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

On Wed, Oct 10, 2012 at 10:26 PM, Bruce Momjian <bruce@momjian.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?"