This is really interesting. Thank you.

I implemented multiversion concurrency control in Java (https://github.com/samsquire/multiversion-concurrency-contro...) and I am yet to get to the point where I'm distributing it between machines for multimachine transactions.

I have a simple read timestamp that is monotonically increasing. If I knew the read timestamps of other shards, distributed by raft log I could implement transactions across shards.