Thread: LISTEN NOTIFY sometimes huge delay

Hi all,

I have a table with time series data and on this table a trigger for 
notifies:

containers_notify AFTER INSERT ON containers FOR EACH ROW EXECUTE 
PROCEDURE containers_notify('containers_notify_collector')

and the function does:

PERFORM pg_notify(CAST(TG_ARGV[0] AS text), row_to_json(NEW)::text);

so that another application (java) fetches every inserted row as a JSON 
for further processing every half second:

...listenStatement.execute("LISTEN 'containers_notify_collector'");
...PGNotification notifications[] = 
((org.postgresql.PGConnection)notifyPGCon.getUnderlyingConnection()).getNotifications(); 


This works as a charm but occasionally (I think with more load on the 
system) the notifications are received much time (up to hours!) after 
the INSERTs.
Nevertheless no notifications become lost, they are only very late! The 
delay grows, seems as a queue grows, but the java process tries to fetch 
the notifications fairly fast,
so there should be no queue growing..

Versions:
PostgreSQL 10.12 on x86_64-pc-linux-gnu, compiled by 
x86_64-pc-linux-gnu-gcc (Gentoo 6.4.0-r1 p1.3) 6.4.0, 64-bit
JDBC 42.2.23

The commit of the application inserting the data is ok/fast. So the 
insert of the data is not slowed down.
Are the notifications delivered asynchronously to the commit/trigger?

Thanks for any help,

Peter

"Peter Eser HEUFT [Germany]" <peter.eser@heuft.com> writes:
> I have a table with time series data and on this table a trigger for 
> notifies:
> containers_notify AFTER INSERT ON containers FOR EACH ROW EXECUTE 
> PROCEDURE containers_notify('containers_notify_collector')
> and the function does:
> PERFORM pg_notify(CAST(TG_ARGV[0] AS text), row_to_json(NEW)::text);

> This works as a charm but occasionally (I think with more load on the 
> system) the notifications are received much time (up to hours!) after 
> the INSERTs.
> Nevertheless no notifications become lost, they are only very late! The 
> delay grows, seems as a queue grows, but the java process tries to fetch 
> the notifications fairly fast,

Hm.  We've not previously had reports of late notifications.  One idea
that comes to mind is that the server won't deliver notifications as
long as the client has an open transaction, so is it possible your
listening process sometimes forgets to close its transaction?

> Versions:
> PostgreSQL 10.12 on x86_64-pc-linux-gnu, compiled by 
> x86_64-pc-linux-gnu-gcc (Gentoo 6.4.0-r1 p1.3) 6.4.0, 64-bit
> JDBC 42.2.23

That's pretty old.  We've made a number of changes to the LISTEN/NOTIFY
code since then; although in reading the commit log entries about them,
nothing is said about long-delayed notifications.

            regards, tom lane