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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.
Yes, it is Free and Open Source. If you didn't know, free/open-source software can be paid if distributed under GPL or some other open source licenses. Also "free" doesn't necessarily mean that software is provided for zero cost. Donations help to keep project alive in long term.
NOTE:
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