Re: Asynchronous Communication across two independent components

On Tuesday 25 March 2003 09:15, Damien Dougan wrote:
> Darko,
>
> Thanks for the response, but I'm not sure if your suggestions will meet my
> needs - as the page you referenced seems to suggest that these operations
> must all go over the same connection (and I have a Sender and a Receiver on
> separate connections).
>
How about sharing connections between sender and receiver. After all
PGConnection is only pointer that can be same for Sender and Receiver.
How do you mean to connect Sender and Receiver ? How should postgres know
which Sender will post results to which Receiver ?

The only way to have sender and receiver on different connections is to write
some kind of own server that will connect server and receiver to "Pair", but
I did newer heard for such "construction" in SQL server philosophy.

It is also possible to store the "results" from Sender to tables, and
Receiver can periodically check if something happened and get the results!

It all seems a little fuzzy to me. Maybe You can explain it with more details.

> 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 = 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
> mechanisms would do the trick for me - but there appears to be two problems
> with the PQsendQuery and PQgetResult.
>
> (1) You cannot call PQsendQuery on the same connection until PQgetResult
> has returned null.

That is true, but why do you wish to send another query if you send them all
in one PQSendQuery ?

>
> (2) You cannot call PQgetResult on a different connection from PQsendQuery
> to obtain the results of a query.
>
> So whilst your solution would be fine for asynchronous comms within a
> single 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 which 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.
>
Unfortunately RAISE NOTICE can only function through same connection, and if
You insist on different connection, this will not help.

> Damien
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 5: Have you checked our extensive FAQ?
>
> http://www.postgresql.org/docs/faqs/FAQ.html