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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.
However, there is already a tool that downloads epubs from a Dropbox / Gdrive folder https://github.com/fsantini/KoboCloud I can't say that I've tried it though.
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Hence, a higher number means a more popular project.