the more specific the dictionary is, the lesser the chance of it fitting what the user browses.
Previously servers would cache compressed versions of your static resources.
Whereas now they either have to compress on-the-fly or have a massive cache of not only your most recent static JavaScript blob, but also all past blobs and versions compressed using different combinations of them as a dictionary.
This could easily 10x resources needed for serving static html/CSS/js.
o11c•4h ago