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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Makes a request conditional based on the E-Tag."><title>IF_NONE_MATCH in hyper::header - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="hyper" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc constant"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../hyper/index.html">hyper</a><span class="version">0.14.27</span></h2></div><div class="sidebar-elems"><h2><a href="index.html">In hyper::header</a></h2></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../../hyper/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Constant <a href="../index.html">hyper</a>::<wbr><a href="index.html">header</a>::<wbr><a class="constant" href="#">IF_NONE_MATCH</a><button id="copy-path" title="Copy item path to clipboard"><img src="../../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../../src/http/header/name.rs.html#156-987">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><code>pub const IF_NONE_MATCH: <a class="struct" href="struct.HeaderName.html" title="struct hyper::header::HeaderName">HeaderName</a>;</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Makes a request conditional based on the E-Tag.</p>
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<p>The If-None-Match HTTP request header makes the request conditional. For
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GET and HEAD methods, the server will send back the requested resource,
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with a 200 status, only if it doesn’t have an ETag matching the given
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ones. For other methods, the request will be processed only if the
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eventually existing resource’s ETag doesn’t match any of the values
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listed.</p>
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<p>When the condition fails for GET and HEAD methods, then the server must
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return HTTP status code 304 (Not Modified). For methods that apply
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server-side changes, the status code 412 (Precondition Failed) is used.
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Note that the server generating a 304 response MUST generate any of the
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following header fields that would have been sent in a 200 (OK) response
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to the same request: Cache-Control, Content-Location, Date, ETag,
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Expires, and Vary.</p>
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<p>The comparison with the stored ETag uses the weak comparison algorithm,
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meaning two files are considered identical not only if they are
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identical byte to byte, but if the content is equivalent. For example,
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two pages that would differ only by the date of generation in the footer
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would be considered as identical.</p>
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<p>When used in combination with If-Modified-Since, it has precedence (if
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the server supports it).</p>
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<p>There are two common use cases:</p>
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<ul>
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<li>For <code>GET</code> and <code>HEAD</code> methods, to update a cached entity that has an associated ETag.</li>
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<li>For other methods, and in particular for <code>PUT</code>, <code>If-None-Match</code> used with
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the <code>*</code> value can be used to save a file not known to exist,
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guaranteeing that another upload didn’t happen before, losing the data
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of the previous put; this problems is the variation of the lost update
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problem.</li>
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</ul>
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</div></details></section></div></main></body></html> |