RE: [HACKERS] Transactions involving multiple postgres foreign servers

From: "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>
To: 'Masahiko Sawada' <sawada(dot)mshk(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Antonin Houska <ah(at)cybertec(dot)at>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: RE: [HACKERS] Transactions involving multiple postgres foreign servers
Date: 2017-12-28 02:40:37
Message-ID: 0A3221C70F24FB45833433255569204D1F87302E@G01JPEXMBYT05
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From: Masahiko Sawada [mailto:sawada(dot)mshk(at)gmail(dot)com]
> I've updated documentation of patches, and fixed some bugs. I did some
> failure tests of this feature using a fault simulation tool[1] for
> PostgreSQL that I created.
>
> 0001 patch adds a mechanism to track of writes on local server. This is
> required to determine whether we should use 2pc at commit. 0002 patch is
> the main part. It adds a distributed transaction manager (currently only
> for atomic commit), APIs for 2pc and foreign transaction manager resolver
> process. 0003 patch makes postgres_fdw support atomic commit using 2pc.
>
> Please review patches.

I'd like to join the review and testing of this functionality. First, some comments after taking a quick look at 0001:

(1)
Why don't you use the existing global variable MyXactFlags instead of the new TransactionDidWrite? Or, how about using XactLastRecEnd != 0 to determine the transaction did any writes? When the transaction only modified temporary tables on the local database and some data on one remote database, I think 2pc is unnecessary.

(2)
If TransactionDidWrite is necessary, I don't think you need to provide setter functions, because other transaction state variables are accessed globally without getter/setters. And you didn't create getter function for TransactionDidWrite.

(3)
heap_multi_insert() doesn't modify TransactionDidWrite. Is it sufficient to just remember heap modifications? Are other modifications on the coordinator node covered such as TRUNCATEand and REINDEX?

Questions before looking at 0002 and 0003:

Q1: Does this functionality work when combined with XA 2pc transactions?

Q2: Does the atomic commit cascade across multiple remote databases? For example:
* The local transaction modifies data on remote database 1 via a foreign table.
* A trigger fires on the remote database 1, which modifies data on remote database 2 via a foreign table.
* The local transaction commits.

Regards
Takayuki Tsunakawa

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