Re: eXtensible Transaction Manager API

I think there are ways to reduce the cost of this.  Some distributed
systems have solved it by retreating from snapshot isolation and going
back to using locks.  This can improve scalability if you've got lots
of transactions but they're very short and rarely touch the same data.
Locking individual data elements (or partitions) doesn't require a
central system that knows about all commits - each individual node can
just release locks for each transaction as it completes.  More
generally, if you could avoid having to make a decision about whether
transaction A precedes or follows transaction B unless transaction A
and B actually touch the same data - an idea we already use for SSI -
then you could get much better scalability.  But I doubt that can be
made to work without a deeper rethink of our transaction system.