Relip's Geek blog

Apache Mod_Watch & Munin Graphs showing vhost accesses

While this is more of an apache topic than a munin one, the main reason I wanted to compile the mod_watch module for apache is so I can graph vhost usage and access. Munin will then show graphs of how much a vhost is used. Also, while there are other how-to websites out there for this, they seem to drop off and disappear after a few years.

mod_watch itself is not supported and is actually quite difficult to get a hold of. I was using this on Debian Etch and Lenny on a x32 system. Copying the mod_watch.so and running on x64 failed so I needed to recomiple on a x64 debian squeeze.

The main reference I used was: http://stubbedtoe.co.nz/index.php/2009/09/apache-vhost-monitoring-with-munin/ This worked perfectly. There was an issue with debian squeeze which will be sorted below.

Compile

apt-get install apache2-dev libcrypt-ssleay-perl libwww-perl make
wget http://wiki.kartbuilding.net/mod_watch-4.3_apache22_mod.tar.gz
#gunzip and tar -xvf
nano Makefile.dso
//change APXS location to:
APXS=/usr/bin/apxs2

make -f Makefile.dso build
make -f Makefile.dso install 

Add and config mod_watch apache module

vi /etc/apache2/mods-available/watch.load
//add in
LoadModule watch_module /usr/lib/apache2/modules/mod_watch.so

a2enmod watch
vi /etc/apache2/sites-enabled/01-default  (or whatever vhost you want)
       <Location /watch-info>
         SetHandler watch-info
           Order deny,allow
           Deny from all
           Allow from 127.0.0.1 ::1
       </Location>
       <Location /watch-list>
        SetHandler watch-list
           Order deny,allow
           Deny from all
           Allow from 127.0.0.1 ::1
       </Location>

/etc/init.d/apache2 restart

Checking mod_watch for errors

elinks http://localhost/watch-list
#check for output

mod_watch Errors received

in error.log with Debian Squeeze running 2.6.32-5-amd64

tail -f /var/log/apache2/error.log
//I noticed the following warns when I accessed any vhosts with mod_watch enabled
[Fri Mar 18 22:35:36 2011] [crit] (20014)Internal error: shGetLockedEntry(7fca84e5aae8, "SERVER") failed in watchPostReadRequestHash()
[Fri Mar 18 22:35:36 2011] [error] shVerifyString(7fca84e5aae8, 7fca83c946f8) failed!
[Fri Mar 18 22:35:36 2011] [crit] (20014)Internal error: shGetLockedEntry(7fca84e5aae8, "monitor2.burkesys.com") failed in watchLogHash()
[Fri Mar 18 22:35:36 2011] [error] shVerifyString(7fca84e5aae8, 7fca83c94718) failed!
[Fri Mar 18 22:35:36 2011] [crit] (20014)Internal error: shGetLockedEntry(7fca84e5aae8, "SERVER") failed in watchLogHash()

resolve mod_watch errors

Googling led me to http://forums.cacti.net/viewtopic.php?p=186639#p186639

cd /root/mod_watch-4.3_apache22_mod/
vi SharedHash.c
//on line 102 change to look like:
shVerifyString(struct shTable *tp, char *str)
{
   /* if ((char *) tp->shared <= str && str < (char *) tp->eshared) { */
   if ((char *) apr_shm_baseaddr_get(tp->shared) <= str && str < (char *) tp->eshared) {
       if (str + strlen(str) < (char *) tp->eshared)
           return str;
   }

So now browing various vhosts on the server showed no errors in error.log. Using elinks (apt-get install elinks) going to http://localhost/watch-list showed results.

Munin Plugin for Mod_Watch

cd /usr/share/munin/plugins
wget http://wiki.kartbuilding.net/apache_watch
chmod 755 apache_watch
vi apache_watch
//change my $URL = exists $ENV{'url'} ? $ENV{'url'} : "http://monitor2.burkesys.com:%d/watch-list"; to:
my $URL = exists $ENV{'url'} ? $ENV{'url'} : "http://server.com:%d/watch-list";

ln -s /usr/share/munin/plugins/apache_watch /etc/munin/plugins/apache_watch_requests

That should do it. Once mod_watch is working and accessible munin should pick it up ok. References: http://stubbedtoe.co.nz/index.php/2009/09/apache-vhost-monitoring-with-munin/ 
http://exchange.munin-monitoring.org/plugins/apache_watch_/version/1 
http://forums.cacti.net/about15475.html

This is a nice apache module. I'm surprised that there is no maintained similar apache module. I've seen another similar munin plugin that parses /var/log/apache2/vhost%_access.log to generate these graphs, however it requires separate log files for each vhost and parses the logs every so often. Any more info /queries / suggestions drop us a mail sburke[at]burkesys.com