Re: Logical Replication of sequences

On Mon, 3 Nov 2025 at 16:44, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Mon, Nov 3, 2025 at 12:35 AM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> >
>
> Some inor comments on 0001.
> 1.
> + /*
> + * Acquire LogicalRepWorkerLock in LW_EXCLUSIVE mode to block the apply
> + * worker (holding LW_SHARED) from reading or updating
> + * last_seqsync_start_time. See ProcessSyncingSequencesForApply().
> + */
> + LWLockAcquire(LogicalRepWorkerLock, LW_EXCLUSIVE);
>
> Is it required to have LW_EXCLUSIVE lock here? In the function
> ProcessSyncingSequencesForApply(), apply_worker access/update
> last_seqsync_start_time only once it ensures that sequence sync worker
> has exited. I have made changes related to this in the attached to
> show you what I have in mind.

Modified

> 2.
> + /*
> + * Worker needs to process sequences across transaction boundary, so
> + * allocate them under long-lived context.
> + */
> + oldctx = MemoryContextSwitchTo(TopMemoryContext);
> +
> + seq = palloc0_object(LogicalRepSequenceInfo);
> …
> ...
> + /*
> + * Allocate in a long-lived memory context, since these
> + * errors will be reported after the transaction commits.
> + */
> + oldctx = MemoryContextSwitchTo(TopMemoryContext);
> + mismatched_seqs = lappend_int(mismatched_seqs, seqidx);
>
> At the above and other places in syncworker, we don't need to use
> TopMemoryContext; rather, we can use ApplyContext allocated via
> SequenceSyncWorkerMain()->SetupApplyOrSyncWorker()->InitializeLogRepWorker().

Modified

> 3.
> ProcessSyncingTablesForApply(current_lsn);
> + ProcessSyncingSequencesForApply();
>
> I am not sure if the function name ProcessSyncingSequencesForApply is
> appropriate. For tables, we do some work for concurrently running
> tablesync workers and launch new as well but for sequences, we don't
> do any work for sequences that are already being synced. How about
> ProcessSequencesForSync()?

Changed it to ProcessSequencesForSync

> 4.
> +      /* Should never happen. */
> +      elog(ERROR, "Sequence synchronization worker not expected to
> process relations");
>
> The first letter of the ERROR message should be small. How about:
> "sequence synchronization worker is not expected to process
> relations"? I have made this change in the attached.

Modified

> 5.
> @@ -5580,7 +5606,8 @@ start_apply(XLogRecPtr origin_startpos)
> * idle state.
> */
> AbortOutOfAnyTransaction();
> - pgstat_report_subscription_error(MySubscription->oid, !am_tablesync_worker());
> + pgstat_report_subscription_error(MySubscription->oid,
> + !am_tablesync_worker());
>
> Why this change?

This is not required, removed this change

> 6.
> @@ -264,6 +267,8 @@ extern bool
> logicalrep_worker_launch(LogicalRepWorkerType wtype,
> Oid userid, Oid relid,
> dsm_handle subworker_dsm,
> bool retain_dead_tuples);
> +extern void launch_sync_worker(LogicalRepWorkerType wtype, int nsyncworkers,
> + Oid relid, TimestampTz *last_start_time);
> extern void logicalrep_worker_stop(LogicalRepWorkerType wtype, Oid subid,
> Oid relid);
> All the other functions except the newly added one are from
> launcher.c. So, this one should be after those, no? It should be after
> the InvalidateSyncingRelStates() declaration.

Modified

> Apart from above, please find attached top-up patch to improve
> comments and some other cosmetic stuff.

Thanks, I have merged them.

The attached v20251103 patch has the changes for the same.

Regards,
Vignesh