RE: BUG #16481: Stored Procedure Triggered by Logical Replication is Unable to use Notification Events

Just to help those who find themselves in the same situation, there is a simple application-level workaround which consists in listening to the notifications in the replica when they are issued by the master and vice versa.

We are programming in .NET and we use a code like this:

Code in the replica side:

var csb = new NpgsqlConnectionStringBuilder
{
Host = "master",
Database = "MasterDB",
Port = 5432,
Username = "postgres",
Password = "XXXXXXX",

};
var connection = new NpgsqlConnection(csb.ConnectionString);
connection.Open();
using (var command = new NpgsqlCommand("listen \"Table\"", connection))
{
command.ExecuteNonQuery();
}
connection.Notification += PostgresNotification;

So you can listen from the replica every changed raised by a trigger on the master from the replica side on the table "Table".

CREATE TRIGGER master_trigger
AFTER INSERT OR DELETE OR UPDATE
ON public."TABLE"
FOR EACH ROW
EXECUTE PROCEDURE public.master_notice();

ALTER TABLE public."Tabele"
ENABLE ALWAYS TRIGGER master_trigger;

CREATE FUNCTION public. master_notice ()
RETURNS trigger
LANGUAGE 'plpgsql'
COST 100
VOLATILE NOT LEAKPROOF
AS $BODY$
BEGIN
PERFORM pg_notify('Table', Cast(NEW."ID" as text));
RETURN NEW;
END;
$BODY$;

ALTER FUNCTION public.incupdate1_notice()
OWNER TO postgres;

I hope that help someone, because the bug last from "years". I tried in version 10 11 and 12, so it is present since 2017-10-05 and I can't see any solution on 13 beta.

Best Regards.
Fabio.

-----Original Message-----
From: Vianello Fabio
Sent: lunedì 8 giugno 2020 11:14
To: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Cc: pgsql-bugs(at)lists(dot)postgresql(dot)org; pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: RE: BUG #16481: Stored Procedure Triggered by Logical Replication is Unable to use Notification Events

Hi Kyotaro Horiguchi, thanks for you helps.
We have a question about the bug. Why there isn't any solution in the HEAD?

This bug last since 10.4 version and I can't understand why it is not fixed in the HEAD yet.

BR.
Fabio Vianello.

-----Original Message-----
From: Kyotaro Horiguchi [mailto:horikyota(dot)ntt(at)gmail(dot)com]
Sent: lunedì 8 giugno 2020 10:28
To: Vianello Fabio <fabio(dot)vianello(at)salvagninigroup(dot)com>; pgsql-bugs(at)lists(dot)postgresql(dot)org; pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: BUG #16481: Stored Procedure Triggered by Logical Replication is Unable to use Notification Events

Hello.

It seems to me a bug.

At Fri, 05 Jun 2020 11:05:14 +0000, PG Bug reporting form <noreply(at)postgresql(dot)org> wrote in
> The following bug has been logged on the website:
>
> Bug reference: 16481
> Logged by: Fabio Vianello
> Email address: fabio(dot)vianello(at)salvagninigroup(dot)com
> PostgreSQL version: 12.3
> Operating system: Windows 10
> Description:
>
> About the bug BUG #15293, on PostgreSQL version 10.4 and 11.2 as
> describe below, we found the same issue on the PostgreSQL version 12.3.

The HEAD behaves the same way.

> Is it a feature?
> Becasue in the documentation we didn't found any constraint that says
> that we can not use NOTIFY/LISTEN on logical replication tables.
>
> "When using logical replication a stored procedure executed on the
> replica is unable to use NOTIFY to send messages to other listeners.
> The stored procedure does execute as expected but the pg_notify()
> doesn't appear to have any effect. If an insert is run on the replica
> side the trigger executes the stored procedure as expected and the
> NOTIFY correctly notifies listeners.

The message is actually queued, but logical replication worker doesn't signal that to listener backends. If any ordinary session sent a message to the same listener after that, both messages would be shown at once.

That can be fixed by calling ProcessCompletedNotifies() in apply_handle_commit. The function has a code to write out notifications to connected clients but it doesn't nothing on logical replication workers.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center
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