I'm writing server using python and ZMQ. REQ-REP pattern. Sometimes, if the server crashes or restarts, then it stops seeing any messages sent to it at all. What could be the problem?
Q : What could be the problem?

The nature of a principally unsafe, mutual-deadlock prone distributed-Finite-State-Automaton is.
Given the REP dies during the REQ waiting for a response, deadlock
Given the REQ dies during the REP waiting for a request, chances are to restore REQ/REP quickstep
Given the REQ dies before the REP sends the response, deadlock
Given the REQ dies during the REP sending the response, deadlock
Given the REP dies before the REQ sending a request, chances are to restore REQ/REP quickstep
ZMQ server doesn't see any messages