Search Postgresql Archives

Re: Unexpected Backend PID reported by Notification

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 6/11/24 08:05, Dominique Devienne wrote:
Hi. I have a unit test using a single connection, that simulates a
client interacting with a server via a PostgreSQL "queue", i.e. a
non-writable table with SECURITY DEFINER procedures to mediate writes
to that table, with those PROC-initiated updates triggering
pg_notify() messages (via an UPDATE trigger).

The test is passing, I get all the side-effects and notifications I
expect. BUT...
For some reason, the backend_pid reported on the notification object
itself (i.e. PGnotify::be_pid),
is different from the one reported for the (sole) connection the unit
test is using (PQbackendPID()).

How can that be?
Are Stored PROCs running in a different backend?
Are Triggers running in a different backend?

Any doc pointers to explain this behavior?

https://www.postgresql.org/docs/current/sql-notify.html

"It is common for a client that executes NOTIFY to be listening on the same notification channel itself. In that case it will get back a notification event, just like all the other listening sessions. Depending on the application logic, this could result in useless work, for example, reading a database table to find the same updates that that session just wrote out. It is possible to avoid such extra work by noticing whether the notifying session's server process PID (supplied in the notification event message) is the same as one's own session's PID (available from libpq). When they are the same, the notification event is one's own work bouncing back, and can be ignored."

Looks to me like are seeing the correct thing, a client session that is different from the server process.


Thanks. --DD

PS: v14 server on RedHat; v16 libpq on Windows
PPS: Below's a snippet of my test code, which shows actual PID values:

     auto perreq_notif = c.notification();
     BOOST_REQUIRE(perreq_notif);
     BOOST_CHECK_EQUAL(perreq_notif.channel(), req.channel());
     /*
     ** In fact I get perreq_notif.backend_pid() == N + c.backend_pid() !!!
     ** Is the fact the pg_notify() is done from a trigger the reason???
     ** e.g. [4053957 != 4053955]
     BOOST_CHECK_EQUAL(perreq_notif.backend_pid(), c.backend_pid());
     */
     BOOST_CHECK_EQUAL(perreq_notif.payload(), "...");



--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx






[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Index of Archives]     [Postgresql Jobs]     [Postgresql Admin]     [Postgresql Performance]     [Linux Clusters]     [PHP Home]     [PHP on Windows]     [Kernel Newbies]     [PHP Classes]     [PHP Databases]     [Postgresql & PHP]     [Yosemite]

  Powered by Linux