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

From: MARK CALLAGHAN <mdcallag(at)gmail(dot)com>
To: Markus Wanner <markus(at)bluegap(dot)ch>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.
Date: 2011-03-18 13:16:24
Message-ID: AANLkTi=B+cWE-pQJQM=zoA9=Zr25W7zPPMFvvBe11O4E@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-hackers
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

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