What reliability guarantees (if any) does ZMQ make for PUB/SUB over epgm?

Question

I've got an app sending messages on an epgm PUB socket to one or more epgm SUB sockets. Things mostly work, but if a subscribing application is left up long enough, it will generally end up missing a message or a few messages. (My messages have sequence numbers, so I can tell if any are missing or out of order.) Based on my reading of the ZMQ docs, I would have thought that the "reliable multicast" nature of epgm would prevent this from happening, that after a SUB socket gets one message, it's guaranteed to keep getting them until shutdown or until major network troubles (ie, the connection is maxed out).

Anyway, that's the context, but the question is simply the title: What reliability guarantees (if any) does ZMQ make for PUB/SUB over epgm?

Answer 1

ZeroMQ makes exactly one guarantee: all messages are complete - you will never receive partial messages. It makes no guarantee of reliability. You should check out the documentation of the high water mark (HWM) behavior, which is the most common cause for dropped messages, as illustrated by the suicidal snail.