Nagios is one of the oldest open-source monitoring tools. Many products evolved from Nagios concepts.
Architecture:
Nagios core runs checks. Plugins perform actual tests. This separation lets you monitor almost anything.
Key features:
- Host and service monitoring
- Contact notifications
- Event handlers for automatic remediation
- Dependencies to suppress downstream alerts
- Web interface for status
Limitations:
- Configuration is text-file based
- Scaling requires additional architecture
- Metrics storage is basic
Nagios works well for availability monitoring.