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From: Markus Wanner <markus@bluegap.ch>
To: MARK CALLAGHAN <mdcallag@gmail.com>
Cc: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Andrew Dunstan <andrew@dunslane.net>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: Aidan Van Dyk <aidan@highrise.ca>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.
Date: Fri, 18 Mar 2011 15:19:56 +0100
Message-ID: <4D836A0C.3000306@bluegap.ch> (raw)
In-Reply-To: <AANLkTi=B+cWE-pQJQM=zoA9=Zr25W7zPPMFvvBe11O4E@mail.gmail.com>
References: <E1PwMns-00047O-UU@gemulon.postgresql.org>
	<4D741441.2090704@dunslane.net>
	<1299454125.1696.6138.camel@ebony>
	<4D74896C.5030402@enterprisedb.com>
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	<4D74F3AE.9000802@dunslane.net>
	<4D74FDEC.5040406@enterprisedb.com>
	<4D74FF15.1070804@dunslane.net>
	<1299513979-sup-1987@alvh.no-ip.org>
	<4D832571.5030808@bluegap.ch>
	<AANLkTi=B+cWE-pQJQM=zoA9=Zr25W7zPPMFvvBe11O4E@mail.gmail.com>

Mark,

On 03/18/2011 02:16 PM, MARK CALLAGHAN wrote:
> 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

Oh, okay, good to know who to blame ;-)  However, I didn't mean to
offend anybody.

> 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.

Sure?

Their documentation [1] isn't entirely clear on that first: "the master
blocks after the commit is done and waits until at least one
semisynchronous slave acknowledges that it has received all events for
the transaction" and the "slave acknowledges receipt of a transaction's
events only after the events have been written to its relay log and
flushed to disk".

But then continues to say that "[the master is] waiting for
acknowledgment from a slave after having performed a commit", so this
indeed sounds like the transaction is visible to other sessions before
the slave ACKs.

So, semi-sync may show temporary inconsistencies in case of a master
failure.  Wow!

Regards

Markus Wanner


[1] MySQL 5.5 reference manual, 17.3.8. Semisynchronous Replication:
http://dev.mysql.com/doc/refman/5.5/en/replication-semisync.html



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  Cc: markus@bluegap.ch, mdcallag@gmail.com, alvherre@commandprompt.com, andrew@dunslane.net, heikki.linnakangas@enterprisedb.com, aidan@highrise.ca, simon@2ndquadrant.com, pgsql-hackers@postgresql.org
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