Module mod_file_cache

This module should be used with care. You can easily create a broken site using mod_file_cache, so read this document carefully.

Caching frequently requested files that change very infrequently is a technique for reducing server load. mod_file_cache provides two techniques for caching frequently requested static files. Through configuration directives, you can direct mod_file_cache to either open then mmap()a file, or to pre-open a file and save the file's open file handle. Both techniques reduce server load when processing requests for these files by doing part of the work (specifically, the file I/O) for serving the file when the server is started rather than during each request.

mod_file_cache is not compiled into the server by default. To use mod_file_cache you have to enable the following line in the server build Configuration file:

    AddModule  modules/experimental/mod_file_cache.o

Notice: You cannot use this for speeding up CGI programs or other files which are served by special content handlers. It can only be used for regular files which are usually served by the Apache core content handler.

This module is an extension of and borrows heavily from the mod_mmap_static module in Apache 1.3.

Summary

mod_file_cache caches a list of statically configured files via MMapFile or CacheFile directives in the main server configuration.

Not all platforms support both directives. For example, Apache on Windows does not currently support the MMapStatic directive, while other platforms, like AIX, support both. You will receive an error message in the server error log if you attempt to use an unsupported directive. If given an unsupported directive, the server will start but the file will not be cached. On platforms that support both directives, you should experiment with both to see which works best for you.

`MmapFile` Directive

The MmapFile directive of mod_file_cache maps a list of statically configured files into memory through the system call mmap(). This system call is available on most modern Unix derivates, but not on all. There are sometimes system-specific limits on the size and number of files that can be mmap()d, experimentation is probably the easiest way to find out.

This mmap()ing is done once at server start or restart, only. So whenever one of the mapped files changes on the filesystem you have to restart the server (see the Stopping and Restarting documentation). To reiterate that point: if the files are modified in place without restarting the server you may end up serving requests that are completely bogus. You should update files by unlinking the old copy and putting a new copy in place. Most tools such as rdist and mv do this. The reason why this modules doesn't take care of changes to the files is that this check would need an extra stat() every time which is a waste and against the intent of I/O reduction.

`CacheFile` Directive

The CacheFile directive of mod_file_cache opens an active handle or file descriptor to the file (or files) listed in the configuration directive and places these open file handles in the cache. When the file is requested, the server retrieves the handle from the cache and passes it to the sendfile() (or TransmitFile() on Windows), socket API.

Insert more details about sendfile API...

This file handle caching is done once at server start or restart, only. So whenever one of the cached files changes on the filesystem you have to restart the server (see the Stopping and Restarting documentation). To reiterate that point: if the files are modified in place without restarting the server you may end up serving requests that are completely bogus. You should update files by unlinking the old copy and putting a new copy in place. Most tools such as rdist and mv do this.

MMapFile

Syntax: MMapFile filename [filename] ...
Default: None
Context: server-config
Override: Not applicable
Status: Experimental
Module: mod_file_cache
Compatibility: Only in Apache 1.3 (via mod_mmap_statis) or later.

The MMapFile directive maps one or more files (given as whitespace separated arguments) into memory at server startup time. They are automatically unmapped on a server shutdown. When the files have changed on the filesystem at least a HUP or USR1 signal should be send to the server to re-mmap them.

Be careful with the filename arguments: They have to literally match the filesystem path Apache's URL-to-filename translation handlers create. We cannot compare inodes or other stuff to match paths through symbolic links etc. because that again would cost extra stat() system calls which is not acceptable. This module may or may not work with filenames rewritten by mod_alias or mod_rewrite.

Example:

  MMapFile /usr/local/apache/htdocs/index.html

CacheFile

Syntax: CacheFile filename [filename] ...
Default: None
Context: server-config
Override: Not applicable
Status: Experimental
Module: mod_file_cache
Compatibility: Only available in Apache 2.0 or later.

The CacheFile directive opens handles to one or more files (given as whitespace separated arguments) and places these handles into the cache at server startup time. Handles to cached files are automatically closed on a server shutdown. When the files have changed on the filesystem, the server should be restarted to to re-cache them.