The Fedora Repository makes it possible to design custom event-driven application workflows. For instance, a common task involves use case is sending content to an external search engine or triplestore. Other repositories may wish to generate derivative content such as creating a set of smaller images from a high-resolution master.
Because Fedora publishes modification events on a JMS topic using a local ActiveMQ broker, one can write custom listener applications to handle these various workflows. By default, the repository's JMS broker supports both the OpenWire and STOMP protocols, which means that it is possible to write client listeners (or consumers ) in a wide variety of languages, including PHP, Python, Ruby and JAVA, among others.
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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 one such 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:
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XPathBuilder xpath = new XPathBuilder("/rdf:RDF/rdf:Description/rdf:type[@rdf:resource='http://fedora.info/definitions/v4/rest-api#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?accept=application/json&transform=default")
.to("http4:localhost:8080/solr/core/update"); |
In this examplespecific 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.
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Please note that the hostnames used for fedora and solr here are entirely 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.
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Some additional processing must be done to transform an application/n-triples response into a valid application/sparql-update payload before sending to an external triplestore such as Fuseki or Sesame. The fcrepo component contains some processors in org.fcrepo.camel.processor to handle this case. The examples below assume that messages have already been routed based on eventType (see below) and passed to the appropriate queue.
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ActiveMQ supports “virtual destinations”, allowing your broker to automatically forward messages from one location to another. If fedora4 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:
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