We get asked a lot by customers to install and setup monit. Its not an overly hard task, in fact its pretty darned easy.

Monit is brilliant for monitoring and restarting services when they are down, it can alert you or just restart after 5 failed connects/attempts. It handles everything from disk space, to memory, services, and other things. Its very easy to configure and very customizable.

To install ..

Debian/Ubuntu based:

apt-get install monit

Centos:

Instructions are here http://www.howtoforge.com/server-monitoring-with-munin-and-monit-on-centos-5.2 . Basically use rpmforge. and install from there.

Do your basic config, and anything else you may need/want.

Now for the tomcat part – this is based on tomcat being in /usr/local/tomcat where our typical setup script puts everything.

Tomcat, method a (recomended):

Simply run the below snippet to enable monit monitoring of tomcat. This method requires the least work and changes to configurations. Typically monit prefers a bid file to monitor a service as described in method b, but this way works just as well so long as the http port connector is enabled.

echo "check host tomcat with address localhost
 stop program = "/etc/init.d/tomcat stop"
 start program = "/etc/init.d/tomcat restart"
 if failed port 8080 and protocol http
 then start
" > /etc/monit.d/tomcat 
/etc/init.d/monit restart

Tomcat, method b:

Use this method if you dont have a suitable http connector enabled for your tomcat instance, but be aware that pid files can be left in an inconsistent state in some cases. Which may then require manual intervention anyway. Add a tomcat instance into your config for monit that looks like this (change gid/uid tomcat runs )

check process tomcat with pidfile "/var/run/tomcat/tomcat.pid"
start program = "/usr/local/tomcat/bin/startup.sh"
as uid tomcat gid tomcat
stop program = "/usr/local/tomcat/bin/shutdown.sh"
as uid tomcat gid tomcat
if failed port 8080 then alert
if failed port 8080 for 5 cycles then restart

Then edit your catalina.sh and set

CATALINA_PID to be /var/run/tomcat/tomcat.pid
JAVA_HOME=/usr/java/jdk

Add in at the top

Then of course create the pid directory

mkdir /var/run/tomcat/
chown tomcat.tomcat /var/run/tomcat/

MySQL

Add this to your my.cnf under [mysqld]

pid-file=/var/run/mysqld/mysqld.pid

and this to your monit config

check process mysql with pidfile /var/run/mysqld/mysqld.pid
   group database
   start program = "/etc/init.d/mysql start"
   stop program = "/etc/init.d/mysql stop"
   if failed host 127.0.0.1 port 3306 protocol mysql then restart
   if 5 restarts within 5 cycles then timeout
   depends on mysql_rc

 check file mysql_rc with path /etc/init.d/mysql
   group database
   if failed checksum then unmonitor
   if failed permission 755 then unmonitor
   if failed uid root then unmonitor
   if failed gid root then unmonitor



Disk Space

Add the following to your monit config

check filesystem with path /dev/xvda1
      if space usage > 95% then alert
      if inode usage > 95% then alert

SSH

Add this to your monit config (change the port if yours is different)

check process sshd with pidfile /var/run/sshd.pid
   start program  "/etc/init.d/sshd start"
   stop program  "/etc/init.d/sshd stop"
   if failed port 22 protocol ssh then restart
   if 5 restarts within 5 cycles then timeout

Now it should all work once you have reloaded your monit config.

For more examples check out http://mmonit.com/wiki/Monit/ConfigurationExamples