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authorEelco Dolstra <eelco.dolstra@logicblox.com>2016-09-14T14·00+0200
committerEelco Dolstra <eelco.dolstra@logicblox.com>2016-09-14T14·36+0200
commit90ad02bf626b885a5dd8967894e2eafc953bdf92 (patch)
tree7af8764fe95e2093f9c99fc5827a2f9ffde8dd5d /doc/manual
parenta75d11a7e6984b3df15da9677fbd49ee8de7a9c3 (diff)
Enable HTTP/2 support
The binary cache store can now use HTTP/2 to do lookups. This is much
more efficient than HTTP/1.1 due to multiplexing: we can issue many
requests in parallel over a single TCP connection. Thus it's no longer
necessary to use a bunch of concurrent TCP connections (25 by
default).

For example, downloading 802 .narinfo files from
https://cache.nixos.org/, using a single TCP connection, takes 11.8s
with HTTP/1.1, but only 0.61s with HTTP/2.

This did require a fairly substantial rewrite of the Downloader class
to use the curl multi interface, because otherwise curl wouldn't be
able to do multiplexing for us. As a bonus, we get connection reuse
even with HTTP/1.1. All downloads are now handled by a single worker
thread. Clients call Downloader::enqueueDownload() to tell the worker
thread to start the download, getting a std::future to the result.
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