Detecting slow readers with zmq(zeromq)

Question

I'm trying to replace a small homegrown messaging system, and are playing around a bit with zmq . I'll be needing to detect slow readers, and boot/disconnect them - slow readers pretty much meaning a particular consumer whos queue size is above a certain threshold.

So far it seems zmq blocks every consumer if one of them is a bit slow (fair enough) - but I can't find any way to detect a potential slow consumer. Anyone have any experience with wether and how this is possible with zmq - or have any other broker-less messaging system to recccommend ?

Answer 1

As of zeromq-2.0.7, you can set the ZMQ_HWM option on a ZMQ_PUB socket to control the maximum number of messages that can be queued for a subscriber. Once the high-water mark has been reached, all further messages destined for that subscriber will be dropped until the queue size drops back below the high-water mark. This limits the amount of memory dedicated to what you call a slow reader.

However, because the ZeroMQ library exposes sockets, not clients, there is no way for you to identify and forcibly disconnect unwanted clients without modifying the library itself.