LevelDB

	Important
	Since JBoss Fuse 6.3, the Camel LevelDB (camel-leveldb) component is deprecated on all operating systems except for Red Hat Enterprise Linux. In future, the Camel LevelDB component will be supported only on Red Hat Enterprise Linux.

Available as of Camel 2.10

Leveldb is a very lightweight and embedable key value database. With Camel, it provides persistent support for various Camel features, such as Aggregator.

Current features it provides:

LevelDBAggregationRepository is an AggregationRepository which on the fly persists the aggregated messages. This ensures that you will not loose messages, as the default aggregator will use an in memory only AggregationRepository.

It has the following options:

The repositoryName option must be provided. Then either the persistentFileName or the levelDBFile must be provided.

LevelDBAggregationRepository will only preserve any Serializable compatible message body data types. Message headers must be primitive, string, number or similar. If a data type is not such a type its dropped and a WARN is logged. And it only persists the Message body and the Message headers. The Exchange properties are not persisted.

The LevelDBAggregationRepository will by default recover any failed Exchange. It does this by having a background tasks that scans for failed Exchanges in the persistent store. You can use the checkInterval option to set how often this task runs. The recovery works as transactional which ensures that Camel will try to recover and redeliver the failed Exchange. Any Exchange which was found to be recovered will be restored from the persistent store and resubmitted and send out again.

The following headers is set when an Exchange is being recovered/redelivered:

Only when an Exchange has been successfully processed it will be marked as complete which happens when the confirm method is invoked on the AggregationRepository. This means if the same Exchange fails again it will be kept retried until it success.

You can use option maximumRedeliveries to limit the maximum number of redelivery attempts for a given recovered Exchange. You must also set the deadLetterUri option so Camel knows where to send the Exchange when the maximumRedeliveries was hit.

You can see some examples in the unit tests of camel-leveldb, for example this test.

In this example we want to persist aggregated messages in the target/data/leveldb.dat file.

public void configure() throws Exception {
    // create the leveldb repo
    LevelDBAggregationRepository repo = new LevelDBAggregationRepository("repo1", "target/data/leveldb.dat");

    // here is the Camel route where we aggregate
    from("direct:start")
        .aggregate(header("id"), new MyAggregationStrategy())
            // use our created leveldb repo as aggregation repository
            .completionSize(5).aggregationRepository(repo)
            .to("mock:aggregated");
}

The same example but using Spring XML instead:

<!-- a persistent aggregation repository using camel-leveldb -->
<bean id="repo" class="org.apache.camel.component.leveldb.LevelDBAggregationRepository">
    <!-- store the repo in the leveldb.dat file -->
    <property name="persistentFileName" value="target/data/leveldb.dat"/>
    <!-- and use repo2 as the repository name -->
    <property name="repositoryName" value="repo2"/>
</bean>

<!-- aggregate the messages using this strategy -->
<bean id="myAggregatorStrategy" class="org.apache.camel.component.leveldb.LevelDBSpringAggregateTest$MyAggregationStrategy"/>

<!-- this is the camel routes -->
<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">

    <route>
        <from uri="direct:start"/>
        <!-- aggregate using our strategy and leveldb repo, and complete when we have 5 messages aggregated -->
        <aggregate strategyRef="myAggregatorStrategy" aggregationRepositoryRef="repo" completionSize="5">
            <!-- correlate by header with the key id -->
            <correlationExpression><header>id</header></correlationExpression>
            <!-- send aggregated messages to the mock endpoint -->
            <to uri="mock:aggregated"/>
        </aggregate>
    </route>

</camelContext>

To use LevelDB in your camel routes you need to add the a dependency on camel-leveldb.

If you use maven you could just add the following to your pom.xml, substituting the version number for the latest & greatest release (see the download page for the latest versions).

<dependency>
  <groupId>org.apache.camel</groupId>
  <artifactId>camel-leveldb</artifactId>
  <version>2.17.0.redhat-630177</version>
</dependency>