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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 is not publicly available. If you click on this link you get a 404 page: https://github.com/EpicGames/UnrealEngine You actually have to go and subscribe, agree to their terms, and then you get to view the source code. It happens to be free, but it is by no means "public".
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.