Re: [sca-assembly] [Issue 251] Some early thoughts

[Eric Johnson <eric@tibco.com>]

A quick response to this:

On 12/14/10 12:41 AM, Anish Karmarkar wrote:
> The pub-sub model is different than this. A consumer may express 
> interest in certain events, but there is absolutely no guarantee that 
> an event may ever be delivered to it. Similarly, a producer may 
> produce events, but there is not guarantee that any consumer is either 
> listening for those events or even if a consumer is listening, it may 
> decide to just drop it on the floor and not take any action based on 
> that event. Furthermore, these kind of connections are meant to be 
> many-to-many. As far as cardinality goes, the cardinality has to be 
> wrt how many channels the producer/consumer is connected to regardless 
> of how many consumer/producers are on those channels. This makes 
> cardinality in pub-sub tricky. As far as cardinality upper bound goes, 
> what is the difference between a consumer connected to 2 channels each 
> with 5 producers and the same consumer connected to a single channel 
> with 10 producers? 

Seems like the above notion confuses the M-to-N relationship of 
(publishers & subscribers) "attached" to a channel, with the different 
scenario of attaching a producer to multiple channels.  With the latter, 
in the generic case, the runtime will likely need to send messages to 
both channels - thus a receiving consumer will see the same message 
*twice* because a producer is wired to two channels, and the consumer is 
wired to the same two channels.  Consequently, I see a dramatic 
difference between allowing a consumer to consume from multiple 
channels, and allowing both a producer and a consumer to attach to 
multiple channels.

In the second case, I'm guaranteeing the ability to achieve a relatively 
useless outcome - a consumer who is ("best effort", or whatever quality 
of service is in use) guaranteed to receive two or more copies of the 
same event.

Why?

-Eric.