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Datastore isn't proprietary, AppScale (http://www.appscale.com) has built a handy OSS version which runs above Cassandra.

Disclaimer: I work at GCP, so I'm geeked about open approaches to solving problems :)

Excellent point Miles (and as you know I work in GCP too), I would add https://cloudplatform.googleblog.com/2015/05/AppScale-and-Ap... and https://www.youtube.com/embed/2fRTgosMtw4?rel=0&controls=0&s... (in both of which, of course, you feature quite prominently:-)...