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From: Damien Dougan <damien.dougan@mobilecohesion.com>
Organization: Mobile Cohesion
To: Darko Prenosil <darko.prenosil@finteh.hr>, pgsql-interfaces@postgresql.org
Subject: Re: Asynchronous Communication across two independent components
Date: Tue, 25 Mar 2003 09:15:48 +0000
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	<200303242127.01355.darko.prenosil@finteh.hr>
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Darko,

Thanks for the response, but I'm not sure if your suggestions will meet my=
=20
needs - as the page you referenced seems to suggest that these operations=
=20
must all go over the same connection (and I have a Sender and a Receiver on=
=20
separate connections).

Some comments embedded below ...

On Monday 24 March 2003 9:27 pm, you wrote:

> You can send whole bunch of SQL commands using PQsendQuery, not only one !
> After that you can get more than one PGResults depending on what commands
> you send to Postgres. You only need to terminate it with ";".
> For example:
> PQSendQuery("SELECT * FROM pg_table; BEGIN TRANSACTION; DELETE FROM tbl
> WHERE id =3D 1; COMMIT;");
> When PQSendQuery command is executed, queries are just send to server.
> Non-blocking means: program will not block and wait for results to come !=
!!
> After that you should use combination of PQconsumeInput, PQisBusy and
> PQgetResult. You can even cancel rest of the queries with PQrequestCancel.
> It is all explained at:
> http://developer.postgresql.org/docs/postgres/libpq-async.html , and it
> works perfectly (spiking from my experience) !!!

I did have a read through this before posting - and I was hoping the above=
=20
mechanisms would do the trick for me - but there appears to be two problems=
=20
with the PQsendQuery and PQgetResult.

(1) You cannot call PQsendQuery on the same connection until PQgetResult ha=
s=20
returned null.

(2) You cannot call PQgetResult on a different connection from PQsendQuery =
to=20
obtain the results of a query.

So whilst your solution would be fine for asynchronous comms within a singl=
e=20
connection, I'm afraid its not what I'm looking for!

> If You need to wait to see if first command passes, and then send another,
> threads and non-blocking commands can't help You! Then You simple must
> execute with PQExec and wait for result !
>
> > I also looked at the LISTEN/NOTIFY, which seemed promising. However,
> > there does not appear to be any mechanism to identify the event that
> > caused the notification. I was hoping to be able to have the Receiver
> > handle the notification by going to the appropriate relation and picking
> > up the data which was now ready for it - but there's no way to know whi=
ch
> > entry it is!
>
> You can RAISE NOTICE inside of the function("stored procedure") with
> custom text. But You do not need this. PQsendQuery with multiple sql
> commands is what you need !
>

Thanks for the RAISE NOTICE heads-up - I'll take a look at this.

Damien