The resequencer pattern, shown in Figure 52, enables you to resequence messages according to the sequence number stored in an NMR property. The ServiceMix EIP resequencer pattern maps to the Apache Camel resequencer configured with the stream resequencing algorithm.

Figure 52. Resequencer Pattern

The sequence of messages emitted from the resequencer is determined by the value of the sequence number property: messages with a low sequence number are emitted first and messages with a higher number are emitted later. By default, the sequence number is read from the org.apache.servicemix.eip.sequence.number property in ServiceMix, but you can customize the name of this property using the eip:default-comparator element in ServiceMix.

The equivalent concept in Apache Camel is a sequencing expression, which can be any message-dependent expression. When migrating from ServiceMix EIP, you normally define an expression that extracts the sequence number from a header (a Apache Camel header is equivalent to an NMR message property). For example, to extract a sequence number from a seqnum header, you can use the simple expression, header.seqnum.

Example 26 shows how to define a resequencer using the ServiceMix EIP component.

<eip:resequencer
  service="sample:Resequencer"
  endpoint="ResequencerEndpoint"
  comparator="#comparator"
  capacity="100"
  timeout="2000">
  <eip:target>
    <eip:exchange-target service="sample:SampleTarget" />
  </eip:target>
</eip:resequencer>

<!-- Configure default comparator with custom sequence number property -->
<eip:default-comparator xml:id="comparator" 
                        sequenceNumberKey="seqnum"/>

Example 27 shows how to define an equivalent route using Apache Camel XML configuration.

<route>
  <from uri="jbi:endpoint:sample:Resequencer:ResequencerEndpoint"/>
  <resequencer>
    <simple>header.seqnum</simple>
    <to uri="jbi:service:sample:SampleTarget" />
    <stream-config capacity="100" timeout="2000"/>
  </resequencer>
</route>

Example 28 shows how to define an equivalent route using the Apache Camel Java DSL.

from("jbi:endpoint:sample:Resequencer:ResequencerEndpoint").
    resequencer(header("seqnum")).
    stream(new StreamResequencerConfig(100, 2000L)).
    to("jbi:service:sample:SampleTarget");