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doc: Explain the nginx ETag patch

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https://github.com/NixOS/nixpkgs/issues/25485
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Chris Martin 2019-04-30 22:10:11 -06:00
parent 9d71cd0520
commit 1478feb7aa
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doc/package-notes.xml
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@ -918,4 +918,52 @@ citrix_receiver.override {
</para>
</section>
</section>
<section xml:id="sec-nginx">
<title>Nginx</title>
<para>
<link xlink:href="https://nginx.org/">Nginx</link> is a
reverse proxy and lightweight webserver.
</para>
<section xml:id="sec-nginx-etag">
<title>ETags on static files served from the Nix store</title>
<para>
HTTP has a couple different mechanisms for caching to prevent
clients from having to download the same content repeatedly
if a resource has not changed since the last time it was requested.
When nginx is used as a server for static files, it implements
the caching mechanism based on the
<link xlink:href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified"><literal>Last-Modified</literal></link>
response header automatically; unfortunately, it works by using
filesystem timestamps to determine the value of the
<literal>Last-Modified</literal> header. This doesn't give the
desired behavior when the file is in the Nix store, because all
file timestamps are set to 0 (for reasons related to build
reproducibility).
</para>
<para>
Fortunately, HTTP supports an alternative (and more effective)
caching mechanism: the
<link xlink:href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag"><literal>ETag</literal></link>
response header. The value of the <literal>ETag</literal> header
specifies some identifier for the particular content that the
server is sending (e.g. a hash). When a client makes a second
request for the same resource, it sends that value back in an
<literal>If-None-Match</literal> header. If the ETag value is
unchanged, then the server does not need to resend the content.
</para>
<para>
As of NixOS 19.09, the nginx package in Nixpkgs is patched such
that when nginx serves a file out of <filename>/nix/store</filename>,
the hash in the store path is used as the <literal>ETag</literal>
header in the HTTP response, thus providing proper caching functionality.
This happens automatically; you do not need to do modify any
configuration to get this behavior.
</para>
</section>
</section>
</chapter>