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