Re: [EXT] Re: NOTIFY

Same page:

"Secondly, if a listening session receives a notification signal while it
is within a transaction, the notification event will not be delivered to
its connected client until just after the transaction is completed (either
committed or aborted)."

I can see where "processed" can be misleading but it is referring to an
implementation detail within the depths of the inter-session
communications. At the application layer if your application client
connection has been delivered the notification your session is considered
to have processed it and will, apparently, report that delivery to the
application client connection back to the queue.

Maybe: "There is a queue that holds notifications that have been sent to,
but yet delivered by, all listening sessions." Though we'd need to ensure
that the target of the delivery (i.e., the application connection) is
clear...

David J.

On Mon, Sep 28, 2020 at 5:33 PM Greg Rychlewski (LCL) <
Greg(dot)Rychlewski(at)loblaw(dot)ca> wrote:

> Thank you. For number 2 my wording was poorly chosen, but I was thinking
> about this statement in the documentation:
>
>
>
> “There is a queue that holds notifications that have been sent but not
> yet processed by all listening sessions.”
>
>
>
> I’m not sure how the queue is notified that the notification has been
> processed. I was referring to that as the “acknowledgement”.
>
>
>
> *From: *"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>
> *Date: *Monday, September 28, 2020 at 8:07 PM
> *To: *"Greg Rychlewski (LCL)" <Greg(dot)Rychlewski(at)loblaw(dot)ca>
> *Cc: *"pgsql-novice(at)postgresql(dot)org" <pgsql-novice(at)postgresql(dot)org>
> *Subject: *[EXT] Re: NOTIFY
>
>
>
> *CAUTION:** External email. Do not click links or open attachments unless
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>
> On Mon, Sep 28, 2020 at 3:35 PM Greg Rychlewski (LCL) <
> Greg(dot)Rychlewski(at)loblaw(dot)ca> wrote:
>
> Hi,
>
>
>
> I’m considering using NOTIFY to let my application react to changes in a
> table. If I only send the channel name without a payload, could I run into
> this issue:
>
>
>
> 1. The table is updated and a notification is sent to my app
> 2. My app reads the table, but hasn’t sent acknowledgement of the
> notification yet
> 3. The table is updated again, but the notification gets dropped
> because it’s the same as (1), which hasn’t been acknowledged by the app
>
>
>
> If this happened then there would be new data in my table but my app
> wouldn’t read it until the next time there was an update.
>
>
>
> I don't understand where you are getting point 2 from - the pub/sub model
> doesn't require subscribers to acknowledge the notifications.
>
>
>
> For point 3 you seem to have mis-read the NOTIFY documentation, which
> states:
>
>
>
> "If the same channel name is signaled multiple times with identical
> payload strings within the same transaction, only one instance of the
> notification event is delivered to listeners."
>
>
>
> The part about "within the same transaction" solves your concurrency
> concern, and doesn't say anything about acknowledgements.
>
>
>
> David J.
>
>
>
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