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Re: [PATCH 8/8] Introduce wal decoding via catalog timetravel

From: Christopher Browne <cbbrowne(at)gmail(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Greg Stark <stark(at)mit(dot)edu>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org, hlinnakangas(at)vmware(dot)com
Subject: Re: [PATCH 8/8] Introduce wal decoding via catalog timetravel
Date: 2012-10-11 22:41:00
Message-ID: CAFNqd5VY9aKZtPSEyzOTMsGAhfFHKaGNCgY0D0wZvqjC0Dtt1g@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Oct 10, 2012 at 10:26 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> How does Slony write its changes without causing serialization replay
> conflicts?

It uses a sequence to break any ordering conflicts at the time that
data is inserted into its log tables.

If there are two transactions, A and B, that were "fighting" over a
tuple on the origin, then either:

a) A went first, B went second, and the ordering in the log makes that
order clear;
b) A succeeds, then B fails, so there's no conflict;
c) A is doing its thing, and B is blocked behind it for a while, then
A fails, and B gets to go through, and there's no conflict.

Switch A and B as needed.

The sequence that is used establishes what is termed a "compatible
ordering."  There are multiple possible compatible orderings; ours
happen to interleave transactions together, with the sequence
guaranteeing absence of conflict.

If we could get commit orderings, then a different but still
"compatible ordering" would be to have each transaction establish its
own internal sequence, and apply things in order based on
(commit_tx_order, sequence_within).
-- 
When confronted by a difficult problem, solve it by reducing it to the
question, "How would the Lone Ranger handle this?"


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Subject: Re: [PATCH 8/8] Introduce wal decoding via catalog timetravel
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