[zeromq-dev] XPUB subscription notifications

[Dan Goes]

Hi,

I've been using an XPUB ZMQ socket on a server with a collection of client SUBscribers, with zeromq-3.2.0. I've noticed that when a subscriber subscribes to a specific topic, if another subscriber has previously subscribed to that topic, no notification is generated on the server's XPUB socket. Digging around a little bit, I found that this is working as intended, and is built into the XPUB port (cf. comments on lines 75 and 76 of xpub.cpp in zeromq-3.2.0).

It isn't really useful to me to have the subscription message be a binary "is anybody/nobody subscribed to this topic?" notification. If the XPUB port were notified of every subscription and unsubscription, then users of the XPUB port would have *at least* the above binary functionality, in addition to knowing exactly how many are subscribed at any time. Is there a reason for this behavior? Can it be changed to notify of each subscription and unsubscription?

Thank you,

Dan
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