31 lines
4.9 KiB
HTML
31 lines
4.9 KiB
HTML
<!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 `http`."><meta name="keywords" content="rust, rustlang, rust-lang, IF_NONE_MATCH"><title>http::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="../../dark.css"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="shortcut icon" href="../../favicon.ico"><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='../../http/index.html'><div class='logo-container'><img src='../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='../index.html'>http</a>::<wbr><a href='index.html'>header</a></p><script>window.sidebarCurrent = {name: 'IF_NONE_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"><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><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='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#83-85' title='goto source code'>[src]</a></span><span class='in-band'>Constant <a href='../index.html'>http</a>::<wbr><a href='index.html'>header</a>::<wbr><a class="constant" href=''>IF_NONE_MATCH</a></span></h1><pre class='rust const'>pub const IF_NONE_MATCH: <a class="struct" href="../../http/header/struct.HeaderName.html" title="struct http::header::HeaderName">HeaderName</a>;</pre><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></section><section id="search" class="content hidden"></section><section class="footer"></section><script>window.rootPath = "../../";window.currentCrate = "http";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html> |