Horton

Re: Slow catchup of 2PC (twophase) transactions on replica in LR

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Ajin Cherian <itsajin@gmail.com>
Cc: Давыдов Виталий <v.davydov@postgrespro.ru>, Heikki Linnakangas <hlinnaka@iki.fi>, pgsql-hackers@lists.postgresql.org
Date: 2024-04-04T05:37:49Z
Lists: pgsql-hackers

Commits

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  1. Fix random failure in 021_twophase.

  2. Allow altering of two_phase option of a SUBSCRIPTION.

  3. Doc: use true|false rather than on|off for "failover" option

  4. Support an optional asynchronous commit mode, in which we don't flush WAL

On Thu, Apr 4, 2024 at 10:53 AM Ajin Cherian <itsajin@gmail.com> wrote:
>
> On Wed, Mar 6, 2024 at 1:29 AM Давыдов Виталий <v.davydov@postgrespro.ru> wrote:
>>
>> In usual work, the subscription has two_phase = on. I have to change this option at catchup stage only, but this parameter can not be altered. There was a patch proposal in past to implement altering of two_phase option, but it was rejected. I think, the recreation of the subscription with two_phase = off will not work.
>>
>>
>
> The altering of two_phase was restricted because if there was a previously prepared transaction on the subscriber when the two_phase was on, and then it was turned off, the apply worker on the subscriber would re-apply the transaction a second time and this might result in an inconsistent replica.
> Here's a patch that allows toggling two_phase option provided that there are no pending uncommitted prepared transactions on the subscriber for that subscription.
>

I think this would probably be better than the current situation but
can we think of a solution to allow toggling the value of two_phase
even when prepared transactions are present? Can you please summarize
the reason for the problems in doing that and the solutions, if any?

-- 
With Regards,
Amit Kapila.