SDCH

Dictionary-based compression algorithm

For the Partido Socialdemocracia Chilena, also known by the initials SDCH, see Chilean Social Democracy Party.

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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

[edit]

* HTTP compression
* SPDY

## References

[edit]

1. ^ Butler, Jon; Wei-Hsin Lee; McQuade, Bryan; Mixter, Kenneth. "A Proposal for Shared Dictionary Compression Over HTTP" (PDF). Google.
2. ^ "SDCH Mailing List". Google Groups.
3. ^ "Intent to Unship: SDCH". Retrieved 2017-05-02.

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