[zeromq-dev] Question about multiple Pub/Sub sockets

[Martin Sustrik]

Hi Gonzalo,

> I apologize in advance for the soup letter. Please bear with me for just
> three short paragraphs.
> 
> Say I have one process A, from which I wish to notify about certain
> events by using Pub/Sub sockets, so that I can decouple the notifier
> from the receivers of those notifications. A would generate events E, F
> and G.
> 
> On the receiver side, say I have processes P and Q; P is interested in
> events E and F, Q is interested in events F and G. I could have process
> A open three Pub sockets, for events E, F and G; then from P I would
> open Sub sockets E and F, and from Q I would open Sub sockets F and G.
> Presto! A can just go ahead and send out notifications, without worrying
> if there are 1, 2 or 10 processes listening to them.
> 
> Now, let's say I want to add a new process B, which generates events F,
> G and H. So for the case of event F, I now have two processes (A and B)
> generating the same event, and two processes (P and Q) interested in
> that event. How would I do this? Although P and Q can open the same Sub
> socket for event F, I don't think it would be possible for both A and B
> to open the same Pub socket for that event. Or am I wrong?
> 
> The only way I can think of implementing this pattern would be to have
> one intermediate process N which acts as the central notifier. A and B
> send all events to N (using Downstream sockets), which is now the ONLY
> process opening Pub sockets and sending out notifications. This would
> work, and it may even be "right"; the only concern would be that I now
> centralize all notifications in a single point of failure.

Yes. You are right. If there are multiple senders and multiple receivers 
you have to have a forwarder process in the middle otherwise you would 
have to manually setup all the connections between senders and receivers.

Et voila! There's such a process implemented in 0MQ already! Have a look 
at zmq_forwarder(7).

For the example of code have a look at chatroom example:

http://github.com/sustrik/zeromq-chat

(check README file for detailed instructions)

A side note: You may want to have "event types" managed automatically, 
guarantee the ordering etc. In such a case use a single PUB socket and 
tag the events by event type. For instance, message containing event of 
type A may look something like this:

A|my-event-specific-data

On the SUB side you can subscribe for a particular message types:

zmq_setsockopt (s, ZMQ_SUBSCRIBE, "A|", 2);
zmq_setsockopt (s, ZMQ_SUBSCRIBE, "B|", 2);

etc.

HTH
Martin