Horton

RE: Slow catchup of 2PC (twophase) transactions on replica in LR

Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>

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From: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
To: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, 'Amit Kapila' <amit.kapila16@gmail.com>
Cc: Ajin Cherian <itsajin@gmail.com>, Давыдов Виталий <v.davydov@postgrespro.ru>, Heikki Linnakangas <hlinnaka@iki.fi>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2024-07-05T09:08:17Z
Lists: pgsql-hackers

Commits

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  1. Fix random failure in 021_twophase.

  2. Allow altering of two_phase option of a SUBSCRIPTION.

  3. Doc: use true|false rather than on|off for "failover" option

  4. Support an optional asynchronous commit mode, in which we don't flush WAL

Dear Amit,

Sorry, I forgot to say one content.

> > But that is not a good reason for this operation to stop workers
> > first. Instead, we should prohibit this operation if any worker is
> > present. The reason is that there is always a chance that if any
> > worker is alive, it can prepare a new transaction after we have
> > checked for the presence of any prepared transactions.
> 
> I used the function because it internally waits until all workers are exited.
> But OK, I modified like you suggested (logicalrep_workers_find() is used).

Based on the reason, after the above modification, test codes prior to v14
sometimes failed because backend could execute ALTER SUBSCRIPTION ... SET (two_phase).
So I added lines in test codes to poll until workers are exited, e.g.,

```
+# Alter subscription two_phase to false
+$node_subscriber->safe_psql('postgres',
+    "ALTER SUBSCRIPTION tap_sub_copy DISABLE;");
+$node_subscriber->poll_query_until('postgres',
+    "SELECT count(*) = 0 FROM pg_stat_activity WHERE backend_type = 'logical replication worker'"
+);
+$node_subscriber->safe_psql(
+       'postgres', "
+    ALTER SUBSCRIPTION tap_sub_copy SET (two_phase = false);
+    ALTER SUBSCRIPTION tap_sub_copy ENABLE;");
```

Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/