# SDCH

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{{Short description|Dictionary-based compression algorithm}}
{{For|the Partido Socialdemocracia Chilena, also known by the initials SDCH|Chilean Social Democracy Party}}
{{More citations needed|date=July 2016}}
'''SDCH''' ('''Shared Dictionary Compression for HTTP'''), pronounced "[sandwich](/source/sandwich)", is a [data compression](/source/data_compression) algorithm created by [Google](/source/Google), based on [VCDIFF](/source/VCDIFF) (RFC 3284).

SDCH achieves its effectiveness by using pre-negotiated dictionaries to "warm up" its internal state prior to encoding or decoding. These may either be already stored locally, or uploaded from a source and then cached.

It was supported natively in [Google Chrome](/source/Google_Chrome), Chromium, and Android, as well as on Google websites.<ref>{{cite web|url=https://lists.w3.org/Archives/Public/ietf-http-wg/2008JulSep/att-0441/Shared_Dictionary_Compression_over_HTTP.pdf|title=A Proposal for Shared Dictionary Compression Over HTTP|publisher=Google|last1=Butler|first1=Jon|author2=Wei-Hsin Lee|last3=McQuade|first3=Bryan|last4=Mixter|first4=Kenneth}}</ref><ref>{{cite web|url=https://groups.google.com/forum/#!forum/SDCH|title=SDCH Mailing List|publisher=Google Groups}}</ref>

SDCH compression was removed from Google Chrome, and other Chromium products, in version 59 (2017-06-05).<ref>{{cite web|url=https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/nQl0ORHy7sw|title=Intent to Unship: SDCH|accessdate=2017-05-02}}</ref>

Due to the diffing results and the data being compressed with the same coding, SDCH dictionaries aged relatively quickly and compression density became quickly worse than with the usual non-dictionary compression such as [GZip](/source/GZip). This created extra effort in production to keep the dictionaries fresh and reduced its applicability. Modern dictionary coding such as Shared [Brotli](/source/Brotli) has a more effective solution for this that fixes the dictionary aging problem.

==See also==
*[HTTP compression](/source/HTTP_compression)
*[SPDY](/source/SPDY)

==References==
{{Reflist}}

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Category:Data compression

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Adapted from the Wikipedia article [SDCH](https://en.wikipedia.org/wiki/SDCH) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/SDCH?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.