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Camel Messaging

For simple message-consuming applications, writing special-purpose applications may be an excellent choice. In contrast, once a repository begins making use of more complex message-based workflows or when there are multiple listener applications to manage, many repositories use systems such as Apache Camel to simplify the handling of these messages.

Camel makes use of "components" to integrate various services using a terse, domain specific language (DSL) that can be expressed in Java, XML, Scala or Groovy. There exists an fcrepo-camel component designed to work specifically with a Fedora4 repository. This makes it possible to model Solr indexing in only a few lines of code like so:

XPathBuilder xpath = new XPathBuilder("/rdf:RDF/rdf:Description/rdf:type[@rdf:resource='http://fedora.info/definitions/v4/indexing#Indexable']")
xpath.namespace("rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#")

from("activemq:topic:fedora")
  .to("fcrepo:localhost:8080/fedora/rest")
  .filter(xpath)
    .to("fcrepo:localhost:8080/fedora/rest?transform=default")
    .to("http4:localhost:8080/solr/core/update");

In this specific case, the XPath filtering predicate is just an example; you can, of course, use many different Predicate languages, including XQuery, SQL or various Scripting Languages.

This same logic can also be expressed using the Spring XML extensions:

<route>
  <from uri="activemq:topic:fedora"/>
  <to uri="fcrepo:localhost:8080/fedora/rest"/>
  <filter>
    <xpath>/rdf:RDF/rdf:Description/rdf:type[@rdf:resource='http://fedora.info/definitions/v4/indexing#Indexable']</xpath>
    <to uri="fcrepo:localhost:8080/fedora/rest?transform=default"/>
    <to uri="http4:localhost:8080/solr/core/update"/>
  </filter>
</route>

Or, in Scala:

val xpath = new XPathBuilder("/rdf:RDF/rdf:Description/rdf:type[@rdf:resource='http://fedora.info/definitions/v4/indexing#Indexable']")
xpath.namespace("rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#")
 
"activemq:topic:fedora" ==> {
    to("fcrepo:localhost:8080/fedora/rest")
    filter(xpath) {
        to("fcrepo:localhost:8080/fedora/rest?transform=default")
        to("http4:localhost:8080/solr/core/update")
    }
}

Please note that the hostnames used for Fedora and Solr in the snippets above are 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 an arbitrarily large number of independent systems.

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.

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After the project has been built (mvn install), you will find the WAR file in ./target. That file can simply be copied to the webapps directory of your Jetty/Tomcat server.

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Camel

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Messaging

For simple message-consuming applications, writing special-purpose applications may be an excellent choice. In contrast, once a repository begins making use of more complex message-based workflows or when there are multiple listener applications to manage, many repositories use systems such as Apache Camel to simplify the handling of these messages.

Camel makes use of "components" to integrate various services using a terse, domain specific language (DSL) that can be expressed in Java, XML, Scala or Groovy. There exists an fcrepo-camel component designed to work specifically with a Fedora4 repository. This makes it possible to model Solr indexing in only a few lines of code like soThe Fedora project distributes camel routes for several common repository tasks as part of the fcrepo-camel-toolbox project, for use with Karaf version 4.x. Additional information is available on the Integration Services page. Detailed installation instructions are available as part of the project README and follow this pattern:

XPathBuilder xpath = new XPathBuilder("/rdf:RDF/rdf:Description/rdf:type[@rdf:resource='http://fedora.info/definitions/v4/indexing#Indexable']")
xpath.namespace("rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#")

from("activemq:topic:fedora")
  .to("fcrepo:localhost:8080/fedora/rest")
  .filter(xpath)
    .to("fcrepo:localhost:8080/fedora/rest?transform=default")
    .to("http4:localhost:8080/solr/core/update");

In this specific case, the XPath filtering predicate is just an example; you can, of course, use many different Predicate languages, including XQuery, SQL or various Scripting Languages.

This same logic can also be expressed using the Spring XML extensions:

<route>
  <from uri="activemq:topic:fedora"/>
  <to uri="fcrepo:localhost:8080/fedora/rest"/>
  <filter>
    <xpath>/rdf:RDF/rdf:Description/rdf:type[@rdf:resource='http://fedora.info/definitions/v4/indexing#Indexable']</xpath>
    <to uri="fcrepo:localhost:8080/fedora/rest?transform=default"/>
    <to uri="http4:localhost:8080/solr/core/update"/>
  </filter>
</route>

Or, in Scala:

val xpath = new XPathBuilder("/rdf:RDF/rdf:Description/rdf:type[@rdf:resource='http://fedora.info/definitions/v4/indexing#Indexable']")
xpath.namespace("rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#")
 
"activemq:topic:fedora" ==> {
    to("fcrepo:localhost:8080/fedora/rest")
    filter(xpath) {
        to("fcrepo:localhost:8080/fedora/rest?transform=default")
        to("http4:localhost:8080/solr/core/update")
    }
}

Please note that the hostnames used for Fedora and Solr in the snippets above are 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 an arbitrarily large number of independent systems.

Monitoring Your Camel Routes

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