Apache Camel: 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:

Policy	Description
Round Robin	The exchanges is selected in a round robin fashion. This is a well known and classic policy. This spreads the load even.
Random	A random endpoint is selected for each exchange
Sticky	Sticky load balancing using an Expression to calculate a correlation key to perform the sticky load balancing; rather like jsessionid in the web or JMSXGroupID in JMS.
Topic	Topic which sends to all destinations (rather like JMS Topics)
Failover	Camel 2.0: In case of failures the exchange is tried on the next endpoint.

Round Robin

Using the Fluent Builders

from("direct:start").loadBalance().
roundRobin().to("mock:x", "mock:y", "mock:z");

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.

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

from("direct:start")
    // here we will load balance if IOException was thrown
    // any other kind of exception will result in the Exchange as failed
    // to failover over any kind of exception we can just omit the exception
    // in the failOver DSL
    .loadBalance().failover(IOException.class)
        .to("direct:x", "direct:y", "direct:z");

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

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:

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

Using This Pattern

If you would like to use this EIP Pattern then please read the Getting Started, you may also find the Architecture useful particularly the description of Endpoint and URIs. Then you could try out some of the Examples first before trying this pattern out.

Load Balancer

Build in load balancing policies

Round Robin

Failover

Using failover in Spring DSL

Using This Pattern

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