Revision 1143391 of ETag

<div>{{HTTPSidebar}}</div>

<p>The <strong><code>ETag</code></strong> HTTP response header is an identifier for a specific version of a resource. It allows caches to be more efficient, and saves bandwidth, as a web server does not need to send a full response if the content has not changed. On the other side, if the content has changed, etags are useful to help prevent simultaneous updates of a resource from overwriting each other ("mid-air collisions").</p>

<p>If the resource at a given URL changes, a new <code>Etag</code> value must be generated. Etags are therefore similar to fingerprints and might also be used for tracking purposes by some servers. A comparison of them allows to quickly determine whether two representations of a resource are the same, but they might also be set to persist indefinitely by a tracking server.</p>

<table class="properties">
 <tbody>
  <tr>
   <th scope="row">Header type</th>
   <td>{{Glossary("Response header")}}</td>
  </tr>
  <tr>
   <th scope="row">{{Glossary("Forbidden header name")}}</th>
   <td>no</td>
  </tr>
 </tbody>
</table>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">
ETag: W/"&lt;etag_value&gt;"
ETag: "&lt;etag_value&gt;"
</pre>

<h2 id="Directives">Directives</h2>

<dl>
 <dt><code>W/</code> {{optional_inline}}</dt>
 <dd><code>'W/'</code> (case-sensitive) indicates that a <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Conditional_requests#Weak_validation">weak validator</a> is used. Weak validators are easy to generate but are far less useful for comparisons. Strong validators are ideal for comparisons but can be very difficult to generate efficiently. Weak <code>Etag</code> values of two representations of the same resources might be semantically equivalent, but not byte-for-byte identical.</dd>
 <dt>"&lt;etag_value&gt;"</dt>
 <dd>Entity tags uniquely representing the requested resources. They are a string of ASCII characters placed between double quotes (Like <code>"675af34563dc-tr34"</code>). The method by which <code>ETag</code> values are generated is not specified. Oftentimes, a hash of the content, a hash of the last modification timestamp, or just a revision number is used. For example, MDN uses a hash of hexadecimal digits of the wiki content.</dd>
</dl>

<h2 id="Examples">Examples</h2>

<pre>
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
ETag: W/"0815"</pre>

<h3 id="Avoiding_mid-air_collisions">Avoiding mid-air collisions</h3>

<p>With the help of the <code>ETag</code> and the {{HTTPHeader("If-Match")}} headers, you are able to detect mid-air edit collisions.</p>

<p>For example when editing MDN, the current wiki content is hashed and put into an <code>Etag</code> in the response:</p>

<pre>
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4</pre>

<p>When saving changes to a wiki page (posting data), the {{HTTPMethod("POST")}} request will contain the {{HTTPHeader("If-Match")}} header containing the <code>ETag</code> values to check freshness against.</p>

<pre>
If-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"</pre>

<p>If the hashes don't match, it means that the document has been edited in-between and a {{HTTPStatus("412")}}<code> Precondition Failed</code> error is thrown.</p>

<h3 id="Caching_of_unchanged_resources">Caching of unchanged resources</h3>

<p>Another typical use case of the <code>ETag</code> header is to cache resources that are unchanged. If a user visits a given URL again (that has an <code>ETag</code> set), and it is <em>stale</em>, that is too old to be considered usable, the client will send the value of its <code>ETag</code> along in an {{HTTPHeader("If-None-Match")}} header field:</p>

<pre>
If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"</pre>

<p>The server compares the client's <code>ETag</code> (sent with <code>If-None-Match</code>) with the <code>ETag</code> for its current version of the resource and if both values match (that is, the resource has not changed), the server send back a {{HTTPStatus("304")}}<code> Not Modified</code> status, without any body, which tells the client that the cached version of the response is still good to use (<em>fresh</em>).</p>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Title</th>
  </tr>
  <tr>
   <td>{{RFC("7232", "ETag", "2.3")}}</td>
   <td>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<p class="hidden">The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out <a href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</p>

<p>{{Compat("http/headers/etag")}}</p>

<h2 id="See_also">See also</h2>

<ul>
 <li>{{HTTPHeader("If-Match")}}</li>
 <li>{{HTTPHeader("If-None-Match")}}</li>
 <li>{{HTTPStatus("304")}}<code> Not Modified</code></li>
 <li>{{HTTPStatus("412")}}<code> Precondition Failed</code></li>
 <li>
  <p><a href="https://www.w3.org/1999/04/Editing/">W3C Note: Editing the Web – Detecting the Lost Update Problem Using Unreserved Checkout</a></p>
 </li>
</ul>