| <!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."><meta name="keywords" content="rust, rustlang, rust-lang, IF_NONE_MATCH"><title>IF_NONE_MATCH in http::header - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.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><a class="sidebar-logo" href="../../http/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../../http/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In http::header</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Constant <a href="../index.html">http</a>::<wbr><a href="index.html">header</a>::<wbr><a class="constant" href="#">IF_NONE_MATCH</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../../src/http/header/name.rs.html#156-987">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><pre class="rust const"><code>pub const IF_NONE_MATCH: <a class="struct" href="struct.HeaderName.html" title="struct http::header::HeaderName">HeaderName</a>;</code></pre></div><details class="rustdoc-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> |
| <p>The If-None-Match HTTP request header makes the request conditional. For |
| GET and HEAD methods, the server will send back the requested resource, |
| with a 200 status, only if it doesn’t have an ETag matching the given |
| ones. For other methods, the request will be processed only if the |
| eventually existing resource’s ETag doesn’t match any of the values |
| listed.</p> |
| <p>When the condition fails for GET and HEAD methods, then the server must |
| return HTTP status code 304 (Not Modified). For methods that apply |
| server-side changes, the status code 412 (Precondition Failed) is used. |
| Note that the server generating a 304 response MUST generate any of the |
| following header fields that would have been sent in a 200 (OK) response |
| to the same request: Cache-Control, Content-Location, Date, ETag, |
| Expires, and Vary.</p> |
| <p>The comparison with the stored ETag uses the weak comparison algorithm, |
| meaning two files are considered identical not only if they are |
| identical byte to byte, but if the content is equivalent. For example, |
| two pages that would differ only by the date of generation in the footer |
| would be considered as identical.</p> |
| <p>When used in combination with If-Modified-Since, it has precedence (if |
| the server supports it).</p> |
| <p>There are two common use cases:</p> |
| <ul> |
| <li>For <code>GET</code> and <code>HEAD</code> methods, to update a cached entity that has an associated ETag.</li> |
| <li>For other methods, and in particular for <code>PUT</code>, <code>If-None-Match</code> used with |
| the <code>*</code> value can be used to save a file not known to exist, |
| guaranteeing that another upload didn’t happen before, losing the data |
| of the previous put; this problems is the variation of the lost update |
| problem.</li> |
| </ul> |
| </div></details></section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="http" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
