Folks,
I have an application that would I would like to react immediately to
changes in a PostgreSQL database. I would like to use a combination of
TRIGGERs and LISTEN/NOTIFY to achieve this.
I am connecting to my database using:
(define (NH . a)
(printf "NOTIFICATION: ~s~%" a))
(define pgc
(postgresql-connect
#:server "my-host"
#:user "me"
#:database "the-db"
#:password "souper secret password"
#:notification-handler NH))
Then setting up a “listener” with:
(query pgc "LISTEN x")
;; (query pgc "NOTIFY x, 'woo'") ; [1]
If I issue a "NOTIFY x;" on an psql prompt... nothing happens.
Until I do a database query (even “SELECT 1” is good enough).
I can therefore “poll” the database for notifications with:
(define (poll-notify)
(query pgc "select 1")
(sleep 3)
(poll-notify))
(thread poll-notify)
But I want NH to be called as soon (as possible) as the “NOTIFY” is
issued (subject to the caveats in [2]). This either to happen
automagically OR by being able to obtain an evt that I can sync on.
I can find a place where the handlers are delayed -- I assume due to
being inside a transaction or lock -- but I cannot find a handle to
anything syncable.
Any help would be appreciated.
[1] Ironically: if this query is included, NH is called, because the
notifcation is posted before the query returns.
[2] http://www.postgresql.org/docs/9.4/static/sql-notify.html
Regards,
Tim
--
Tim Brown CEng MBCS <tim.br...@cityc.co.uk>
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