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The Apache HTTP server provides a series of caching features designed to improve server performance in a variety of ways.

Three-state RFC2616 HTTP cache

Mod_cache and its provider module mod_cache_disk provide intelligent HTTP-aware caching. The content itself is stored in the cache, and mod_cache is designed to comply with all the various HTTP headers and options that control content cacheability.

Mod_cache is designed for simple and complex cache configurations where you can handle proxy content, dynamic local content, or need to accelerate access to local files on potentially slow disks.

Dual-state key / value shared object cache

Shared object cache API (socache) and its provider module provide a server-wide key / value shared object cache. These modules are designed to cache low-level data, such as SSL sessions and authentication credentials.

The back end allows data to be stored in shared memory server-wide, or in a cache within the data center, such as memcache or distcache.

Dedicated file caching

Mod_file_cache provides the ability to preload files into memory when the server starts, and can improve access time and save file handles on frequently accessed files because you do not need to go to disk on every request.

Three-state RFC2616 HTTP cache

The HTTP protocol includes built-in support for the inline caching mechanism described in Section 13 of RFC2616, and the mod_cache module can be used to take advantage of this feature.

Unlike the simple two state key / value caches, where the content disappears completely when it is no longer fresh, the HTTP cache includes a mechanism to retain stale content and ask the source server if this stale content has changed, and if not, refresh it again.

Entries in the HTTP cache have one of the following three states:

Fresh

If the content is new enough (younger than its fresh life), it is considered Fresh. The HTTP cache can provide new content for free without making any calls to the source server.

Stale

If the content is too old (earlier than its freshness life cycle), it is considered Stale. The HTTP cache should contact the original server and check that the content is still new before providing obsolete content to the client.

If the original server is still invalid, the original server will respond with replacement content, or ideally, the source server will respond with code to inform that the cached content is still new without having to generate or send the content again. The content becomes new again, and the loop continues.

The HTTP protocol allows caching to provide obsolete data in some cases, such as a 5xx error while trying to refresh the data using the source server, or another request that is already in the process of refreshing a given entry. In these cases, a warning header is added to the response.

Non Existent

If the cache is full, the option to remove content from the cache to make room is retained. Content can be deleted at any time, old or new. The htcacheclean tool can be run at once or deployed as a daemon to keep the cache size within a given size or number of inode. The tool attempts to delete the old content before attempting to delete the new content.

These are the details of how to use Apache HTTP Server's caching function, please pay attention to other related articles!

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