Apache Module mod_cache

Summary

mod_cache implements an RFC 2616 compliant HTTP content caching filter, with support for the caching of content negotiated responses containing the Vary header.

RFC 2616 compliant caching provides a mechanism to verify whether stale or expired content is still fresh, and can represent a significant performance boost when the origin server supports conditional requests by honouring the If-None-Match HTTP request header. Content is only regenerated from scratch when the content has changed, and not when the cached entry expires.

As a filter, mod_cache can be placed in front of content originating from any handler, including flat files (served from a slow disk cached on a fast disk), the output of a CGI script or dynamic content generator, or content proxied from another server.

In the default configuration, mod_cache inserts the caching filter as far forward as possible within the filter stack, utilising the quick handler to bypass all per request processing when returning content to the client. In this mode of operation, mod_cache may be thought of as a caching proxy server bolted to the front of the webserver, while running within the webserver itself.

When the quick handler is switched off using the CacheQuickHandler directive, it becomes possible to insert the CACHE filter at a point in the filter stack chosen by the administrator. This provides the opportunity to cache content before that content is personalised by the mod_include filter, or optionally compressed by the mod_deflate filter.

Under normal operation, mod_cache will respond to and can be controlled by the Cache-Control and Pragma headers sent from a client in a request, or from a server within a response. Under exceptional circumstances, mod_cache can be configured to override these headers and force site specific behaviour, however such behaviour will be limited to this cache only, and will not affect the operation of other caches that may exist between the client and server, and as a result is not recommended unless strictly necessary.

RFC 2616 allows for the cache to return stale data while the existing stale entry is refreshed from the origin server, and this is supported by mod_cache when the CacheLock directive is suitably configured. Such responses will contain a Warning HTTP header with a 110 response code. RFC 2616 also allows a cache to return stale data when the attempt made to refresh the stale data returns an error 500 or above, and this behaviour is supported by default by mod_cache. Such responses will contain a Warning HTTP header with a 111 response code.

mod_cache requires the services of one or more storage management modules. The following storage management modules are included in the base Apache distribution:

mod_cache_disk

Implements a disk based storage manager. Headers and bodies are stored separately on disk, in a directory structure derived from the md5 hash of the cached URL. Multiple content negotiated responses can be stored concurrently, however the caching of partial content is not supported by this module. The htcacheclean tool is provided to list cached URLs, remove cached URLs, or to maintain the size of the disk cache within size and inode limits.

mod_cache_socache

Implements a shared object cache based storage manager. Headers and bodies are stored together beneath a single key based on the URL of the response being cached. Multiple content negotiated responses can be stored concurrently, however the caching of partial content is not supported by this module.

Further details, discussion, and examples, are provided in the Caching Guide.