Re: [GENERAL] NOTIFY command impact - Mailing list pgsql-general

On 02/21/2017 02:43 PM, Rob Brucks wrote:
> That's a great thought!
>
> Comparing between systems (one where I don't issue "notify" and one where I do every minute) yields the same thing:
one8k file named "0000". 
>
> If that's truly where notifications are stored, then it looks like I should be good, at least for storage.  But I
wonderif that file is only used to store notify commands during shutdown/startup? 

 From the source of async.c:

During start or reboot, clean out the pg_notify directory.

I would guess the 0000 file is like a WAL file it is a pre-initialized
file filled with 0(?)'s

>
> Or if there are any considerations for memory usage…
>
> --Rob
>
> On 2/21/17, 4:38 PM, "Adrian Klaver" <adrian.klaver@aklaver.com> wrote:
>
>     On 02/21/2017 02:19 PM, Rob Brucks wrote:
>     > I did find a post a while back saying they were discarded, but I wanted
>     > to double-check.
>     >
>     >
>     >
>     > I performed some tests to see if listens worked AFTER the notify was
>     > issued, they were not.  This leads me to believe that the messages are
>     > discarded when a listen does not yet exist.
>
>     Seems the thing to do would be to monitor the size of :
>
>     $PG_DATA/pg_notify/
>
>     >
>     >
>     >
>     > --Rob
>     >
>     >
>     >
>     > *From: *"David G. Johnston" <david.g.johnston@gmail.com>
>     > *Date: *Tuesday, February 21, 2017 at 3:38 PM
>     > *To: *Adrian Klaver <adrian.klaver@aklaver.com>
>     > *Cc: *Rob Brucks <rob.brucks@rackspace.com>,
>     > "pgsql-general@postgresql.org" <pgsql-general@postgresql.org>
>     > *Subject: *Re: [GENERAL] NOTIFY command impact
>     >
>     >
>     >
>     > On Tue, Feb 21, 2017 at 2:17 PM, Adrian Klaver
>     > <adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>> wrote:
>     >
>     >     On 02/21/2017 01:07 PM, Rob Brucks wrote:
>     >
>     >
>     >         Do you see any long-term problems with constantly issuing "NOTIFY"
>     >         commands every 30 seconds without an associated "LISTEN" command?
>     >
>     >
>     >
>     >     Depending on how long 'long term' is:
>     >
>     >     https://www.postgresql.org/docs/9.6/static/sql-notify.html
>     >
>     >     "There is a queue that holds notifications that have been sent but
>     >     not yet processed by all listening sessions
>     >
>     >
>     >
>     > ​Its not clear in the OP that this is the case (it seems to be) but the
>     > documentation is non-specific as to what happens when "# of listeners" =
>     > 0;  I suspect that said messages are created and then immediately
>     > discarded - though apparently they do make it over to the standby server
>     > ​ too - and likely also immediately discarded there as well.
>     >
>     >
>     >
>     > David J.
>     >
>     >
>     >
>
>
>     --
>     Adrian Klaver
>     adrian.klaver@aklaver.com
>
>


--
Adrian Klaver
adrian.klaver@aklaver.com