PUB/SUB filtering on publisher side

[vsmelov]

How does per-topic filtering works?

When I use PUB/SUB and subscribe for some specific topics, does filtering happen on PUB side or on SUB side?
I cannot find any definite answer in ZMQ docs.

Here they say that it's done on SUB (what is inefficient)

https://learning-0mq-with-pyzmq.readthedocs.io/en/latest/pyzmq/patterns/pubsub.html

The current version of zmq supports filtering of messages based on topics at subscriber side. This is usually set via socketoption.

In the current versions of ØMQ, filtering happens at the subscriber side, not the publisher side.

But here they say that it's done on PUB side.

https://stackoverflow.com/questions/26533016/pyzmq-how-do-you-filter-at-publisher-side

#2068

How does it really work?
Can you add more explanations to ZMQ Guide?

Thank you :-)

[somdoron]

Both actually, most of the time. Subscriber is always filtering, publisher filter for most transports, but not all (e.g pgm filtering is only on subscriber side).

Subscriber filtering is done for two reasons:

• Not all transports support publisher-side filtering.
• Immediate unsubscribe - you might receive a message matching a subscription you just unsubscribed because the publisher didn't process the unsubscribe command yet.

[bluca]

Question answered, closing