e data exchange
 defined by their protocol:
</p><ul><li> client/server: The client, first, sends data to the server
                    (the request) and, then, 
                    the server sends data to this client
                    (the reply);
</li><li> publish and subscribe: The publisher sends data to all subscribers
                           (the topic);
</li><li> pusher/worker: The pusher sends data to the worker (the request)
                   without receiving a reply.
</li></ul><p>We call t