Re: speed up a logical replica setup

On Mon, Jun 24, 2024, at 3:47 AM, Hayato Kuroda (Fujitsu) wrote:
> > pg_createsubscriber fails on a dbname containing a space. Use
> > appendConnStrVal() here and for other params in get_sub_conninfo(). See the
> > CVE-2016-5424 commits for more background. For one way to test this
> > scenario,
> > see generate_db() in the pg_upgrade test suite.
>
> Thanks for pointing out. I made a fix patch. Test code was also modified accordingly.

Patch looks good to me. I have one suggestion

-# Mostly Ported from 002_pg_upgrade.pl, but this returns a generated dbname.
+# Extracted from 002_pg_upgrade.pl.

and 2 small fixes:

- 'logical replication works on database $db1');
+ "logical replication works on database $db1");

-is($result, qq(row 1), 'logical replication works on database $db2');
+is($result, qq(row 1), "logical replication works on database $db2");

> > > +static char *
> > > +create_logical_replication_slot(PGconn *conn, struct LogicalRepInfo *dbinfo)
> > > +{
> > > + PQExpBuffer str = createPQExpBuffer();
> > > + PGresult *res = NULL;
> > > + const char *slot_name = dbinfo->replslotname;
> > > + char *slot_name_esc;
> > > + char *lsn = NULL;
> > > +
> > > + Assert(conn != NULL);
> > > +
> > > + pg_log_info("creating the replication slot \"%s\" on database \"%s\"",
> > > + slot_name, dbinfo->dbname);
> > > +
> > > + slot_name_esc = PQescapeLiteral(conn, slot_name, strlen(slot_name));
> > > +
> > > + appendPQExpBuffer(str,
> > > + "SELECT lsn FROM
> > pg_catalog.pg_create_logical_replication_slot(%s, 'pgoutput', false, false, false)",
> >
> > This is passing twophase=false, but the patch does not mention prepared
> > transactions. Is the intent to not support workloads containing prepared
> > transactions? If so, the documentation should say that, and the tool likely
> > should warn on startup if max_prepared_transactions != 0.
>
> IIUC, We decided because it is a default behavior of logical replication. See [1].
> +1 for improving a documentation, but not sure it is helpful for adding output.
> I want to know opinions from others.

Documentation says

When two-phase commit is enabled, prepared transactions are sent to the
subscriber at the time of PREPARE TRANSACTION, and are processed as two-phase
transactions on the subscriber too. Otherwise, prepared transactions are sent to
the subscriber only when committed, and are then processed immediately by the
subscriber.

Hence, the replication should be working for prepared transactions even if it
created the slot with twophase = false. IIRC the user won't be able to change it
later. As Amit said in a previous email, once the command
ALTER SUBSCRIPTION ... SET (two_phase = on) is supported, users can change it
after running pg_createsubscriber. The other option is to add a command-line
option to enable or disable it.

--
Euler Taveira
EDB https://www.enterprisedb.com/