On Fri, 2018-02-23 at 04:57 +0000, Mark wrote: > Hi all. > > According to http://api.zeromq.org/4-2:zmq-socket with a ZMQ_REQ > socket I can connect to multiple endpoints and the semantics of > zmq_send() are: > > "Each request sent is round-robined among all services, and > each reply received is matched with the last issued request. > > If no services are available, then any send operation on the > socket shall block until at least one service becomes > available. The REQ socket shall not discard messages." > > IOWs the message should get to one of the connected endpoints > eventually. > > But I'm not seeing that behavior. Rather, the message never arrives > at > any of the available endpoints. > > To demonstrate this I hacked up the hello world examples such that > the > client connects to two endpoints: the first nominates a port that > nothing is listening on and the second nominates a port that the > server is listening on. What I'm seeing is that the message exchange > never occurs. > > I have attached both programs as they are minorly different from the > examples on the zmq website. > > If I swap the connect order in the client such that the "up" server > comes first, then the message exchange works. After further testing I > deduce that attempting to exchange with second and subsequent connect > endpoints are the problem.
That's because it's round-robin, and the connection is async - so it will wait on the first server to respond, and it never does so it's blocked there. Sounds like what you really want is "fail-over" - IE if the first does not respond, try the second. That might work if you tune the tcp reconnect options to have a small timeout, so that the pipe is removed - by default it's quite large. Note sure it will work - try it. -- Kind regards, Luca Boccassi
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