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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.
You could build your own versions if thats a concern. Or use one of the free pre-C++17 implementations (like I do in my C++11 codebase). https://github.com/mpark/variant
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Hence, a higher number means a more popular project.