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6.2. Publish-Subscribe Channel

Overview
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A publish-subscribe channel, shown in Figure 6.2, “Publish Subscribe Channel Pattern”, 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 6.2. Publish Subscribe Channel Pattern

Publish subscribe channel pattern
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Publish subscribe channel pattern

Components that support publish-subscribe channel
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The following Apache Camel components support the publish-subscribe channel pattern:
  • JMS
  • ActiveMQ
  • XMPP
  • SEDA for working with SEDA in the same CamelContext which can work in pub-sub, but allowing multiple consumers.
  • VM as SEDA, but for use within the same JVM.

JMS
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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
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See chapter "JMS" in "Apache Camel Component Reference" for more details.

ActiveMQ
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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
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See chapter "ActiveMQ" in "Apache Camel Component Reference" for more details.

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

Static subscription lists
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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.

Java DSL example
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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");
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Note
This only works for the InOnly message exchange pattern.

XML configuration example
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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>
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