[zeromq-dev] Reliability question
sustrik at 250bpm.com
Mon Aug 2 09:18:09 CEST 2010
Hannes Schmidt wrote:
> Ok, I got it. I was erroneously working under the assumption that
> pub-sub in 0MQ is reliable. This assumption was not justified because
> the documentation did not say 0mq was reliable. That said, and as you
> already mentioned, it might help users to explicitly state that fact.
> Many messaging systems claim that they are reliable and users might
> initially expect reliability from every messaging system. Interestingly,
> the systems I have come across implement reliability either at great
> cost or simply reduce the chance of failure somewhat. After having read
> your suggestions, I started reading a book on this matter. It seems that
> ordered and reliable multicast messaging can and should be implemented
> on top of an unreliable, non-ordered implementation. I say 'should'
> because doing it that way nicely separates concerns and gives users a
> chance to pick and choose from different strengths of ordering and
> reliability constraints.
The main problem here is that with fully reliable multicast a single
hung up subscriber can cause hang-up of the whole system (by consuming
all the resources on the publisher node and thus bringing it to standstill).
That's the reason behind 0MQ's design of unreliable PUB/SUB.
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