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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.
It's available under the CC BY 4.0 license, you can find the source here:
https://github.com/SystemsApproach/book
There are instructions in the README to build epub, html and other formats (including a giant manpage).
NOTE:
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Hence, a higher number means a more popular project.