pcache.pl -- Periodic file caching for Apache httpd
Copyright (c) JAIST FTP Admins <[email protected]>

= Overview

This script realizes file caching for the Apache httpd. It is aimed
at servers delivering large files. It collects requests, priodically
choose files to be cached, and asynchronously caches them in SSD. It
does not cause client's wait for caching files differently from
mod_disk_cache.

= How to use

It needs a dedicated user account and a directory writable by the
user. You must create it and set it to $prefix in the script. You
must set the document root to $docroot, the caching directory in SSD
to $cacheroot, and the total amount of caches to $cache_total_size.

$cache_total_size should be less than 80% of the real capacity. The
script often removes files opened by the httpd, and they consume
space until it closes them. The script can't take the space into
account, so $cache_total_size should be a reduced value.

You should modify $pipe_buf and $maxsymlinks so they match PIPE_BUF
and MAXSYMLINKS respectively in /usr/include/sys/param.h in your OS.
As you have noticed, the script works on Unix-like OS only.

You must install the script in an appropriate directory, change its
owner to the user, and set the setuid bit on it. You must specify
the CustomLog directive in httpd.conf as follows.

    CustomLog "|/somewhere/pcache.pl" "%>s %O %f"

You must install the mod_logio to use %O directive in the format
because %B does not return actual transferred bytes. In addition, I
recommend you to make the BufferedLogs directive enabled. It can
much reduce overhead of piped logging.

You must specify RewriteCond and RewriteRule directives in
httpd.conf to allow the httpd to fetch caches. If $cacheroot is
"/ftp/.cache", you must specify them as follows.

    RewriteCond /ftp/.cache$1 -f
    RewriteRule ^(.*)$ /ftp/.cache$1

If there are symbolic links under $docroot, you must set the
FollowSymLinks option to the $cacheroot directory. The script
copies these links under $cacheroot if necessary.

When you restart the httpd gracefully, the script start to
work. After it collects requests for 2 hours, it chooses files to be
cached and copy them under $cacheroot.

= Configuration

== Virtual hosts

If you have virtual hosts, you should not place any CustomLog
directives inside <VirtualHost> sections so the script can handle
all requests. Every document root must be placed under one directry
and $docroot must have the directory. You must place the RewriteCond
and RewriteRule directives shown above inside every <VirtualHost>
section to allow the httpd to get caches.

== Excluding files to be cached

$min_file_size and $exclude_pattern specify files undesirable to be
cached. The script has a long delay to update caches after the
originals are updated. This can cause inconsistent states in some
repositories. If files will be overwritten, you should avoid them
from being cached.

== Generating statistics

The script periodically generates two files to show statictics. One
is $prefix/data/hitrate.txt having the hit rate of the cache every 5
minutes. Another is $prefix/data/raking.json having ratios of
traffic of each category of files every 30 minutes. You can use
latter to generate a graph like one you can see in
http://ftp.jaist.ac.jp/.

You can control the intervals with $hitrate_interval and
$rank_interval. If you do not need these files, you can set 0 to
either or both of the variables. If you need ranking.json, you must
properly define the function path_to_rank to map path names to their
categories.

=== Cleaning up

The script choose files to be cached and remove ineffective cached
files every 2 hours, but does not remove empty directries and
dangling symbolic links in the caching directory. The cleanup
process of them runs at 6 am by default. This time is specified by
$cache_cleanup_time.

== Turning algorithm

The script chooses files to be cached based on the exponentital
moving average of transferred bytes of each file. It calculates the
average by adding 2% of transferred bytes for 2 hours to 98% of the
previous average. These parameters are defined by $cache_ratio and
$cahce_interval.

Too large $cache_ratio and/or too small $cache_interval cause
frequent replacement of cached files. It resoluts in removing opened
files and increasing unusable spaces in the caching directory. You
should avoid the frequent replacement, but too infrequent
replacement lowers the efficency of caches.

The script generates logs recording addition and removal of cached
files and the hit rate in $prefix/log. You can consult them to
adjust those parameters adequate for your site.

The traffic ratios are calculated by the same algolithm and
diferrent parameters. $rank_ratio and $rank_interval are used in
this case. If you want to see a short term trend, you can choose
larger $rank_ratio and/or smaller $rank_interval.

There are some parameters about costs of the script. $max_records
specifies the number of files recorded for candidates of cached
files. The script examins whether every candidate still exists or
not, on choosing cached files. You should not choose too large
number, but it should be enough larger than the number of cached
files.

The script collects all requests to calculate the statistics by
default. You can set the sampling rate of requests with
$sampling_rate. You can specify an integer more than 1 to pick one
of the number of requests. A large number reduces not only the CPU
usage but also collectness of the statistics.