Re: Listen / Notify rewrite

On Thu, Nov 12, 2009 at 11:30 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Joachim Wieland <joe@mcknight.de> writes:
>> However I share Greg's concerns that people are trying to use NOTIFY
>> as a message queue which it is not designed to be.
>
> Yes.  Particularly those complaining that they want to have very large
> payload strings --- that's pretty much a dead giveaway that it's not
> being used as a condition signal.
>
> Now you might say that yeah, that's the point, we're trying to enable
> using NOTIFY in a different style.  The problem is that if you are
> trying to use NOTIFY as a queue, you will soon realize that it has
> the wrong semantics for that --- in particular, losing notifies across
> a system crash or client crash is OK for a condition notification,
> not so OK for a message queue.  The difference is that the former style
> assumes that the authoritative data is in a table somewhere, so you can
> still find out what you need to know after reconnecting.  If you are
> doing messaging you are likely to think that you don't need any backing
> store for the system state.
>
> So while a payload string for NOTIFY has been on the to-do list since
> forever, I have to think that Greg's got a good point questioning
> whether it is actually a good idea.

I think there could be cases where the person writing the code can
know, extrinsic to the system, that lost notifications are OK, and
still want to deliver a payload.  But I think the idea of enabling a
huge payload is not wise, as it sounds like it will sacrifice
performance for a feature that is by definition not essential to
anyone who is using this now.  A small payload seems like a reasonable
compromise.

...Robert