[zeromq-dev] ZMQ 3, multicasting and subscription forwarding

Martin Sustrik sustrik at 250bpm.com
Wed Aug 31 09:05:30 CEST 2011


Hi Emmanuel,

> I have a question about ZMQ-3, subscription forwarding and multicasting.
> Using ZMQ-3, with subscription forwarding, in a PUB-SUB framework,
> subscription filtering is done on the publisher side.
> I understand  this when using unicast network transport.
> However, if the transport is a multicasting protocol (epgm), I don't
> really understand how the filtering could be done by the publisher.
> In such a case, all subscribers and the publisher belong to the same
> multicast group. How is it possible to send a message to only some members
> of the same multicast group?

No, it is not possible. How it works is that the subscription is 
forwarded upstream if possible. If it gets to a hop where forwarding it 
further is not possible (eg. multicast) subscription lands there and 
starts filtering on the receiving side of the multicast transport.

For the detailed description of the mechanism have a look here:

http://www.250bpm.com/pubsub#toc5

Martin



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