It sounds like it commits in the foreground, but it can fail due to contention -- in which case nothing happens. So the client might have to retry indefinitely,

It sounds like it commits in the foreground, but it can fail due to contention -- in which case nothing happens. So the client might have to retry indefinitely, absent some extra complexity for gaining exclusive access (a lock). It would still have to have a system for retiring transactions, at which point the client knows it went through. You would still have to wait for all the shards to respond. At least, that's what I think they are describing, it's all a bit hand wavey and suspicious.