Commit ca2d8500 authored by Roy T. Fielding's avatar Roy T. Fielding
Browse files

Add documentation for mod_expires. 

Submitted by: Ken "Rodent of Unusual Size" Coar <coar@decus.org>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@77581 13f79535-47bb-0310-9956-ffa450edef68
parent 4cd6fdca

docs/manual/mod/mod_expires.html

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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">

<HTML>

 <HEAD>

  <TITLE>Apache module mod_expires</TITLE>

 </HEAD>

 <BODY>

  <!--#include virtual="header.html" -->

  <H1>Module mod_expires</H1>

  <P>

  This module is contained in the <CODE>mod_expires.c</CODE> file, and

  is <STRONG>not</STRONG> compiled in by default.  It provides for the

  generation of <CODE>Expires</CODE> headers according to user-specified

  criteria.

  </P>

  <H2>Summary</H2>

  <P>

  This module controls the setting of the <CODE>Expires</CODE> HTTP

  header in server responses.  The expiration date can set to be

  relative to either the time the source file was last modified, or to

  the time of the client access.

  </P>

  <P>

  The <CODE>Expires</CODE> HTTP header is an instruction to the client

  about the document's validity and persistence.  If cached, the document

  may be fetched from the cache rather than from the source until this

  time has passed.  After that, the cache copy is considered

  &quot;expired&quot; and invalid, and a new copy must be obtained from

  the source.

  </P>

  <H2>Directives</H2>

  <P>

  <MENU>

   <LI><A

        HREF="#expiresactive"

       >ExpiresActive</A>

   </LI>

   <LI><A

        HREF="#expiresbytype"

       >ExpiresByType</A>

   </LI>

   <LI><A

        HREF="#expiresdefault"

       >ExpiresDefault</A>

   </LI>

  </MENU>

  <HR>

  <A NAME="expiresactive">

   <H2>ExpiresActive directive</H2>

  </A>

  <!--%plaintext &lt;?INDEX {\tt ExpiresActive} directive&gt; -->

  <P>

  <STRONG>Syntax:</STRONG> ExpiresActive <EM>boolean</EM>

  <BR>

  <STRONG>Context:</STRONG> server config, virtual host, directory, .htaccess

  <br>

  <STRONG>Override:</STRONG> Indexes

  <br>

  <STRONG>Status:</STRONG> Extension

  <br>

  <STRONG>Module:</STRONG> mod_expires

  </P>

  <P>

  This directive enables or disables the generation of the

  <CODE>Expires</CODE> header for the document realm in question.  (That

  is, if found in an <CODE>.htaccess</CODE> file, for instance, it

  applies only to documents generated from that directory.)  If set to

  <EM><CODE>Off</CODE></EM>, no <CODE>Expires</CODE> header will be

  generated for any document in the realm (unless overridden at a lower

  level, such as an <CODE>.htaccess</CODE> file overriding a server

  config file).  If set to <EM><CODE>On</CODE></EM>, the header will be

  added to served documents according to the criteria defined by the

  <A

   HREF="expiresbytype"

  >ExpiresByType</A>

  and

  <A

   HREF="#expiresdefault"

  >ExpiresDefault</A>

  directives (<EM>q.v.</EM>).

  </P>

  <P>

  Note that this directive does not guarantee that an

  <CODE>Expires</CODE> header will be generated.  If the criteria aren't

  met, no header will be sent, and the effect will be as though this

  directive wasn't even specified.

  </P>

  <HR>

  <A NAME="expiresbytype">

   <H2>ExpiresByType directive</H2>

  </A>

  <!--%plaintext &lt;?INDEX {\tt ExpiresByType} directive&gt; -->

  <P>

  <STRONG>Syntax:</STRONG> ExpiresByType <EM>mime-type &lt;code&gt;seconds</EM>

  <BR>

  <STRONG>Context:</STRONG> server config, virtual host, directory, .htaccess

  <br>

  <STRONG>Override:</STRONG> Indexes

  <br>

  <STRONG>Status:</STRONG> Extension

  <br>

  <STRONG>Module:</STRONG> mod_expires

  </P>

  <P>

  This directive defines the value of the <CODE>Expires</CODE> header

  generated for documents of the specified type (<EM>e.g.</EM>,

  <CODE>text/html</CODE>).  The second argument sets the number of

  seconds that will be added to a base time to construct the expiration

  date.

  </P>

  <P>

  The base time is either the last modification time of the file, or the

  time of the client's access to the document.  Which should be used is

  specified by the <CODE><EM>&lt;code&gt;</EM></CODE> field;

  <STRONG>M</STRONG> means that the file's last modification time should

  be used as the base time, and <STRONG>A</STRONG> means the client's

  access time should be used.

  </P>

  <P>

  The difference in effect is subtle.  If <EM>M</EM> is used, all current

  copies of the document in all caches will expire at the same time,

  which can be good for something like a weekly notice that's always

  found at the same URL.  If <EM>A</EM> is used, the date of expiration

  is different for each client; this can be good for image files that

  don't change very often, particularly for a set of related documents

  that all refer to the same images (<EM>i.e.</EM>, the images will be

  accessed repeatedly within a relatively short timespan).

  </P>

  <P>

  <STRONG>Example:</STRONG>

  </P>

  <P>

  <PRE>

   ExpiresActive On                  # enable expirations

   ExpiresByType image/gif A2592000  # expire GIF images after a month

                                     #  in the client's cache

   ExpiresByType text/html M604800   # HTML documents are good for a

                                     #  week from the time they were

                                     #  changed, period

  </PRE>

  </P>

  <P>

  Note that this directive only has effect if <CODE>ExpiresActive

  On</CODE> has been specified.  It overrides, for the specified MIME

  type <EM>only</EM>, any expiration date set by the

  <A

   HREF="#expiresdefault"

  >ExpiresDefault</A>

  directive.

  </P>

  <HR>

  <A NAME="expiresdefault">

   <H2>ExpiresDefault directive</H2>

  </A>

  <!--%plaintext &lt;?INDEX {\tt ExpiresDefault} directive&gt; -->

  <P>

  <STRONG>Syntax:</STRONG> ExpiresDefault <EM>&lt;code&gt;seconds</EM>

  <BR>

  <STRONG>Context:</STRONG> server config, virtual host, directory, .htaccess

  <br>

  <STRONG>Override:</STRONG> Indexes

  <br>

  <STRONG>Status:</STRONG> Extension

  <br>

  <STRONG>Module:</STRONG> mod_expires

  </P>

  <P>

  This directive sets the default algorithm for calculating the

  expiration time for all documents in the affected realm.  It can be

  overridden on a type-by-type basis by the

  <A

   HREF="#expiresbytype"

  >ExpiresByType</A>

  directive.  See the description of that directive for details about

  the syntax of the argument.

  </P>

  <!--#include virtual="footer.html" -->

 </BODY>

</HTML>