A publish-subscribe channel, shown in Figure 16, is a message channel that enables multiple subscribers to consume any given message. This is in contrast with a point-to-point channel. Publish-subscribe channels are frequently used as a means of broadcasting events or notifications to multiple subscribers.

Figure 16. Publish Subscribe Channel Pattern

The following Apache Camel components support the publish-subscribe channel pattern:

In JMS, a publish-subscribe channel is represented by a topic. For example, you can specify the endpoint URI for a JMS topic called StockQuotes as follows:

jms:topic:StockQuotes

See JMS in EIP Component Reference for more details.

In ActiveMQ, a publish-subscribe channel is represented by a topic. For example, you can specify the endpoint URI for an ActiveMQ topic called StockQuotes, as follows:

activemq:topic:StockQuotes

See ActiveMQ in EIP Component Reference for more details.

The XMPP (Jabber) component supports the publish-subscribe channel pattern when it is used in the group communication mode. See XMPP in EIP Component Reference for more details.

If you prefer, you can also implement publish-subscribe logic within the Apache Camel application itself. A simple approach is to define a static subscription list, where the target endpoints are all explicitly listed at the end of the route. However, this approach is not as flexible as a JMS or ActiveMQ topic.

The following Java DSL example shows how to simulate a publish-subscribe channel with a single publisher, seda:a, and three subscribers, seda:b, seda:c, and seda:d:

from("seda:a").to("seda:b", "seda:c", "seda:d");

	Note
	This only works for the InOnly message exchange pattern.

The following example shows how to configure the same route in XML:

<camelContext id="buildStaticRecipientList" xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="seda:a"/>
    <to uri="seda:b"/>
    <to uri="seda:c"/>
    <to uri="seda:d"/>
  </route>
</camelContext>