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.

References[edit]

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

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[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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