Discovery refers to either a client (producer or consumer) establishing a 'transport connector' to the broker or a broker establishing 'network connector' to another broker without explicit static configuration of broker (IP or hostname).

The scheme 'multicast' refers to either listening for or advertising discovery events on a multicast address. zeroconf is similar to multicast, except for protocol specific to Apple's Rendezvous (bonjour).

Broker

   <networkConnectors>
     <networkConnector name="default" uri="multicast://default"/>
   </networkConnectors>

The above configuration in the broker xml file will make it automatically establish network connectors to other brokers that advertise themselves on the multicast address: when this broker gets a discovery event from another broker, the event has information that enables it to establish network connector to that broker.

Self-advertisement is facilitated by the following config:

 
  <transportConnectors>
      <transportConnector name="default" uri="tcp://localhost:61616" discoveryUri="multicast://default"/>
   </transportConnectors>

Client

An example usage of discovery on the client side (using the transport 'discovery' that uses discovery agent) ...

 <transportConnectors>
      <transportConnector name="default" uri="discovery:(multicast://default)"/>
   </transportConnectors>

ps: discovery (which is a confusing name) is just like a 'reliable' transport such as failover. If it is unable to connect to a broker, it will try another broker (assuming it
heard its advertisement over multicast).