Julian Edwards wrote: > On Thursday 08 October 2009 22:21:20 Aaron Bentley wrote: >> I don't think so. It's no coincidence that Muharem presented a Job system >> as a use case at the Epic. Polling will never be as responsive or efficient >> as messaging. > > I disagree with the word "never". > > I am a huge fan of messaging systems and I have a lot of experience > with them and also with polling-based solutions. > > Consider the situation where you have several updates causing diff > requests on the same branch. You're only interested in the last > change; with a message for each diff request, calculating the diffs > in-between is a waste as you will throw them away with subsequent diff > processing. If you were to periodically poll, you would see only the > most recent change and work with that. > > This is a slightly contrived example but you get my drift.
A very interesting example .. usually solved with a counting
queue ingress/egress filter i.e. one that maintains a "lookup
table" that answers the following question:
How many messages with a given ID do I have in the queue of
interest?
The "lookup table" counters are incremented/decremented upon
enqueueing/dequeueing the messages.
A dequeued message is *processed* if the "lookup table" counter for its
ID is zero and discarded otherwise.
> Basically, there is a trade-off between responsiveness and efficient
> processing.
While I tend to agree, I'd also like to point out that messaging is a
vast area and that there's a (well known) pattern for almost every
situation :)
<my$0.02>
IMHO polling is not a good approach in 99.9% of all cases unless
you're building a system with (pseudo) real-time properties.
</my$0.02>
Best regards
--
Muharem Hrnjadovic <[email protected]>
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