System Monitoring with Monit

In a DevOps environment, you can't watch your server 24/7. Monit is an autonomous "watchdog" that ensures your infrastructure stays healthy even while you sleep.

1. What is Monit?

Monit is a small, open-source utility for managing and monitoring Unix systems. It can:

• Automatic Restarts: If Nginx or MongoDB crashes, Monit restarts them.
• Resource Monitoring: If CPU usage stays above 90% for too long, it can alert you.
• File Integrity: It can watch for changes in critical configuration files.
• Network Checks: It can verify if a website or API is actually responding.

2. Installation

Monit is available on most Linux distributions.

# On Ubuntu/Debian
sudo apt update
sudo apt install monit

# Start and Enable Monit
sudo systemctl start monit
sudo systemctl enable monit

3. Configuring Monit

Monit's power lies in its configuration file, usually located at /etc/monit/monitrc. The syntax is designed to be "human-readable."

Example: Monitoring Nginx

Add this to your config to ensure your "Front Door" never stays closed:

monitrc

check process nginx with pidfile /var/run/nginx.pid
    start program = "/usr/bin/systemctl start nginx"
    stop program  = "/usr/bin/systemctl stop nginx"
    if failed port 80 protocol http then restart
    if 5 restarts within 5 cycles then timeout

Example: Monitoring System Resources

monitrc

check system $HOST
    if loadavg (5min) > 3 then alert
    if memory usage > 80% then alert
    if cpu usage (user) > 70% for 3 cycles then alert

4. The Monit Web Dashboard

Monit comes with a built-in web interface that allows you to see the status of all your services in one place.

1. Find the set httpd port 2812 section in monitrc.
2. Enable it by uncommenting the lines and setting your username/password.
3. Access it at http://your-server-ip:2812.

5. Monit vs. PM2: When to use which?

Feature	PM2	Monit
Primary Focus	Node.js Apps	System Services (Nginx, DBs, RAM)
Language	JavaScript/Node	C (System Native)
Auto-Restart	Yes (Apps)	Yes (Entire Services)
Ease of Use	High for JS Devs	Medium (Requires Linux knowledge)

Master Strategy: Use PM2 to manage your CodeHarborHub Node.js processes and use Monit to manage the underlying server, Nginx, and your Database.

Practice: The "Self-Healing" Test

1. Install Monit and set up a check for your database (e.g., MongoDB).
2. Manually stop your database: sudo systemctl stop mongodb.
3. Wait 1-2 minutes and check the status: sudo monit status.
4. You will see that Monit detected the failure and automatically executed the start command!

Alerting

Monit can be configured to send emails via Gmail or other SMTP services. Setting this up is crucial so that you get a notification on your phone the moment a "Master" system needs your attention.