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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Kibana
Kibana is a modern, beautiful visualization tool that works seamlessly with Logstash and ElasticSearch. It features a no-code setup, realtime analytics, and much more.
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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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