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From: Andrey Borodin <[email protected]>
To: Bharath Rupireddy <[email protected]>
Cc: Laurenz Albe <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: SATYANARAYANA NARLAPURAM <[email protected]>
Subject: Re: An attempt to avoid locally-committed-but-not-replicated-to-standby-transactions in synchronous replication
Date: Mon, 9 May 2022 16:09:30 +0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CALj2ACWoB3k0kfjA7JxJgskVGGeE3jWzmGRPjP9QTRSCgSjhOg@mail.gmail.com>
References: <CALj2ACUrOB59QaE6=jF2cFAyv1MR7fzD8tr4YM5+OwEYG1SNzA@mail.gmail.com>
	<[email protected]>
	<CALj2ACWoB3k0kfjA7JxJgskVGGeE3jWzmGRPjP9QTRSCgSjhOg@mail.gmail.com>



> On 9 May 2022, at 14:20, Bharath Rupireddy <[email protected]> wrote:
> 
> On Tue, Apr 26, 2022 at 11:57 AM Laurenz Albe <[email protected]> wrote:
>> 
>> While this may mitigate the problem, I don't think it will deal with
>> all the cases which could cause a transaction to end up committed locally,
>> but not on the synchronous standby.  I think that only using the full
>> power of two-phase commit can make this bulletproof.
> 
> Not sure if it's recommended to use 2PC in postgres HA with sync
> replication where the documentation says that "PREPARE TRANSACTION"
> and other 2PC commands are "intended for use by external transaction
> management systems" and with explicit transactions. Whereas, the txns
> within a postgres HA with sync replication always don't have to be
> explicit txns. Am I missing something here?

COMMIT PREPARED needs to be replicated as well, thus encountering the very same problem as usual COMMIT: if done during failover it can be canceled and committed data can be wrongfully reported durably written. 2PC is not a remedy to the fact that PG silently cancels awaiting of sync replication. The problem arise in presence of any "commit". And "commit" is there if transactions are there.

> On 9 May 2022, at 14:44, Dilip Kumar <[email protected]> wrote:
> 
> IMHO, making it wait for some amount of time, based on GUC is not a
> complete solution.  It is just a hack to avoid the problem in some
> cases.

Disallowing cancelation of locally committed transactions is not a hack. It's removing of a hack that was erroneously installed to make backend responsible to Ctrl+C (or client side statement timeout).

> On 26 Apr 2022, at 11:26, Laurenz Albe <[email protected]> wrote:
> 
> Is it worth adding additional complexity that is not a complete solution?

Its not additional complexity. It is removing additional complexity that made sync rep interruptible. (But I'm surely talking not about GUCs like synchronous_replication_naptime_before_cancel - wait of sync rep must be indefinite until synchrous_commit\synchronous_standby_names are satisfied )

And yes, we need additional complexity - but in some other place. Transaction can also be locally committed in presence of a server crash. But this another difficult problem. Crashed server must not allow data queries until LSN of timeline end is successfully replicated to synchronous_standby_names.

Best regards, Andrey Borodin.

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