Publish / Subscribe
A PUB channel will send messages to all of the
SUB channels wired to the same resource.
Push / Pull
A PUSH channel will send messages to one of
the PULL channels wired to the same resource.
Request / Reply
A REQ channel will send messages to one of
the REP channels wired to the same
resource. The response from the REP channel
is routed back to the original REQ channel.
