collectd
A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values
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jclouds
A Java Library for controlling cloud resources, with various plugins for many popular cloud providers.
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jmxtrans
Plugs Java VM statistics via JMX to various backend monitoring tools
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KairosDB
KairosDB is a fast distributed scalable time series database written on top of Cassandra. RESTful JSON-based query/results. Includes flot-based visualization.
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Logstash
A modern tool for managing events and logs, with an ElasticSearch backend which allows a high degree of searching and filtering. It includes a shipper in remote systems, indexer in the aggregator machine, and a Web UI.
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Metrics
Captures JVM and application-level metrics for Java
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Monit
An open source utility for managing and monitoring processes, programs, files, directories and filesystems
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Pallet
A tool to provision and maintain servers across various cloud platforms as well as virtual machine platforms, without any dependencies. Provides cloud and operating system independence.
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Sentry
Automate error tracking to find and fix issues before your users even experience a crash. Sentry is open-source, real-time, and loved by 100,000 developers around the world.
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testssl.sh
A command-line tool which checks a server's service on any port for the support of TLS/SSL ciphers, protocols as well as some cryptographic flaws. It's designed to provide clear output for your "is this good or bad" decision.
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Zabbix
An open source, enterprise-class monitoring framework
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