Horton

Re: Conflict detection for update_deleted in logical replication

Amit Kapila <amit.kapila16@gmail.com>

View thread View on postgr.es
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: 2025-05-20T11:41:06Z
Lists: pgsql-hackers

Attachments

On Fri, May 16, 2025 at 5:01 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>

Please find some more comments on the 0001 patch:
1.
We need to know about such transactions
+ * for conflict detection and resolution in logical replication. See
+ * GetOldestTransactionIdInCommit and its use.

Do we need to mention resolution in the above sentence? This patch is
all about detecting conflict reliably.

2. In wait_for_publisher_status(), we use remote_epoch,
remote_nextxid, and remote_oldestxid to compute full transaction id's.
Why can't we send FullTransactionIds for remote_oldestxid and
remote_nextxid from publisher? If these are required, maybe a comment
somewhere for that would be good.

3.
/*
+ * Note it is important to set committs value after marking ourselves as
+ * in the commit critical section (DELAY_CHKPT_IN_COMMIT). This is because
+ * we want to ensure all such transactions are finished before we allow
+ * the logical replication client to advance its xid which is used to hold
+ * back dead rows for conflict detection. See
+ * maybe_advance_nonremovable_xid.
+ */
+ committs = GetCurrentTimestamp();

How does setting committs after setting DELAY_CHKPT_IN_COMMIT help in
advancing client-side xid? IIUC, on client-side, we simply wait for
such an xid to be finished based on the remote_oldestxid and
remote_nextxid sent via the server. So, the above comment is not
completely clear to me. I have updated this and related comments in
the attached diff patch to make it clear. See if that makes sense to
you.

4.
In 0001's commit message, we have: "Furthermore, the preserved commit
timestamps and origin data are essential for
consistently detecting update_origin_differs conflicts." But it is not
clarified how this patch helps in consistently detecting
update_origin_differs conflict?

5. I have tried to add some more details in comments on why
oldest_nonremovable_xid needs to be FullTransactionId. See attached.

-- 
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.