Monit

Monit is a small, open-source utility for managing and monitoring Unix systems. Its primary function is watching processes, files, directories, filesystems, and network connections, and automatically taking corrective action when problems are detected. If a service crashes, Monit restarts it. If disk space runs low, Monit can trigger alerts or cleanup scripts.

The most common use case in hosting is process supervision. Monit watches critical services like Apache, Nginx, MySQL, PHP-FPM, Postfix, Dovecot, and other daemons. If any of these services crash or stop responding, Monit automatically restarts them and sends an alert to the administrator. This self-healing behavior prevents prolonged outages from unattended service failures.

Monit's monitoring capabilities go beyond simple process checking. It can test TCP/UDP port connectivity (verify that a service is actually accepting connections), check HTTP response codes and content, monitor CPU and memory usage per process, watch file checksums for unauthorized changes, and track filesystem usage.

The built-in web interface provides a status dashboard showing all monitored services, their current state, resource usage, and event history. It is deliberately simple: a single page showing green (running) or red (stopped) for each service, with the ability to manually start, stop, or restart services.

Configuration uses a readable, English-like syntax. A typical rule might read: "check process nginx with pidfile /var/run/nginx.pid, start program = /etc/init.d/nginx start, stop program = /etc/init.d/nginx stop, if 5 restarts within 5 cycles then alert." This makes Monit configurations easy to write and understand.

For hosting providers, Monit is a lightweight first line of defense against service outages. It complements full monitoring systems like Zabbix or Nagios by handling automatic recovery locally on each server. Many providers run Monit alongside a centralized monitoring system: Monit handles immediate restarts while the monitoring system provides alerting, trending, and visibility.