Monit是一款功能非常丰富的进程、文件、目录和设备的监测软件，用于Unix平台。它可以自动修复那些已经停止运作的程序，特使适合处理那些由于多种原因导致的软件错误。

　　When you cannot monitor your server for service availability, it is better to take help of automated monitor and restart utility. Last 4 days I was away from my server as I was enjoying my vacation. During this time due to load my lighttpd webserver died but it was restarted automatically within 2 minutes. I had utility configured for monitoring services on a Linux system called monit. It offers all features you ever needed for system monitoring and perform error recovery for UNIX like system.

　　Before monit I had my own shell and perl script for monitoring service. If service failed script will try to restart service and send an automated email to me. However monit is a superior solution.

　　monit is a utility for managing and monitoring processes, files, directories and devices on a Unix system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations. For example, monit can start a process if it does not run, restart a process if it does not respond and stop a process if it uses to much resources. You may use monit to monitor files, directories and devices for changes, such as timestamps changes, checksum changes or size changes.

　　You may also use monit to monitor files, directories and devices on localhost. Monit can monitor these items for changes, such as timestamps changes, checksum changes or size changes. This is also useful for security reasons you can monitor the md5 checksum of files that should not change.

　　Personally I always install and configure monit on all boxes which are under my control.

Install monit under Debian or Ubuntu Linux
　　Use apt-get command to install monit

　　# apt-get install monitOR$ sudo apt-get install monit

Install monit under Red Hat enterprise Linux (source code installation)
　　Many distributions include monit. However monit is not included in official Red hat enterprise Linux. Just download monit source code from official web site using wget command:

　　# cd /opt

　　# wget http://www.tildeslash.com/monit/dist/monit-4.8.2.tar.gz

　　Untar monit

　　# tar -zcvf monit-4.8.2.tar.gz

　　# cd monit-4.8.2

Configure and compile monit:
　　# ./configure

　　# make

　　Install monit

　　# make install

Copy monit configuration file:
　　# cp monitrc /etc/monitrc

　　By default monit is located at /usr/local/bin/monit

How do I Configure monit?
　　monitrc is name of monit configuration file and it is by default located at /etc/monitrc location. However each distribution places file in different location: .

　　=> Source code installation : /etc/monitrc

　　=> Debian / Unentu Linux installation : /etc/monit/monitrc

　　Open monit configuration file and setup values as follows:

　　# vi /etc/monitrc

　　a) Run it as daemon and check the services (such as web, mysql, sshd) at 2-minute

　　intervals.

　　set daemon 120

　　b) Set syslog logging with the ‘daemon’ facility:

　　set logfile syslog facility log_daemon

　　c) Set mail server name to send email alert

　　set mailserver mail.cyberciti.biz

　　Set email format such as from email

　　set mail-format { from: alert@nixcraft.in

　　subject: $SERVICE $EVENT at $DATE

　　message: Monit $ACTION $SERVICE at $DATE on $HOST: $DESCRIPTION.

　　}

　　d) Now most important part, restart lighttpd or apache web server if failed or killed by Linux kernel due to any causes:

　　check process lighttpd with pidfile /var/run/lighttpd.pid

　　group lighttpd

　　start program = "/etc/init.d/lighttpd start"

　　stop program = "/etc/init.d/lighttpd stop"

　　if failed host 75.126.43.232 port 80

　　protocol http then restart

　　if 5 restarts within 5 cycles then timeout

　　Where,

check process lighttpd with pidfile /var/run/lighttpd.pid : You are specifying lighttpd pid file and daemon name
group lighttpd: Specify group name, which is allowed or used to start/restart lighttpd
start program = “/etc/init.d/lighttpd start” : Command to start lighttpd server
stop program = “/etc/init.d/lighttpd stop” : Command to stop lighttpd server
if failed host 127.0.0.1 port 80 : Server IP address and port number (80)
protocol http then restart : If above IP and port failed restart the webserver
if 5 restarts within 5 cycles then timeout : Try to restart 5 times; if monit cannot restart webserver 5 times; just time out to avoid race condition.
　　Here is my mysql server restart configuration directives:

　　check process mysqld with pidfile /var/run/mysqld/mysqld.pid

　　group database

　　start program = "/etc/init.d/mysqld start"

　　stop program = "/etc/init.d/mysqld stop"

　　if failed host 127.0.0.1 port 3306 then restart

　　if 5 restarts within 5 cycles then timeout

　　Here is my sshd server configuration directives:

　　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 host 127.0.0.1 port 22 protocol ssh then restart

　　if 5 restarts within 5 cycles then timeout

　　Replace IP address 127.0.0.1 with your actual IP address. If you are using Debian just start monit:

　　# /etc/init.d/monit start

　　If you are using Red Hat Enterprise Linux, start monit from /etc/inittab file:

　　Open /etc/inittab file:

　　# vi /etc/inittab

　　Append following line:

　　mo:2345:respawn:/usr/local/bin/monit -Ic /etc/monitrc

　　Now start monit:

　　# inittab -q

　　You can verify that monit is started from /var/log/message log file:

　　# tail -f /var/log/messageOutput:

Nov 21 04:39:21 server monit[8759]: Starting monit daemon
Nov 21 04:39:21 server monit[8759]: Monit started
　　If lighttpd died, you will see something as follows in log file:

Nov 21 04:45:13 server monit[8759]: 'lighttpd' process is not running
Nov 21 04:45:13 server monit[8759]: 'lighttpd' trying to restart
Nov 21 04:45:13 server monit[8759]: 'lighttpd' start: /etc/init.d/lighttpd
　　You may use monit to monitor daemon processes or similar programs running on localhost or started from /etc/init.d/ location such as


　　=> Apache Web Server

　　=> SSH Server

　　=> Postfix/Sendmail MTA

　　=> MySQL etc

Further readings
monit man page
monit official web site and documenation