Fcrepo Component

The fcrepo: component provides access to an external Fedora4 Object API for use with Apache Camel.

URI format

fcrepo:hostname[:port][/resourceUrl][?options]

By default this endpoint connects to fedora repositories on port 80.

FcrepoEndpoint options

Name	Default Value	Description
contentType	null	Set the Content-Type header
accept	null	Set the Accept header for content negotiation
fixity	false	Whether GET requests should check the fixity of non-RDF content
metadata	true	Whether GET requests should retrieve RDF descriptions of non-RDF content
preferOmit	null	If set, this populates the Prefer: HTTP header with omitted values. For single values, the standard LDP values and the corresponding Fcrepo extensions can be provided in short form (without the namespace).
preferInclude	null	If set, this populates the Prefer: HTTP header with included values. For single values, the standard LDP values and the corresponding Fcrepo extensions can be provided in short form (without the namespace).
throwExceptionOnFailure	true	Option to disable throwing the HttpOperationFailedException in case of failed responses from the remote server. This allows you to get all responses regardless of the HTTP status code.

Examples

A simple example for sending messages to an external Solr service:

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").routeId("triplestore-router")
  .process(new EventProcessor())
  .choice()
    .when(header(FCREPO_EVENT_TYPE).contains("https://www.w3.org/ns/activitystreams#Delete"))
      .to("direct:remove")
    .when(header(FCREPO_RESOURCE_TYPE).contains("http://fedora.info/definitions/v4/indexing#Indexable"))
      .to("direct:update")
    .otherwise()
      .to("direct:remove");

from("direct:update").routeId("triplestore-updater")
  .to("fcrepo:localhost:8080/fcrepo/rest?accept=application/n-triples")
  .process(new SparqlUpdateProcessor())
  .to("http4:triplestore-host:8080/dataset/update");

from("direct:remove").routeId("triplestore-remover")
  .process(new SparqlDeleteProcessor())
  .to("http4:triplestore-host:8080/dataset/update");

Or, using the Spring DSL:

<bean id="updateProcessor" class="org.fcrepo.camel.processor.SparqlUpdateProcessor"/>
<bean id="deleteProcessor" class="org.fcrepo.camel.processor.SparqlDeleteProcessor"/>
<bean id="eventProcessor" class="org.fcrepo.camel.processor.EventProcessor"/>

<camelContext xmlns="http://camel.apache.org/schema/blueprint">
  <route id="triplestore-router">
    <from uri="activemq:topic:fedora"/>
    <process ref="eventProcessor"/>
    <choice>
      <when>
        <simple>${header[CamelFcrepoEventType].contains("https://www.w3.org/ns/activitystreams#Delete")}</simple>
        <to uri="direct:remove"/>
      </when>
      <when>
        <simple>${header[CamelFcrepoResourceType].contains("http://fedora.info/definitions/v4/indexing#Indexable")}</simple>
        <to uri="direct:update"/>
      </when>
      <otherwise>
        <to uri="direct:remove"/>
      </otherwise>
    </choice>
  </route>

  <route id="triplestore-updater">
    <from uri="direct:update"/>
    <to uri="fcrepo:localhost:8080/rest?accept=application/n-triples"/>
    <process ref="updateProcessor"/>
    <to uri="http4:triplestore-host:8080/dataset/update"/>
  </route>

  <route id="triplestore-remover">
    <from uri="direct:remove"/>
    <process ref="deleteProcessor"/>
    <to uri="http4:triplestore-host:8080/dataset/update"/>
  </route>
</camelContext>

Please Note: as in this example, if you plan to handle ResourceDeletion events, you should expect any requests back to fedora (via fcrepo:) to respond with a 410 Gone error, so it is recommended that you route your messages accordingly.

Setting basic authentication

Name	Default Value	Description
authUsername	null	Username for authentication
authPassword	null	Password for authentication
authHost	null	The host name for authentication

Configuring the fcrepo component

In addition to configuring the fcrepo component with URI options on each request, it is also sometimes convenient to set up component-wide configurations. This can be done via Spring (or Blueprint), like so:

<bean id="fcrepo" class="org.fcrepo.camel.FcrepoComponent">
  <property name="authUsername" value="${fcrepo.authUsername}"/>
  <property name="authPassword" value="${fcrepo.authPassword}"/>
  <property name="authHost" value="${fcrepo.authHost}"/>
  <property name="baseUrl" value="${fcrepo.baseUrl}"/>
</bean>

Message headers

Name	Type	Description
Exchange.HTTP_METHOD	String	The HTTP method to use
Exchange.CONTENT_TYPE	String	The ContentType of the resource. This sets the Content-Type header, but this value can be overridden directly on the endpoint.
Exchange.ACCEPT_CONTENT_TYPE	String	This sets the Accept header, but this value can be overridden directly on the endpoint.
FcrepoHeaders.FCREPO_AGENT	List	A collection of agents that generated this event.
FcrepoHeaders.FCREPO_BASE_URL	String	The base url used for accessing Fedora. Note: users are encouraged to use the FCREPO_URI header instead.
FcrepoHeaders.FCREPO_DATE_TIME	String	A datetime string formatted in ISO 8601 corresponding to the instant of the event.
FcrepoHeaders.FCREPO_EVENT_ID	String	A unique identifier for this event.
FcrepoHeaders.FCREPO_EVENT_TYPE	List	A set of URIs corresponding to the event type.
FcrepoHeaders.FCREPO_IDENTIFIER	String	The resource path, appended to the endpoint uri. Note: users are encouraged to use the FCREPO_URI header instead.
FcrepoHeaders.FCREPO_NAMED_GRAPH	String	Sets a URI for a named graph when used with the processor.Sparql* classes. This may be useful when storing data in an external triplestore.
FcrepoHeaders.FCREPO_PREFER	String	This sets the Prefer header on a repository request. The full header value should be declared here, and it will override any value set directly on an endpoint.
FcrepoHeaders.FCREPO_RESOURCE_TYPE	List	A set of URIs corresponding to the resource type.
FcrepoHeaders.FCREPO_URI	String	The full resource URI. Note: if this is defined, it takes precedence over any values set with FCREPO_IDENTIFIER and FCREPO_BASE_URL.

If these headers are used with the Spring DSL or with the Simple language, the header values can be used directly with the following values:

Name	Value
FcrepoHeaders.FCREPO_AGENT	CamelFcrepoAgent
FcrepoHeaders.FCREPO_BASE_URL	CamelFcrepoBaseUrl
FcrepoHeaders.FCREPO_DATE_TIME	CamelFcrepoDateTime
FcrepoHeaders.FCREPO_EVENT_ID	CamelFcrepoEventId
FcrepoHeaders.FCREPO_EVENT_TYPE	CamelFcrepoEventType
FcrepoHeaders.FCREPO_IDENTIFIER	CamelFcrepoIdentifier
FcrepoHeaders.FCREPO_NAMED_GRAPH	CamelFcrepoNamedGraph
FcrepoHeaders.FCREPO_PREFER	CamelFcrepoPrefer
FcrepoHeaders.FCREPO_RESOURCE_TYPE	CamelFcrepoResourceType
FcrepoHeaders.FCREPO_URI	CamelFcrepoUri

These headers can be removed as a group like this in the Java DSL: removeHeaders("CamelFcrepo*")

Message body

Camel will store the HTTP response from the Fedora4 server on the OUT body. All headers from the IN message will be copied to the OUT message, so headers are preserved during routing. Additionally, Camel will add the HTTP response headers to the OUT message headers.

Response code

Camel will handle the HTTP response code in the following ways:

• Response code in the range 100..299 is a success.
• Response code in the range 300..399 is a redirection and will throw a FcrepoOperationFailedException with the relevant information.
• Response code is 400+ is regarded as an external server error and will throw an FcrepoOperationFailedException with the relevant information.

Resource path

The path for fcrepo resources can be set in several different ways. If the CamelFcrepoUri header is set, that will be used as the full path of the Fedora resource. If that header is not set, the value of the CamelFcrepoIdentifier header will be appended to either the endpoint URI or the value of the CamelFcrepoBaseUrl header (the CamelFcrepoBaseUrl header takes precedence, if defined).

It is generally a good idea to set the endpoint URI to fedora's REST API endpoint and then use the appropriate header to set the path of the intended resource.

For example, each of these routes will request the resource at http://localhost:8080/rest/a/b/c/abcdef:

from("direct:start")
  .setHeader("CamelFcrepoIdentifier", "/a/b/c/abcdef")
  .to("fcrepo:localhost:8080/rest");

// CamelFcrepoUri and CamelFcrepoIdentifier headers are undefined
from("direct:start")
  .to("fcrepo:localhost:8080/rest/a/b/c/abcdef");

// and CamelFcrepoIdentifier is not defined
from("direct:start")
  .setHeader("CamelFcrepoUri", "http://localhost:8080/rest/a/b/c/abcdef")
  .to("fcrepo:localhost:8080/rest")

FcrepoOperationFailedException

This exception contains the following information:

• The requested URL
• The HTTP status code
• The HTTP status line (text of the status code)

How to set the HTTP method

The endpoint will always use the GET method unless explicitly set in the Exchange.HTTP_METHOD header. Other methods, such as PUT, PATCH, POST, and DELETE are available and will be passed through to the Fedora server. Here is an example:

from("direct:start")
    .setHeader(Exchange.HTTP_METHOD, constant("POST"))
    .to("fcrepo:localhost:8080/fcrepo4/rest")
    .to("mock:results");

And the equivalent Spring sample:

<camelContext xmlns="http://activemq.apache.org/camel/schema/spring">
  <route>
    <from uri="direct:start"/>
    <setHeader headerName="Exchange.HTTP_METHOD">
        <constant>POST</constant>
    </setHeader>
    <to uri="fcrepo:localhost:8080/fcrepo4/rest"/>
    <to uri="mock:results"/>
  </route>
</camelContext>

Getting the response code

You can get the HTTP response code from the fcrepo component by getting the value from the Out message header with Exchange.HTTP_RESPONSE_CODE.

Transactions

The fcrepo-camel component follows the Transactional Client pattern when using transactions with a Fedora Repository. A route can begin using transactions by simply identifying the route as transacted() like so:

from("direct:foo")
  .transacted()
  .to("fcrepo:localhost:8080/rest")
  .process(new MyProcessor())
  .to("fcrepo:localhost:8080/rest")
  .process(new MyOtherProcessor())
  .to("fcrepo:localhost:8080/rest");

A single transaction can span multiple routes so long as the transaction is run within a single thread. That is, if the direct endpoint is used, a transacted workflow may be divided among multiple routes (do not use seda or vm).

In order to enable a transactional client, a TransactionManager must be added to the Spring configuration: for this to work, the built-in FcrepoTransactionManager needs to know the baseUrl of the underlying repository. Authentication information, if necessary, can also be added in the bean configuration.

<bean id="fcrepoTxManager" class="org.fcrepo.camel.FcrepoTransactionManager">
  <property name="baseUrl" value="http://localhost:8080/rest"/>
</bean>

<bean id="fcrepo" class="org.fcrepo.camel.FcrepoComponent">
  <property name="transactionManager" ref="fcrepoTxManager"/>
</bean>

Like with other transactional clients, if an error is encountered anywhere in the route, all transacted operations will be rolled back.

Building the component

The fcrepo-camel compnent can be built with Maven:

mvn clean install

Fcrepo messaging

Fedora4 uses an internal ActiveMQ message broker to send messages about any updates to the repository content. By default, all events are published to a topic called fedora on the local broker. Each message contains an empty body and up to seven different header values:

• org.fcrepo.jms.baseURL
• org.fcrepo.jms.eventType
• org.fcrepo.jms.identifier
• org.fcrepo.jms.resourceType
• org.fcrepo.jms.timestamp
• org.fcrepo.jms.user
• org.fcrepo.jms.userAgent

Both eventType and resourceType are comma-delimited lists of values. The eventType uses the following terms from the ActivityStreams 2.0 Activities vocabulary:

• https://www.w3.org/ns/activitystreams#Create
• https://www.w3.org/ns/activitystreams#Delete
• https://www.w3.org/ns/activitystreams#Update
• https://www.w3.org/ns/activitystreams#Move
• https://www.w3.org/ns/activitystreams#Follow

The resourceType values will include any rdf:type values for the resource in question.

The message body will be formatted as JSON-LD, and users are encouraged to rely on the data found there rather than in the JMS-specific headers.

Typically, an application will unmarshal the payload from a message broker like so:

from("activemq:queue:fedora")
    .unmarshal().json(Jackson)

Additionally, the EventProcessor will populate Message headers with the data in this message. The recommended pattern to use is:

from("activemq:queue:fedora")
    .unmarshal().json(Jackson)
    .process(new EventProcessor())

If you don't need further access to the message body, it is possible to omit the unmarshal().json(Jackson) step.

Examples and more information

For projects that use fcrepo-camel, please refer to the fcrepo-camel-toolbox project.

Furthermore, additional information about designing and deploying fcrepo-based message routes along with configuration options for Fedora's ActiveMQ broker can be found on the fedora project wiki.

Maintainers

Current maintainers:

• Daniel Lamb
• Bethany Seeger