Horton

Re: Conflict detection for update_deleted in logical replication

Dilip Kumar <dilipbalaut@gmail.com>

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From: Dilip Kumar <dilipbalaut@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, shveta malik <shveta.malik@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-02-05T07:44:50Z
Lists: pgsql-hackers 
On Thu, Jan 23, 2025 at 5:17 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
I was reviewing v26 patch set and have some comments so far I reviewed
0001 so most of the comments/question are from this patch.

comments on v26-0001

1.
+ next_full_xid = ReadNextFullTransactionId();
+ epoch = EpochFromFullTransactionId(next_full_xid);
+
+ /*
+ * Adjust the epoch if the next transaction ID is less than the oldest
+ * running transaction ID. This handles the case where transaction ID
+ * wraparound has occurred.
+ */
+ if (oldest_running_xid > XidFromFullTransactionId(next_full_xid))
+ epoch--;
+
+ full_xid = FullTransactionIdFromEpochAndXid(epoch, oldest_running_xid);

I think you can directly use the 'AdjustToFullTransactionId()'
function here, maybe we can move that somewhere else and make that
non-static function.


2.
+ /*
+ * We expect the publisher and subscriber clocks to be in sync using time
+ * sync service like NTP. Otherwise, we will advance this worker's
+ * oldest_nonremovable_xid prematurely, leading to the removal of rows
+ * required to detect update_delete conflict.
+ *
+ * XXX Consider waiting for the publisher's clock to catch up with the
+ * subscriber's before proceeding to the next phase.
+ */
+ if (TimestampDifferenceExceeds(data->reply_time,
+    data->candidate_xid_time, 0))
+ ereport(ERROR,
+ errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("oldest_nonremovable_xid transaction ID may be advanced prematurely"),
+ errdetail("The clock on the publisher is behind that of the subscriber."));


I don't fully understand the purpose of this check. Based on the
comments in RetainConflictInfoData, if I understand correctly,
candidate_xid_time represents the time when the candidate is
determined, and reply_time indicates the time of the reply from the
publisher. Why do we expect these two timestamps to have zero
difference to ensure clock synchronization?

3.
+ /*
+ * Use last_recv_time when applying changes in the loop; otherwise, get
+ * the latest timestamp.
+ */
+ now = data->last_recv_time ? data->last_recv_time : GetCurrentTimestamp();

Can you explain in the comment what's the logic behind using
last_recv_time here?  Why not just compare 'candidate_xid_time' vs
current timestamp?

4.
Comment of v26-0004 doesn't clearly explain that once retention
stopped after reaching 'max_conflict_retention_duration' will it
resume back?


-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

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.