Subject: Re: logical decoding of two-phase transactions
To: Craig Ringer <craig@2ndquadrant.com>,
 Stas Kelvich <s.kelvich@postgrespro.ru>
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Cc: Michael Paquier <michael.paquier@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
 Simon Riggs <simon@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>,
 Tom Lane <tgl@sss.pgh.pa.us>
From: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Message-ID: <11fb4e6c-58f2-7a41-ea58-c285e53c8d19@2ndquadrant.com>
Date: Wed, 15 Mar 2017 08:42:03 +0100
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On 02/03/17 17:34, Petr Jelinek wrote:
> On 02/03/17 13:23, Craig Ringer wrote:
>> On 2 March 2017 at 16:20, Stas Kelvich <s.kelvich@postgrespro.ru> wrote:
>>>
>>>> On 2 Mar 2017, at 11:00, Craig Ringer <craig@2ndquadrant.com> wrote:
>>>>
>>>> We already have it, because we just decoded the PREPARE TRANSACTION.
>>>> I'm preparing a patch revision to demonstrate this.
>>>
>>> Yes, we already have it, but if server reboots between commit prepared (all
>>> prepared state is gone) and decoding of this commit prepared then we loose
>>> that mapping, isn’t it?
>>
>> I was about to explain how restart_lsn works again, and how that would
>> mean we'd always re-decode the PREPARE TRANSACTION before any COMMIT
>> PREPARED or ROLLBACK PREPARED on crash. But...
>>
>> Actually, the way you've implemented it, that won't be the case. You
>> treat PREPARE TRANSACTION as a special-case of COMMIT, and the client
>> will presumably send replay confirmation after it has applied the
>> PREPARE TRANSACTION. In fact, it has to if we want 2PC to work with
>> synchronous replication. This will allow restart_lsn to advance to
>> after the PREPARE TRANSACTION record if there's no other older xact
>> and we see a suitable xl_running_xacts record. So we wouldn't decode
>> the PREPARE TRANSACTION again after restart.
>>

Thinking about this some more. Why can't we use the same mechanism
standby uses, ie, use xid to identify the 2PC? If output plugin cares
about doing 2PC in two phases, it can send xid as part of its protocol
(like the PG10 logical replication and pglogical do already) and simply
remember on downstream the remote node + remote xid of the 2PC in
progress. That way there is no need for gids in COMMIT PREPARED and this
patch would be much simpler (as the tracking would be left to actual
replication implementation as opposed to decoding). Or am I missing
something?

-- 
  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services


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