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Re: Questions about Listen/Notify mechanism

From: Chris Wareham <cwareham(at)visitlondon(dot)com>
To: "max" <maxbox(at)directbox(dot)com>
Cc: <pgsql-jdbc(at)postgresql(dot)org>
Subject: Re: Questions about Listen/Notify mechanism
Date: 2010-07-19 11:23:16
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-jdbc
On 07/18/10 03:33, max wrote:
> Hallo,
> I need to write a Java app which deals with as low as possible network
> resources. That's why the database should inform my Java app about
> changes and I does not have to poll the database every 5 seconds to
> recognize them.
> I read the Postgresql-JDBC documentation about the Listen/Notify
> commands. I do not understand that note
> "A key limitation of the JDBC driver is that it cannot receive
> asynchronous notifications and must poll the backend to check if any
> notifications were issued."
> Does that mean that the driver will poll the database, e.g. every 5
> seconds? If so, that would not be acceptable for me, because that is
> exactly what I want to avoid. The only advantage of using Listen/Notify
> in that case would be that I don't need an extra notification table that
> my trigger would fill. Am I wrong? Oo
> I hope somebody can help me with that.
> Regard,
> Max

Hi Max,

Sounds like you need a message queue in order to implement an event
driven architecture, which would eliminate polling entirely. I'm going
to be looking into this shortly for a project at work, and I'm hoping
to implement the database part of it using Skytools PgQ:

I'm planning on pushing messages to a generic queuing package (HornetQ
is my preferred choice), with AMQP as the message protocol. From what
I've read, this is definitely one of the intended uses of PgQ, although
descriptions of how it can be accomplished seems to be limited to a a
presentation from PGCon 2010 and a handful blog posts at the moment.


Chris Wareham
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