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openITCOCKPIT
openITCOCKPIT is an Open Source system monitoring tool built for different monitoring engines like Nagios, Naemon and Prometheus.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
checkmk is my go to. They have a free version which is pretty feature rich, and a paid, which gets better performance. It is based on nagios but much much better and more easily configured.
openITCOCKPIT (with Nagios/Naemon/Checkmk and Grafana with Graphite for data visualization) - very easy to configure and to use :)
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