PostgreSQL has a useful feature where application programs can send
notifications to each other, this tends to be much "cheaper" than
periodically polling a table for changes.
I've had this working on various CPUs and OSes in a number of programs
since at least 2.2.4, but it appears to have been broken at some point
between 2.6.0 and 2.6.4 with problems persisting through to 3.0.0-rc1
and trunk. The specific fragment of code that's failing looks like this:
result := badPoll;
if PQStatus(DbTF.PQConnection1.Handle) <> CONNECTION_OK then
exit;
pqConsumeResult := PQconsumeInput(DbTF.PQConnection1.Handle);
if PQStatus(DbTF.PQConnection1.Handle) <> CONNECTION_OK then
exit;
result := okPollNo;
notification := PQnotifies(DbTF.PQConnection1.Handle);
if notification <> nil then
From 2.6.4 onwards PQnotifies() always returns nil, despite other
functionality behaving as expected.
I'm working on trying to pin this down a bit better, but would
appreciate any pointers from anybody who remembers working in this area.
The thing that usually causes problems for this feature is if the
database connection is closed and reopened, so that the server no longer
knows that the app wants to receive notifications. I haven't yet
researched what relevant info Postgres exposes.
--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk
[Opinions above are the author's, not those of his employers or colleagues]
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