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Please note that the hostnames used for fedora and solr here are entirely arbitrary. It is quite likely that these systems will be deployed on separate hosts and that the Camel routes will be deployed on yet another host. Camel makes it easy to distribute applications and replicate data asynchronously across multiple hosts

Message Headers

By default, Fedora publishes events to a topic on a local broker. This topic is named "fedora". Each message will contain an empty body and up to five different header values. Those header values are namespaced so they look like this:

  • org.fcrepo.jms.identifier
  • org.fcrepo.jms.eventType
  • org.fcrepo.jms.properties
  • org.fcrepo.jms.timestamp
  • org.fcrepo.jms.baseURL

Both eventType and properties are comma-delimited lists of events or properties. The eventTypes follow the JCR 2.0 specification and include:

 

 

The properties field will list the RDF properties that changed with that event. NODE_REMOVED events contain no properties. The fcrepo component for Camel is configured to recognize these headers and act appropriately.

Examples

 

Supporting Queues

The default configuration is fine for locally-deployed listeners, but it can be problematic in a distributed context. For instance, if the listener is restarted while a message is sent to the topic, that message may be missed. Furthermore, if there is a networking hiccup between fedora's local broker and the remote listener, that too can result in lost messages. Instead, in this case, a queue may be better suited.

ActiveMQ supports “virtual destinations”, allowing your broker to automatically forward messages from one location to another. If fedora4 is deployed in Tomcat, the ActiveMQ configuration will be located in WEB-INF/classes/config/activemq.xml. That file can be edited to include the following block:
Code Block
languagexml
<destinationInterceptors>
  <virtualDestinationInterceptor>
    <virtualDestinations>
      <compositeTopic name="fedora">
        <forwardTo>
          <queue physicalName="fedora"/>
        </forwardTo>
      </compositeTopic>
    </virtualDestinations>
  </virtualDestinationInterceptor>
</destinationInterceptors>

Now a consumer can pull messages from a queue without risk of losing messages.

This configuration, however, will not allow any other applications to read from the original topic. If it is necessary to have /topic/fedora available to consumers, this configuration will be useful:

Code Block
languagexml
<destinationInterceptors>
  <virtualDestinationInterceptor>
    <virtualDestinations>
      <compositeTopic name="fedora" forwardOnly="false">
        <forwardTo>
          <queue physicalName="fedora"/>
        </forwardTo>
      </compositeTopic>
    </virtualDestinations>
  </virtualDestinationInterceptor>
</destinationInterceptors>

Now, both /topic/fedora and /queue/fedora will be available to consumers.

Distributed Brokers

The above example will allow you to distribute the message consumers across multiple machines without missing messages, but it can also be useful to distribute the message broker across multiple machines. This can be especially useful if you want to further decouple the message producers and consumers. It can also be useful for high-availability and failover support.

ActiveMQ supports a variety of distributed broker topologies. To push messages from both the message queue and topic to a remote broker, this configuration can be used:

Code Block
languagexml
<networkConnectors>
  <networkConnector name="fedora_bridge" dynamicOnly="true" uri="static:(tcp://remote-host:61616)">
    <dynamicallyIncludedDestinations>
      <topic physicalName="fedora"/>
      <queue physicalName="fedora"/>
    </dynamicallyIncludedDestinations>
  </networkConnector>
</networkConnectors>

Protocol Support

ActiveMQ brokers support a wide variety of protocols. If Fedora's internal broker is bridged to an external broker, please remember to enable the proper protocols on the remote broker. This can be done like so:

Code Block
languagexml
<transportConnectors>
  <transportConnector name="openwire" uri="tcp://0.0.0.0:61616"/>
  <transportConnector name="stomp" uri="stomp://0.0.0.0:61613"/>
</transportConnectors>


Each transportConnector supports many additional options that can be added to this configuration.

Deployment

Camel routes can be deployed in any JVM container. In order to deploy to Jetty or Tomcat, the route must be built as a WAR file. This command will get you started:

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