Load Balancer

Load Balancer

The Load Balancer Pattern allows you to delegate to one of a number of endpoints using a variety of different load balancing policies.

Build in load balancing policies

Camel has out of the box the following policies:

Round Robin

Using the Fluent Builders

Using the Spring configuration

<bean id = "roundRobinRef" class="org.apache.camel.processor.loadbalancer.RoundRobinLoadBalancer" />
  <camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
    <route>
      <from uri="direct:start"/>
      <loadBalance ref="roundRobinRef">
          <to uri="mock:x"/>        
          <to uri="mock:y"/>       
          <to uri="mock:z"/>          
      </loadBalance>
    </route>
  </camelContext>

or

<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <loadBalance>        
        <roundRobin/>  <!-- This only support for Camel 1.5 -->
        <to uri="mock:x"/>        
        <to uri="mock:y"/>       
        <to uri="mock:z"/>                 
    </loadBalance>
  </route>
</camelContext>

So the above example will load balance requests from direct:start to one of the available mock endpoint instances, in this case using a round robbin policy.
For further examples of this pattern in use you could look at the junit test case

Failover

Available as of Camel 2.0
The failover load balancer is capable of trying the next processor in case an Exchange failed with an exception during processing.
You can configure the failover with a list of specific exception to only failover. If you do not specify any exceptions it will failover over any exceptions. It uses the same strategy for matching exceptions as the Exception Clause does for the onException.

The current implement of failover load balancer is a simple logic which always tries the first endpoint, and in case of an exception being thrown it tries the next in the list, and so forth. It has no state, and the next message will thus always start with the first endpoint.

Here is a sample to failover only if a IOException related exception was thrown:

You can specify multiple exceptions to failover as the option is varargs, for instance:

// enable redelivery so failover can react
errorHandler(defaultErrorHandler().maximumRedeliveries(5));

from("direct:foo").
    loadBalance().failover(IOException.class, MyOtherException.class)
        .to("direct:a", "direct:b");

Using failover in Spring DSL

Failover can also be used from Spring DSL and you configure it as:

   <route errorHandlerRef="myErrorHandler">
      <from uri="direct:foo"/>
      <loadBalance>
          <failover>
              <exception>java.io.IOException</exception>
              <exception>com.mycompany.MyOtherException</exception>
          </failover>
          <to uri="direct:a"/>
          <to uri="direct:b"/>
      </loadBalance>
    </route>