title: zmq pattern for reliable multicast
body: <p>I am struggling to work out how to use zmq to implement the architecture I need. I have a classic publish/subscribe situation except that once client x has subscribed to a topic I need the topic data to be sent to it to be cached if the client dies and resent on reconnect. The data order is important and I can't miss messages should the client be offline for a while.</p>
  
  <p>The PUB/SUB pattern doesn't seem to know about individual clients and will just stop sending to client x if it dies. Plus I can't find out this has happened and cache the messages, or know when it reconnects.</p>
  
  <p>To try to get around this I used the REQ/REP pattern so the clients can announce themselves and have some persistence but this is not ideal for a couple of reasons:</p>
  
  <p>1) The clients must constantly ask "got any data for me?" which offends my sensibilities</p>
  
  <p>2) What happens if there's no data to send to client x but there is to client y? Without zmq I'd have had a thread per client and simply block the one with no data but I can't block client x without also blocking client y in a single thread. </p>
  
  <p>Am I trying to shove a round peg in a square hole, here? Is there some way I can get feedback from PUB saying 'failed to send to client x'? so I can cache the messages instead? Or is there some other pattern I should be using?</p>
  
  <p>Otherwise it's back to low level tcp for me...</p>
  
  <p>Many thanks;</p>
  
  <p>Jeremy</p>