mod_expires

Overview

mod_expires module is responsible for setting of the Expires HTTP header and max-age directive of Cache-Control HTTP header in server responses. The expiration date can be set to relative to the time the source file was last modified or to the time of the client access.

Above HTTP headers inform the user about the document's validity and persistence. If cached, the document may be taken from cache rather than from source until this time has expired. After that the copy in cache is considered "expired" and therefore invalid, and a new copy must be received from the source.

To modify Cache-Control directives other than max-age you may use the Header directive.

Quick start

# enable expiration header
ExpiresActive On
# html expires in a 2 hour
ExpiresByType text/html A7200
# css expires in a 4 hour
ExpiresByType text/css A14400

<Files *.php>
  ExpiresActive On
  ExpiresByType text/html A0
</Files>

ExpiresActive On
# javascript expires in a 30 minutes
ExpiresByType application/x-javascript "access plus 30 minutes"

Related articles and topics

• Web Caching: what is it?
• How mod_cache works?
• Guide: Example of mod_cache application
• Guide: Tuning Wordpress with Helicon Ape on IIS7
• mod_disk_cache
• mod_headers

Alternate Interval Syntax

ExpiresDefault and ExpiresByType directives may be defined using more convenient syntax:

ExpiresDefault "<base> [plus] {<num> <type>}*"
ExpiresByType type/encoding "<base> [plus] {<num> <type>}*"

where <base> is one of the following:

• access
• now (equals 'access')
• modification

The plus keyword is optional. <num> is an integer value and <type> is one of the following: years , months , weeks , days , hours , minutes , seconds .

For example, any of the following directives can be used to make documents expire 1 month after being accessed, by default:

ExpiresDefault "access plus 1 month"
ExpiresDefault "access plus 4 weeks"
ExpiresDefault "access plus 30 days"

The expiry time can be set more precisely by using several ' <num> <type> ' clauses:

ExpiresByType text/html "access plus 1 month 15 days 2 hours"
ExpiresByType image/gif "modification plus 5 hours 3 minutes"

Note! If you want to set Expires header relative to modification date, it will not be set for the content that does not come from a file on disk because there is no modification time for such content.

Directives

ExpiresActive

ExpiresActive directive enables or disables the generation of Expires and Cache-Control headers for specific location (e.g. if directive is found inside .htaccess file, it will be applied only to documents generated from that directory).

Syntax

ExpiresActive On|Off

If set to Off , the headers will not be generated for any documents inside specific location (unless it is overridden at lower level, e.g. .htaccess overrides server config file). If set to On , the headers will be added to processed documents according to the criteria defined by the ExpiresByType and ExpiresDefault directives.

Note! ExpiresActive directive does not guarantee that Expires or Cache-Control header will be generated. If the criteria aren't met, no header will be sent and the effect will be as though the directive was not specified.

ExpiresByType

ExpiresByType directive defines the value of Expires header and max-age directive of Cache-Control header generated for documents of the specified type (e.g., text/html).

Syntax

ExpiresByType MIME-type <code>seconds

Description

• MIME-type parameter may be defined as a regular expression ( notice that Apache does not support this feature ).
• The second argument sets the number of seconds that will be added to the base time to make up the expiration date. Cache-Control: max-age is calculated by subtracting the request time from the expiration date and converting the result to seconds.
  Base time is either the last modification time for the file, or the time of the client's access to the document. Which of them should be used is specified in <code> field: M stands for file's last modification time and A stands for client's access time.

  If M value is used, all current copies of the document in all caches will expire at the same time.
  If A value is used, the expiration date will be different for each client.

Example

# enable expiration
ExpiresActive On

# expire GIF images after a month in client's cache
ExpiresByType image/gif A2592000

# HTML documents are good for a week from the time they were changed
ExpiresByType text/html M604800

# MIME-type with regex statement
ExpiresByType application/.* A3600

Note! This directive will only have effect if ExpiresActive On has been specified. It overrides, for the specified MIME type only, any expiration date set by the ExpiresDefault directive.

ExpiresDefault

ExpiresDefault directive sets the default algorithm for calculating the expiration time for all documents in the affected location (that depends on the location of the directive). It can be overridden on a type-by-type basis by ExpiresByType directive.

Syntax

ExpiresDefault <code>seconds