Subject: Re: Re: [COMMITTERS] pgsql: Efficient
	transaction-controlled synchronous replication.
From: Simon Riggs <simon@2ndQuadrant.com>
To: MARK CALLAGHAN <mdcallag@gmail.com>
Cc: Markus Wanner <markus@bluegap.ch>,
	Alvaro Herrera <alvherre@commandprompt.com>,
	Andrew Dunstan <andrew@dunslane.net>,
	Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>,
	Aidan Van Dyk <aidan@highrise.ca>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
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Date: Fri, 18 Mar 2011 14:19:18 +0000
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On Fri, 2011-03-18 at 13:16 +0000, MARK CALLAGHAN wrote:
> On Fri, Mar 18, 2011 at 9:27 AM, Markus Wanner <markus@bluegap.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.

Thanks for clearing it up Mark.

We should definitely not be calling what we have "semi-sync". The
semantics are very different.

In PostgreSQL other users cannot observe the commit until an
acknowledgement has been received.

-- 
 Simon Riggs           http://www.2ndQuadrant.com/books/
 PostgreSQL Development, 24x7 Support, Training and Services