# Module mod_expires

This module is contained in the `mod_expires.c` file, and is not compiled in by default. It provides for the generation of `Expires` headers according to user-specified criteria.

## Summary

This module controls the setting of the `Expires` 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.

The `Expires` 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 "expired" and invalid, and a new copy must be obtained from the source.

## Directives

- ExpiresActive
- ExpiresByType
- ExpiresDefault

## ExpiresActive directive

Syntax: ExpiresActive boolean
Context: server config, virtual host, directory, .htaccess
Override: Indexes
Status: Extension
Module: mod_expires

This directive enables or disables the generation of the `Expires` header for the document realm in question. (That is, if found in an `.htaccess` file, for instance, it applies only to documents generated from that directory.) If set to `Off`, no `Expires` header will be generated for any document in the realm (unless overridden at a lower level, such as an `.htaccess` file overriding a server config file). If set to `On`, the header will be added to served documents according to the criteria defined by the ExpiresByType and ExpiresDefault directives (q.v.).

Note that this directive does not guarantee that an `Expires` 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.

## ExpiresByType directive

Syntax: ExpiresByType mime-type <code>seconds
Context: server config, virtual host, directory, .htaccess
Override: Indexes
Status: Extension
Module: mod_expires

This directive defines the value of the `Expires` header generated for documents of the specified type (e.g., `text/html`). The second argument sets the number of seconds that will be added to a base time to construct the expiration date.

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>` field; M means that the file's last modification time should be used as the base time, and A means the client's access time should be used.

The difference in effect is subtle. If M 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 A 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 (i.e., the images will be accessed repeatedly within a relatively short timespan).

Example:

       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

Note that this directive only has effect if `ExpiresActive On` has been specified. It overrides, for the specified MIME type only, any expiration date set by the ExpiresDefault directive.

## ExpiresDefault directive

Syntax: ExpiresDefault <code>seconds
Context: server config, virtual host, directory, .htaccess
Override: Indexes
Status: Extension
Module: mod_expires

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 ExpiresByType directive. See the description of that directive for details about the syntax of the argument.