collectd

A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values

     

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Instrumental

Instrumental is a real-time, high-scale application and server monitoring tool with intelligent alerts, a powerful query language, and free development accounts.

     

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Monit

An open source utility for managing and monitoring processes, programs, files, directories and filesystems

     

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Rex

A agent-less server orchestration tool built on Perl

     

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Zabbix

An open source, enterprise-class monitoring framework

     

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Zipkin

A distributed request tracking system, useful for micro-service and distributed architectures written in Scala

     

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