I usually watch the Apache projects. Apache Arrow (https://github.com/apache/arrow/ https://github.com/apache/arrow/) is very interesting and promising project

I usually watch the Apache projects. Apache Arrow (https://github.com/apache/arrow/ https://github.com/apache/arrow/) is very interesting and promising project. It is in early state; you can watch the design decisions taken and how they are implemented. Plus it is multi-language project, you can find code in C++, Java and Python.