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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 on sale on Tindie. But I recommend to look at their github repo. Not many updates and one issue says that example code is nothing you would consider as smart watch. https://github.com/sqfmi/Watchy/releases
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Hence, a higher number means a more popular project.