collectd
A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values
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Consul
Consul makes it simple for services to register themselves and to discover other services via a DNS or HTTP interface. Register external services such as SaaS providers as well
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Monit
An open source utility for managing and monitoring processes, programs, files, directories and filesystems
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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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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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Supervisord Monitor
Supervisord Monitor is multi server monitoring tool for supervisor - watch, start and stop all your supervisord instances and servvices from one place.
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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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Zipkin
A distributed request tracking system, useful for micro-service and distributed architectures written in Scala
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