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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_MATCH in actix_web::http::header - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../../static.files/rustdoc-dd39b87e5fcfba68.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="actix_web" data-themes="" data-resource-suffix="" data-rustdoc-version="1.80.0-nightly (bdbbb6c6a 2024-05-26)" data-channel="nightly" data-search-js="search-d52510db62a78183.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../../static.files/storage-118b08c4c78b968e.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../../static.files/main-20a3ad099b048cf2.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-df360f571f6edeae.css"></noscript><link rel="icon" href="https://actix.rs/favicon.ico"></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" title="show sidebar"></button><a class="logo-container" href="../../../actix_web/index.html"><img src="https://actix.rs/img/logo.png" alt=""></a></nav><nav class="sidebar"><div class="sidebar-crate"><a class="logo-container" href="../../../actix_web/index.html"><img src="https://actix.rs/img/logo.png" alt="logo"></a><h2><a href="../../../actix_web/index.html">actix_web</a><span class="version">4.6.0</span></h2></div><div class="sidebar-elems"><h2><a href="index.html">In actix_web::http::header</a></h2></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1>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><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="https://docs.rs/http/0.2.11/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_MATCH: <a class="struct" href="struct.HeaderName.html" title="struct actix_web::http::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>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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</div></details></section></div></main></body></html>
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