Re: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.

On Fri, Mar 18, 2011 at 9:27 AM, Markus Wanner <markus(at)bluegap(dot)ch> wrote:
> Google invented the term "semi-syncronous" for something that's
> essentially the same that we have, now, I think.  However, I full
> heartedly hate that term (based on the reasoning that there's no
> semi-pregnant, either).

We didn't invent the term, we just implemented something that Heikki
Tuuri briefly described, for example:
http://bugs.mysql.com/bug.php?id=7440

In the Google patch and official MySQL version, the sequence is:
1) commit on master
2) wait for slave to ack
3) return to user

After step 1 another user on the master can observe the commit and the
following is possible:
1) commit on master
2) other user observes that commit on master
3) master blows up and a user observed a commit that never made it to a slave

I do not think this sequence should be possible in a sync replication
system. But it is possible in what has been implemented for MySQL.
Thus it was named semi-sync rather than sync.

--
Mark Callaghan
mdcallag(at)gmail(dot)com