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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="API documentation for the Rust `IF_NONE_MATCH` constant in crate `hyper`."><meta name="keywords" content="rust, rustlang, rust-lang, IF_NONE_MATCH"><title>hyper::header::IF_NONE_MATCH - Rust</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled ><script id="default-settings"></script><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc constant"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../hyper/index.html'><div class='logo-container rust-logo'><img src='../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class="location"><a href="../index.html">hyper</a>::<wbr><a href="index.html">header</a></p><div id="sidebar-vars" data-name="IF_NONE_MATCH" data-ty="constant" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" class="help-button">?</button>
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<a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="in-band">Constant <a href="../index.html">hyper</a>::<wbr><a href="index.html">header</a>::<wbr><a class="constant" href="">IF_NONE_MATCH</a></span><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../../src/http/header/name.rs.html#146-965" title="goto source code">[src]</a></span></h1><pre class="rust const">pub const IF_NONE_MATCH: <a class="struct" href="../../hyper/header/struct.HeaderName.html" title="struct hyper::header::HeaderName">HeaderName</a>;</pre><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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