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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Nagios Core
Nagios Core network monitoring software is a powerful, enterprise-class host, server, application, monitoring and alerting tool
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Netdata
netdata is a system for distributed real-time performance and health monitoring. It provides unparalleled insights, in real-time, of everything happening on the system it runs (including applications such as web and database servers), using modern interactive web dashboards.
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Rex
A agent-less server orchestration tool built on Perl
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Riemann
Riemann aggregates events from servers and applications, notifies exceptions, tracks latency, collects system statistics and more, all with a powerful stream processing language
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Statman
Statman makes it possible to instrument and collect statistics from your high-traffic production Erlang systems with very low overhead. The collected data points are aggregated in the VM and can be sent to services like Graphite, Munin, New Relic, etc.
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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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