Re: terms for database replication: synchronous vs eager

Hi,

Chris Browne wrote:
> The approach that was going to be taken, in Slony-II, to apply locks
> as early as possible so as to find conflicts as soon as possible,
> rather than waiting, seems "eager" to me.

Agreed. WRT locking, one might also call it "pessimistic", but that
sounds so... negative.

I find the "as soon as possible" bit rather weak, instead it's exactly
"before the origin node confirms commit". Of course only conflicts which
could possibly lead to an abort of the transaction in question are taken
into account. A possible definition may be:

"Eager replication systems do only confirm the commit of a transaction
after they have checked for cross-node conflicts, which could require
the transaction to abort. (While lazy systems may confirm the commit
before)."

Note how much less restrictive that definition is, that that of a fully
synchronous system.

> But I'm not sure to what extent that notion has been drawn into the
> Postgres-R work...

My current variant of Postgres-R goes the very same path, using MVCC
instead of locking wherever possible (with the very same effect, but
allowing more concurrency :-) ).

Regards

Markus