[zeromq-dev] Reliability question

[Martin Sustrik]

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.

Martin