We have a routing tier of a few hundred machines. Any of those can serve as transaction coordinator. This is one of the places where having a transaction record

We have a routing tier of a few hundred machines. Any of those can serve as transaction coordinator. This is one of the places where having a transaction record is useful -- in general, our locking and non-transactional read scheme will wait nicely for a 2PC transaction to complete, but in the face of failures, any actor in the system can abort the 2PC transaction by marking the transaction record. There's also a really nice optimization for non-transactional reads that allows them to read even in the event of a staged but not yet committed 2PC transaction (you can prove to yourself that a linearized read can occur on either side, if the 2PC transaction is still pending when the read begins).