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

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Markus Wanner <markus(at)bluegap(dot)ch>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, MARK CALLAGHAN <mdcallag(at)gmail(dot)com>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.
Date: 2011-03-18 22:47:54
Message-ID: AANLkTik6tVXXsY79SepR-rP1M9sN=_-RPwo7W+cfMbAj@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-hackers
On Fri, Mar 18, 2011 at 5:48 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> Well, the idea is that we don't want to let people depend on the
>> value until it's guaranteed to be durably committed.
>
> OK, so if you see it on the replica, you know it is in at least two
> places.  I guess that makes sense.  It kinda "feels" wrong to see a
> view of the replica which is ahead of the master, but I guess it's
> the least of the evils.  I guess we should document it, though, so
> nobody has a false expectation that seeing something on the replica
> means that a connection looking at the master will see something
> that current.

Yeah, it can go both ways: a snapshot taken on the standby can be
either earlier or later in the commit ordering than the master.
That's counterintuitive, but I see no reason to stress about it.  It's
perfectly reasonable to set up a server with synchronous replication
for enhanced durability and also enable hot standby just for
convenience, but without actually relying on it all that heavily, or
only for non-critical reporting purposes.  Synchronous replication,
like asynchronous replication, is basically a high-availability tool.
As long as it does that well, I'm not going to get worked up about the
fact that it doesn't address every other use case someone might want.
We can always add more frammishes in future releases.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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