Horton

Re: Conflict detection for update_deleted in logical replication

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, vignesh C <vignesh21@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-05-16T05:45:01Z
Lists: pgsql-hackers 
On Fri, Apr 25, 2025 at 4:06 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> 2)
> in wait_for_local_flush(), we have
> should_stop_conflict_info_retention() before 'AllTablesyncsReady'
> check. Should we give a discount for table-sync time and avoid doing
> stop-conflict-retention when table-sync is going on? This is because
> table-sync is one time operation (or done only on
> subscription-refresh), so we shall not count time spent in table-sync
> for 'max_conflict_retention_duration'. We can reset our timer if
> table-sync is observed to be going on. Thoughts?
>

Sounds reasonable to me.

>
> 3)
> In get_candidate_xid(), we first set candidate_xid_time and later
> candidate_xid. And between these 2 there are chances that we return
> without updating candidate_xid. See 'Return if the
> oldest_nonremovable_xid cannot be advanced ' comment. That will leave
> 'candidate_xid_time' set to new value while  'candidate_xid' is not
> yet set.
>

Good point. I think we should set 'candidate_xid_time' along with
candidate_xid (just after setting candidate_xid).

> 4)
> Do you think there should be some relation between
> 'xid_advance_interval' and 'max_conflict_retention_duration'? Should
> max of  'xid_advance_interval' be limited by
> 'max_conflict_retention_duration'. Currently  say
> xid_advance_interval' is set to max 3 mins, what if
> 'max_conflict_retention_duration' is set to 2 mins? In that case we
> will not even check for new xids before 3 mins are over, while
> 'max_conflict_retention_duration' sets a  limit of 2 mins for dead
> tuples retention.
>

Right, ideally, the 'xid_advance_interval' should be set to a value
less than 'max_conflict_retention_duration' when no new_xid is found.

BTW, another related point is that when we decide to stop retaining
dead tuples (via should_stop_conflict_info_retention), should we also
consider the case that the apply worker didn't even try to get the
publisher status because previously it decided that
oldest_nonremovable_xid cannot be advanced due to its
OldestActiveTransactionId?

-- 
With Regards,
Amit Kapila.

Commits

  1. Fix intermittent BF failures in 035_conflicts.

  2. Resume conflict-relevant data retention automatically.

  3. Fix intermittent test failure introduced in 6456c6e2c4.

  4. Fix Coverity issue reported in commit a850be2fe.

  5. Add test to prevent premature removal of conflict-relevant data.

  6. Post-commit review fixes for 228c370868.

  7. Add max_retention_duration option to subscriptions.

  8. Detect and report update_deleted conflicts.

  9. Preserve conflict-relevant data during logical replication.

  10. Integrate FullTransactionIds deeper into two-phase code

  11. Improve checks for GUC recovery_target_timeline

  12. Prevent excessive delays before launching new logrep workers.

  13. Keep WAL segments by slot's last saved restart LSN

  14. Rework some code handling pg_subscription data in psql and pg_dump

  15. Use generateClonedIndexStmt to propagate CREATE INDEX to partitions.