The fanout protocol enables a producer to auto-discover broker endpoints and broadcast topic messages to all of the discovered brokers. The fanout protocol gives producers a convenient mechanism for broadcasting messages to multiple brokers that are not part of a network of brokers.

The fanout protocol relies on a discovery agent to build up the list of broker URIs to which it connects.

Example 8.13 shows the syntax for a fanout URI.

fanout://(DiscoveryAgentUri)?Options

DiscoveryAgentUri is URI for the discovery agent used to build up the list of available brokers. Discovery agents are described in Discovery Agents.

The options, ?Options, are specified in the form of a query list. The discovery options are described in Table 8.2. You can also inject transport options as described in Setting options on the discovered transports.

	Tip
	If no options are required, you can drop the parentheses from the URI. The resulting URI would take the form fanout://DiscoveryAgentUri

The fanout protocol supports the transport options described in Table 8.2.

Example 8.14 shows a discovery URI that uses a multicast discovery agent.

fanout://(multicast://default)?initialReconnectDelay=100

The fanout protocol replicates topic messages by sending each topic message to all of the connected brokers. By default, however, the fanout protocol does not replicate queue messages.

For queue messages, the fanout protocol picks one of the brokers at random and sends all of the queue messages to that broker. This is a sensible default, because under normal circumstances, you would not want to create more than one copy of a queue message.

It is possible to change the default behavior by setting the fanOutQueues option to true. This configures the protocol so that it also replicates queue messages.

By default, the fanout protocol does not start sending messages until the producer has connected to a minimum of two brokers. You can customize this minimum value using the minAckCount option.

Setting minimum number of brokers equal to the expected number of discovered brokers ensures that all of the available brokers start receiving messages at the same time. This ensures that no messages are missed if a broker starts up after the producer has started sending messages.

You have to be careful when using the fanout protocol with brokers that are joined in a network of brokers.

The combination of the fanout protocol's broadcasting behavior and the nature of how messages are propagated through a network of brokers makes it likely that consumers will receive duplicate messages. If, for example, you joined four brokers into a network of brokers and connected a consumer listening for messages on topic hello.jason to broker A and connected a producer to broker B to send messages to topic hello.jason, the consumer would get one copy of the messages. If, on the other hand, the producer connects to the network using the fanout protocol, the producer will connect to every broker in the network simultaneously and start sending messages. Each of the four brokers will receive a copy of every message and deliver its copy to the consumer. So, for each message, the consumer will get four copies.