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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.
If you want a project that is friendly to contributors, try WWIV; there is a decades-long openness to dev interaction going all the way back to the late 80's-early 90's when registering the product got you a copy of the C source code for you to freely modify. There was a large number of publicly released mods that you could manually patch into your source. Now it is an open-source project that continues the tradition of collaboration.
NOTE:
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Hence, a higher number means a more popular project.