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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 can also check "free" programs like: https://teachyourselfcs.com/ https://github.com/ossu/computer-science
You have this "awesome" repository on GitHub that has all sorts of interesting topics: https://github.com/sindresorhus/awesome If you scroll down there is a section on programming languages and one in theory (algorithms, math, etc). To be honest I don't know if they are "beginner friendly".
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Hence, a higher number means a more popular project.