# Compression Dictionary Transport — Compression dictionaries

> Algorithms like {{glossary("Brotli compression")}} and {{glossary("Zstandard compression")}} achieve even greater efficiency by allowing the use of dictionaries of commonly encountered strings, so you don't need any copies of them in the compressed resource.

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## Knowledge

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Algorithms like {{glossary("Brotli compression")}} and {{glossary("Zstandard compression")}} achieve even greater efficiency by allowing the use of dictionaries of commonly encountered strings, so you don't need any copies of them in the compressed resource. These algorithms ship with a predefined default dictionary that is used when compressing HTTP responses.

Compression Dictionary Transport builds on this by enabling you to provide your own dictionary which is especially applicable to a particular set of resources. The compression algorithm can then reference it as a source of bytes when compressing and decompressing the resource.

Assuming the references from the previous example are included in that common dictionary, this could be further reduced to this

The dictionary can either be a separate resource that is only required for Compression Dictionary Transport, or it can be a resource that the website needs anyway.

For example, suppose your website uses a JavaScript library. You would typically load a specific version of the library, and might include the version name in the name of the library, like . When the browser loads your page, it will fetch a copy of the library as a subresource.

If you then update to v2 of the library, most of the library's code will probably have stayed the same. So sites can greatly reduce the size of the download for my-library.v2.js by telling the browser to use my-library.v1.js as a compression dictionary for my-library.v2.js. Then all strings that are common between v1 and v2 don't need to be included in the download for v2, because the browser already has them. Most of the download size of my-library.v2.js is then just the delta between the two versions.

Compression Dictionary Transport can achieve an order of magnitude more compression than compression using a default built-in dictionary: see Compression dictionary transport examples for some real-life results.

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