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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.
https://github.com/FreeOpcUa/ Free, seems to focus on the python wrapper, no releases for the C++ lib, but seems to be maintained. https://github.com/FreeOpcUa/opcua-asyncio/releases
http://www.open62541.org/# Found as conan package, c++98 Wrapper for C++17 available: https://github.com/open62541pp/open62541pp
NOTE:
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Hence, a higher number means a more popular project.