One ZeroMQ socket per thread or per call?

Question

As we all know, a ZeroMQ socket shall not be shared among application threads.
context_t instances however can.

I have a multi-threaded-application a

Answer 1

An alternative could be having one dedicated thread for the ZeroMQ communication with some FIFO queue (must be guarded with a mutex or similar, of course...). This dedicated thread should be sleeping as long as the queue is empty and wake up (being signalled appropriately) whenever this state changes.

Depending on general needs, whenever the response for some outgoing message is received, the dedicated thread could simply call some callback (at some dedicated object per thread); be aware that you have a different thread context then, so you might need some means of synchronisation to prevent race conditions.

Alternatively, the sending threads could just wait for the response, being signalled by the ZeroMQ thread on received response (well, this actually is one of those means to prevent race conditions...).