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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_MATCH` constant in crate `actix_web`."><meta name="keywords" content="rust, rustlang, rust-lang, IF_MATCH"><title>actix_web::http::header::IF_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="../../../dark.css"><link rel="stylesheet" type="text/css" href="../../../light.css" id="themeStyle"><script src="../../../storage.js"></script></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><div class="sidebar-elems"><p class='location'><a href='../../index.html'>actix_web</a>::<wbr><a href='../index.html'>http</a>::<wbr><a href='index.html'>header</a></p><script>window.sidebarCurrent = {name: 'IF_MATCH', ty: 'constant', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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'>actix_web</a>::<wbr><a href='../index.html'>http</a>::<wbr><a href='index.html'>header</a>::<wbr><a class="constant" href=''>IF_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='https://docs.rs/http/0.1.7/src/http/header/name.rs.html#84-86' title='goto source code'>[src]</a></span></h1><pre class='rust const'>pub const IF_MATCH: <a class="struct" href="../../../actix_web/http/header/struct.HeaderName.html" title="struct actix_web::http::header::HeaderName">HeaderName</a><code> = </code><code>HeaderName{inner: Repr::Standard(StandardHeader::IfMatch),}</code></pre><div class='docblock'><p>Makes a request conditional based on the E-Tag.</p>
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<p>For GET and HEAD methods, the server will send back the requested
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resource only if it matches one of the listed ETags. For PUT and other
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non-safe methods, it will only upload the resource in this case.</p>
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<p>The comparison with the stored ETag uses the strong comparison
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algorithm, meaning two files are considered identical byte to byte only.
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This is weakened when the W/ prefix is used in front of the ETag.</p>
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<p>There are two common use cases:</p>
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<ul>
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<li>
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<p>For GET and HEAD methods, used in combination with an Range header, it
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can guarantee that the new ranges requested comes from the same resource
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than the previous one. If it doesn't match, then a 416 (Range Not
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Satisfiable) response is returned.</p>
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</li>
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<li>
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<p>For other methods, and in particular for PUT, If-Match can be used to
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prevent the lost update problem. It can check if the modification of a
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resource that the user wants to upload will not override another change
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that has been done since the original resource was fetched. If the
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request cannot be fulfilled, the 412 (Precondition Failed) response is
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returned.</p>
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</li>
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</ul>
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