Apache Module mod_file_cache

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+ + + +

Description:	Caches a static list of files in memory
Status:	Experimental
Module Identifier:	file_cache_module
Source File:	mod_file_cache.c

Summary

+ + +

+ 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.

+ +

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.

Topics

Using mod_file_cache

Using mod_file_cache

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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()ed, 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.

+ + + +

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.

+ + +

Note

Don't bother asking for a directive which recursively + caches all the files in a directory. Try this instead... See the + Include directive, and consider + this command:

+ +

+ find /www/htdocs -type f -print \ + | sed -e 's/.*/mmapfile &/' > /www/conf/mmap.conf +

CacheFile Directive

+ + + + + + +

Description:	Cache a list of file handles at startup time
Syntax:	`CacheFile file-path [file-path] ...`
Context:	server config
Status:	Experimental
Module:	mod_file_cache

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.

+ +

Be careful with the file-path 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

+ CacheFile /usr/local/apache/htdocs/index.html +

+ +

MMapFile Directive

+ + + + + + +

Description:	Map a list of files into memory at startup time
Syntax:	`MMapFile file-path [file-path] ...`
Context:	server config
Status:	Experimental
Module:	mod_file_cache

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.

+ +

Example

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

+ +