Monit is a free open source utility for managing and monitoring, processes, programs, files, directories and filesystems on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful casual actions in error situations. CITATION

Installation
To install monit in to your Debian server, Simply use apt-get:

[server1]# apt-get install monit

Once installed you’ll find the main configuration file located at /etc/monit/monitrc.
Below is my sample configuration file, which will also explain some keywords.

set daemon  120
# poll at 2-minute intervals. Monit will wakeup every two minute to
# monitor things. Time must be given in seconds.

set logfile syslog facility log_daemon
# Set syslog logging.

# Set a default mail from-address for all alert messages emitted by monit.
# All alert mail will be sent to below mail address.
set mail-format { from: server1@thesysad.com }
set alert root@thesysad.com

# You can define your mail-notification format. Do man monit 
mail-format {
    from: server1@thesysad.com
    subject: $SERVICE $EVENT at $DATE
    message: Monit $ACTION $SERVICE at $DATE on $HOST,

    Yours sincerely,
    theSysad Admin
    }

# Make monit start its web-server. So you can acccess it from webrowser.
set httpd port 2812 and
use address test.com

##Monit web-server ACL.
allow localhost       # allow localhost to connect to the server and
allow 172.20.10.217     # allow 172.20.10.217 to connect to the server,
                      # You can give only one per entry


allow admin:monit     # user name and password for authentication.


allow server1:server1   # set multiple user to access through browser.	

# Monitoring the apache2 web services.
# It will check process apache2 with given pid file.
# If process name or pidfile path is wrong then monit will 
# give the error of failed. tough apache2 is running.
check process apache2 with pidfile /var/run/apache2.pid

  #Below is actions taken by monit when service got stuck.
  start program = "/etc/init.d/apache2 start"
  stop program  = "/etc/init.d/apache2 stop"
  # Admin will notify by mail if below of the condition satisfied.
  if cpu is greater than 60% for 2 cycles then alert
  if cpu > 80% for 5 cycles then restart
  if totalmem > 200.0 MB for 5 cycles then restart
  if children > 250 then restart
  if loadavg(5min) greater than 10 for 8 cycles then stop
  if 3 restarts within 5 cycles then timeout
  group server

#Monitoring slapd process.

check process slapd with pidfile /var/ldap/var/run/slapd.pid
    start program = "/etc/init.d/slapd start"
    stop program  = "/etc/init.d/slapd stop"
    if failed port 389 then restart
    if 2 restarts within 3 cycles then timeout
    group server

    # monit will only monit/give alert in this mode.
    mode passive         

# Check Database file, size,permission etc...
check file slapd-database with path /var/ldap/var/openldap-data/mydata.db
   if failed permission 700 then alert
   if failed uid data then alert
   if failed gid data then alert
   if timestamp > 15 minutes then alert
   if size > 100 MB then alert

#Check directory for permission.       
check directory bin with path /bin
    if failed permission 755 then unmonitor
    if failed uid 0 then unmonitor
    if failed gid 0 then unmonitor

#Check host for which services up/down on particular port.
check host test.com with address 192.168.1.9
    if failed icmp type echo with timeout 4 seconds then alert
    if failed port 21 then alert
    if failed port 22 type tcp 22 with timeout 40 seconds then aler
    if failed port 80 protocol http then alert
    if failed port 389 type tcp with timeout 15 seconds then alert

You can also include other configuration files via include directives:

include /etc/monit/default.monitrc
include /etc/monit/mysql.monitrc

After modifying the configuration file you should check for the syntax to make sure they are correct. To do this run:

[server1]#  monit -t

Now you can run monit directly:

[server1]#  monit

Once monit is running you can check for activity with your web-browser.

You can of course use the Debian init script to start the monitoring:

[server1]# /etc/init.d/monit start

For this to work you must enable the service by changing the file /etc/default/monit:

# You must set this variable to for monit to start
startup=1

# To change the intervals which monit should run uncomment
# and change this variable.
# CHECK_INTERVALS=180