Apache Module mod_file_cache

Summary

Caching frequently requested files that change very infrequently is a technique for reducing server load.mod_file_cache provides two techniques for caching frequently requestedstatic files. Through configuration directives, you can directmod_file_cache to either open thenmmap() a file, or to pre-open a file and save the file's openfile 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 themod_mmap_static module in Apache 1.3.

Topics

• Using mod_file_cache

Directives

• CacheFile
• MMapFile

Bugfix checklist

• httpd changelog
• Known issues
• Report a bug

See also

• Comments

Using mod_file_cache

mod_file_cache caches a list of statically configured files viaMMapFile orCacheFile directives in the main server configuration.

Not all platforms support both directives. 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

TheMMapFile directive ofmod_file_cache maps a list of statically configured files into memory through the system callmmap(). This system call is available on most modern Unix derivatives, but not on all. There are sometimes system-specific limits on the size and number of files that can bemmap()ed, experimentation is probably the easiest way to find out.

Thismmap()ing is done once at server start or restart, only. So whenever one of the mapped files changes on the filesystem youhave to restart the server (see theStopping and Restarting documentation). To reiterate that point: if the files are modifiedin 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 asrdist andmv do this. The reason why this modules doesn't take care of changes to the files is that this check would need an extrastat() every time which is a waste and against the intent of I/O reduction.

CacheFile Directive

TheCacheFile directive ofmod_file_cache opens an activehandle orfile 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 thesendfile() (orTransmitFile() 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 youhave to restart the server (see theStopping and Restarting documentation). To reiterate that point: if the files are modifiedin 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 asrdist andmv do this.

CacheFileDirective

TheCacheFile 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 re-cache them.

Be careful with thefile-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 linksetc. because that again would cost extrastat() system calls which is not acceptable. This module may or may not work with filenames rewritten bymod_alias ormod_rewrite.

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

MMapFileDirective

TheMMapFile 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 aHUP orUSR1 signal should be send to the server to re-mmap() them.

Be careful with thefile-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 linksetc. because that again would cost extrastat() system calls which is not acceptable. This module may or may not work with filenames rewritten bymod_alias ormod_rewrite.

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