Can pyzmq pub/sub sockets be used bidirectionally?

Question

I'm using a pyzmq pub/sub socket for a server to advertise notifications to client subscribers. It works nicely but I have a question:

Is there any way to use the same socket to send information back to the server? Or do I need a separate socket for that?

Use case: I just want to allow the server to see who's actively subscribing to notifications, so I was hoping I could allow clients to send back periodic "heartbeat" messages. I have a use case where if no clients are listening, I want the server to spawn one. (This is a multiprocess system that uses localhost only.)

Answer 1

You need a separate socket. From the ZMQ guide (http://zguide.zeromq.org/page:all#Pros-and-Cons-of-Pub-Sub):

Killing back-chatter is essential to real scalability. With pub-sub, it's how the pattern can map cleanly to the PGM multicast protocol, which is handled by the network switch. In other words, subscribers don't connect to the publisher at all, they connect to a multicast group on the switch, to which the publisher sends its messages.

In order for this to work, the PUB socket will not send back data to the subscribers (at least not in a way visible to the user. The heartbeating problem was discussed in-depth in the guide: http://zguide.zeromq.org/page:all#The-Asynchronous-Client-Server-Pattern

Also, check out the 7/MDP and 18/MDP protocols (http://rfc.zeromq.org/spec:7 -- this is also discussed in the guide) if you want to keep track of clients.