Re: Taking into account syncrep position in flush_lsn reported by apply worker - Mailing list pgsql-hackers

On 8/13/24 06:35, Amit Kapila wrote:
> On Mon, Aug 12, 2024 at 3:43 PM Arseny Sher <ars@neon.tech> wrote:
>>
>> Sorry for the poor formatting of the message above, this should be better:
>>
>> Hey. Currently synchronous_commit is disabled for logical apply worker
>> on the ground that reported flush_lsn includes only locally flushed data
>> so slot (publisher) preserves everything higher than this, and so in
>> case of subscriber restart no data is lost. However, imagine that
>> subscriber is made highly available by standby to which synchronous
>> replication is enabled. Then reported flush_lsn is ignorant of this
>> synchronous replication progress, and in case of failover data loss may
>> occur if subscriber managed to ack flush_lsn ahead of syncrep.
>>
> 
> Won't the same can be achieved by enabling the synchronous_commit
> parameter for a subscription?

Nope, because it would force WAL flush and wait for replication to the
standby in the apply worker, slowing down it. The logic missing
currently is not to wait for the synchronous commit, but still mind its
progress in the flush_lsn reporting.

-- cheers, arseny