Re: eXtensible Transaction Manager API (v2)

From: Stas Kelvich <s(dot)kelvich(at)postgrespro(dot)ru>
To: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, David Steele <david(at)pgmasters(dot)net>, Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Oleg Bartunov <obartunov(at)gmail(dot)com>
Subject: Re: eXtensible Transaction Manager API (v2)
Date: 2016-03-16 11:59:35
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On 12 Mar 2016, at 13:19, Michael Paquier <michael(dot)paquier(at)gmail(dot)com> wrote:
> On Fri, Mar 11, 2016 at 9:35 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> IMO this is not committable as-is, and I don't think that it's something
>> that will become committable during this 'fest. I think we'd be well
>> advised to boot it to the 2016-09 CF and focus our efforts on other stuff
>> that has a better chance of getting finished this month.
> Yeah, I would believe that a good first step would be to discuss
> deeply about that directly at PGCon for folks that will be there and
> interested in the subject. It seems like a good timing to brainstorm
> things F2F at the developer unconference for example, a couple of
> months before the 1st CF of 9.7. We may perhaps (or not) get to
> cleaner picture of what kind of things are wanted in this area.

To give overview of xtm coupled with postgres_fdw from users perspective i’ve packed patched postgres with docker
and provided test case when it is easy to spot violation of READ COMMITTED isolation level without XTM.

This test fills database with users across two shards connected by postgres_fdw and inherits the same table. Then
starts to concurrently transfers money between users in different shards:

update t set v = v - 1 where u=%d; -- this is user from t_fdw1, first shard
update t set v = v + 1 where u=%d; -- this is user from t_fdw2, second shard

Also test simultaneously runs reader thread that counts all money in system:

select sum(v) from t;

So in transactional system we expect that sum should be always constant (zero in our case, as we initialize user with zero balance).
But we can see that without tsdtm total amount of money fluctuates around zero.

Stas Kelvich
Postgres Professional:
Russian Postgres Company

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