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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 JAVAJava, among others.
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 JAVAJava, 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:
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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"); |
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