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: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
Cc: Vitaly Davydov <v.davydov@postgrespro.ru>, Ajin Cherian <itsajin@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2024-07-09T05:05:37Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  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 Mon, Jul 8, 2024 at 5:25 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
>
> I see that in 0003/0004, the patch first aborts pending prepared
> transactions, update's catalog, and then change slot's property via
> walrcv_alter_slot. What if there is any ERROR (say the remote node is
> not reachable or there is an error while updating the catalog) after
> we abort the pending prepared transaction? Won't we end up with lost
> prepared transactions in such a case?
>

Considering the above is a problem the other possibility I thought of
is to change the order like abort prepared xacts after slot update.
That is also dangerous because any failure while aborting could make a
slot change permanent whereas the subscription option will still be
old value. Now, because the slot's two_phase property is off, at
commit, it can resend the entire transaction which can create a
problem because the corresponding prepared transaction will already be
present.

One more thing to think about in this regard is what if we fail after
aborting a few prepared transactions and not all?

At this stage, I am not able to think of a good solution for these
problems. So, if we don't get a solution for these, we can document
that users can first manually abort prepared transactions and then
switch off the two_phase option using Alter Subscription command.

-- 
With Regards,
Amit Kapila.