From: Simon Riggs <simon@2ndQuadrant.com>
To: MARK CALLAGHAN <mdcallag@gmail.com>
Cc: Markus Wanner <markus@bluegap.ch>
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 14:19:18 +0000
Message-ID: <1300457958.18619.13780.camel@ebony> (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>
<4D74DDFA.70800@dunslane.net>
<4D74E584.7080409@enterprisedb.com>
<4D74E9FA.7070202@dunslane.net>
<AANLkTinkzJ=UrLbThZy2KeQg+SjwYqj2r0mDomMwirB7@mail.gmail.com>
<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>
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
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To: pgsql-committers@postgresql.org
Cc: simon@2ndQuadrant.com, mdcallag@gmail.com, markus@bluegap.ch, 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.
In-Reply-To: <1300457958.18619.13780.camel@ebony>
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