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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.
KiriKiri-Z is also open source (https://github.com/krkrz/krkrz), but I usually don't look at its source. It helps me prepare in case I ever need to port VNs without access to engine code.
NOTE:
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