How is it guaranteed that a transaction will make progress? It sounds like every (initial state -> a,b,c,d),(initial state = a,b,c,d; modified state -> a,B,C,d) can be aborted by another transaction? Is there some queuing system for transactions, or timed locks?

And for distributed shared systems, not all shards will have all of (a,b,c,d) in order to make a decision.

If one of the shards fails (i.e. write error) how would the transaction on the other shards abort? If there was shard redundancy I can see this becoming less likely, but by no means impossible.

Sure, maybe they can roll back transactions, but that presents a time window of wrong state to the world.