SDCH

SDCH (Shared Dictionary Compression for HTTP) is a data compression algorithm created by Google, based on 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, Chromium, and Android, as well as on Google websites.[1][2]

SDCH compression was removed from Google Chrome, and other Chromium products, in version 59 (2017-06-05).[3]

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. This created extra effort in production to keep the dictionaries fresh and reduced its applicability. Modern dictionary coding such as Shared Brotli has a more effective solution for this that fixes the dictionary aging problem.

## See also

* Brotli
* HTTP compression
* SPDY

## References

1. ↑ Butler, Jon; Wei-Hsin Lee; McQuade, Bryan; Mixter, Kenneth. "A Proposal for Shared Dictionary Compression Over HTTP". Google. http://lists.w3.org/Archives/Public/ietf-http-wg/2008JulSep/att-0441/Shared_Dictionary_Compression_over_HTTP.pdf.
2. ↑ "SDCH Mailing List". Google Groups. https://groups.google.com/forum/#!forum/SDCH.
3. ↑ "Intent to Unship: SDCH". https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/nQl0ORHy7sw. Retrieved 2017-05-02.

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