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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.
If you have epubs from a bunch of sources it may be worth setting up calibre (https://calibre-ebook.com/) on your computer to manage them. I like it after a bit of effort getting kepubs working properly.
send.djazz.se or other similar services
NOTE:
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Hence, a higher number means a more popular project.