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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.
arduino.cc has a lot of tutorials!
If you want to really learn it I'd recommend Bro Codes C guide, that'll make it easier to understand, his C++ guide is a bit longer but once you know C++ it's a matter of going to the Arduino references and looking for the function you need.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.