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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.
I know the hydroxide project started looking into this a couple of years ago too - which should be able to do CardDAV, but it seems that the CalDAV effort stranded too.
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