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Thanks for this overview. Is Cockroach an open-source version of Google's Spanner as well?

Not technically open-source (BSL-licensed), but yes, source available on https://github.com/cockroachdb/cockroach/.

They used to be completely open-source and have changed the license similar to Redis Labs, MongoDB, etc.

And yes, you can consider it as an alternative to Spanner with more usability and running on commodity hardware.