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From: Markus Wanner <markus@bluegap.ch>
To: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: MARK CALLAGHAN <mdcallag@gmail.com>
Cc: Simon Riggs <simon@2ndquadrant.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: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Re: [COMMITTERS] pgsql: Efficient	 transaction-controlled synchronous replication.
Date: Fri, 18 Mar 2011 15:37:26 +0100
Message-ID: <4D836E26.6080503@bluegap.ch> (raw)
In-Reply-To: <4D831A76020000250003BA82@gw.wicourts.gov>
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>
	<4D831A76020000250003BA82@gw.wicourts.gov>

Hi,

On 03/18/2011 02:40 PM, Kevin Grittner wrote:
> Then the only thing you would consider sync replication, as far as I
> can see, is two phase commit

I think waiting for the ACK before actually making the changes from the
transaction visible (COMMIT) would suffice for disallowing such an
inconsistency to manifest.  But obviously, MySQL decided it's not worth
doing that, as it's such a rare event and a short period of time that
may show inconsistencies...

> people's needs.  The guarantee that some people are looking for is
> that a successful commit means that the data has been persisted on
> two separate servers.

Well, MySQL's semi-sync also seems to guarantee that WRT the client
confirmation.  And transactions always appear committed *before* the
client receives the COMMIT acknowledgement, due to the time it takes for
the ACK to arrive at the client.

It's just the commit *before* receiving the slave's ACK, which might
make a transaction visible that's not durable, yet.  But I guess that
simplified implementation for them...

Regards

Markus Wanner



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  Cc: markus@bluegap.ch, Kevin.Grittner@wicourts.gov, mdcallag@gmail.com, simon@2ndquadrant.com, alvherre@commandprompt.com, andrew@dunslane.net, heikki.linnakangas@enterprisedb.com, aidan@highrise.ca, pgsql-hackers@postgresql.org
  Subject: Re: Re: [COMMITTERS] pgsql: Efficient	 transaction-controlled synchronous replication.
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