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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Prevoty Application Monitoring & Protection (AMP)
Prevoty enables DevOps to inject security into your web applications and web services. Protection is provided at runtime from within the application itself, and stays with the app regardless of deployment method (cloud, on-premises, etc). Applications are protected against threats including XSS, SQLi, CSRF, command injection, etc. Works with Puppet, Chef, Ansible, Docker, Splunk, QRadar, ELK, and many others.
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Rex
A agent-less server orchestration tool built on Perl
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SignalFx
SignalFx is an advanced monitoring and alerting solution for modern infrastructure. With SignalFx, development and operations teams monitor, analyze, and alert in real-time on any time series data.
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Zabbix
An open source, enterprise-class monitoring framework
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