The load balancer pattern allows you to delegate message processing to one of several endpoints, using a variety of different load-balancing policies.

The following route distributes incoming messages between the target endpoints, mock:x, mock:y, mock:z, using a round robin load-balancing policy:

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

The following example shows how to configure the same route in XML:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <loadBalance>
        <roundRobin/>
        <to uri="mock:x"/>
        <to uri="mock:y"/>
        <to uri="mock:z"/>
    </loadBalance>
  </route>
</camelContext>

	Note
	In versions of Fuse Mediation Router earlier than 1.4.2.0, the <roundRobin/> tag must appear as the last tag inside the loadBalance element.

The Fuse Mediation Router load balancer supports the following load-balancing policies:

The round robin load-balancing policy cycles through all of the target endpoints, sending each incoming message to the next endpoint in the cycle. For example, if the list of target endpoints is, mock:x, mock:y, mock:z, then the incoming messages are sent to the following sequence of endpoints: mock:x, mock:y, mock:z, mock:x, mock:y, mock:z, and so on.

You can specify the round robin load-balancing policy in Java DSL, as follows:

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

Alternatively, you can configure the same route in XML, as follows:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <loadBalance>
        <roundRobin/>
        <to uri="mock:x"/>        
        <to uri="mock:y"/>       
        <to uri="mock:z"/>          
    </loadBalance>
  </route>
</camelContext>

The random load-balancing policy chooses the target endpoint randomly from the specified list.

You can specify the random load-balancing policy in Java DSL, as follows:

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

Alternatively, you can configure the same route in XML, as follows:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <loadBalance>
        <random/>
        <to uri="mock:x"/>        
        <to uri="mock:y"/>       
        <to uri="mock:z"/>          
    </loadBalance>
  </route>
</camelContext>

The sticky load-balancing policy directs the In message to an endpoint that is chosen by calculating a hash value from a specified expression. The advantage of this load-balancing policy is that expressions of the same value are always sent to the same server. For example, by calculating the hash value from a header that contains a username, you ensure that messages from a particular user are always sent to the same target endpoint. Another useful approach is to specify an expression that extracts the session ID from an incoming message. This ensures that all messages belonging to the same session are sent to the same target endpoint.

You can specify the sticky load-balancing policy in Java DSL, as follows:

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

Alternatively, you can configure the same route in XML, as follows:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <loadBalance>
      <sticky>
        <expression>
          <simple>header.username</simple>
        </expression>
      </sticky>
      <to uri="mock:x"/>        
      <to uri="mock:y"/>       
      <to uri="mock:z"/>          
    </loadBalance>
  </route>
</camelContext>

The topic load-balancing policy sends a copy of each In message to all of the listed destination endpoints (effectively broadcasting the message to all of the destinations, like a JMS topic).

You can use the Java DSL to specify the topic load-balancing policy, as follows:

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

Alternatively, you can configure the same route in XML, as follows:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <loadBalance>
        <topic/>
        <to uri="mock:x"/>        
        <to uri="mock:y"/>       
        <to uri="mock:z"/>          
    </loadBalance>
  </route>
</camelContext>

Available as of Fuse Mediation Router 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 exceptions that trigger failover. If you do not specify any exceptions, failover is triggered by any exception. The failover load balancer uses the same strategy for matching exceptions as the onException exception clause.

	Enable stream caching if using streams
	If you use streaming, you should enable Stream Caching when using the failover load balancer. This is needed so the stream can be re-read when failing over.

The failover load balancer supports the following options:

The following example is configured to fail over, only if an IOException exception is 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 optionally specify multiple exceptions to fail over, as follows:

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

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

You can configure the same route in XML, as follows:

<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>

The following example shows how to fail over in round robin mode:

from("direct:start")
    // Use failover load balancer in stateful round robin mode,
    // which means it will fail over immediately in case of an exception
    // as it does NOT inherit error handler. It will also keep retrying, as
    // it is configured to retry indefinitely.
    .loadBalance().failover(-1, false, true)
    .to("direct:bad", "direct:bad2", "direct:good", "direct:good2");

You can configure the same route in XML, as follows:

<route>
    <from uri="direct:start"/>
    <loadBalance>
        <!-- failover using stateful round robin,
        which will keep retrying the 4 endpoints indefinitely.
        You can set the maximumFailoverAttempt to break out after X attempts -->
        <failover roundRobin="true"/>
        <to uri="direct:bad"/>
        <to uri="direct:bad2"/>
        <to uri="direct:good"/>
        <to uri="direct:good2"/>
    </loadBalance>
</route>

In many enterprise environments, where server nodes of unequal processing power are hosting services, it is usually preferable to distribute the load in accordance with the individual server processing capacities. A weighted round robin algorithm or a weighted random algorithm can be used to address this problem.

The weighted load balancing policy allows you to specify a processing load distribution ratio for each server with respect to the others. You can specify this value as a positive processing weight for each server. A larger number indicates that the server can handle a larger load. The processing weight is used to determine the payload distribution ratio of each processing endpoint with respect to the others.

The parameters that can be used are

The following Java DSL examples show how to define a weighted round-robin route and a weighted random route:

// Java
// round-robin
from("direct:start")
  .loadBalance().weighted(true, "4:2:1" distributionRatioDelimiter=":")
  .to("mock:x", "mock:y", "mock:z");

//random
from("direct:start")
  .loadBalance().weighted(false, "4,2,1")
  .to("mock:x", "mock:y", "mock:z");

You can configure the round-robin route in XML, as follows:

<!-- round-robin -->
<route>
  <from uri="direct:start"/>
  <loadBalance>
    <weighted roundRobin="true" distributionRatio="4:2:1" distributionRatioDelimiter=":" />
    <to uri="mock:x"/>
    <to uri="mock:y"/>
    <to uri="mock:z"/>
  </loadBalance>
</route>

You can use a custom load balancer (eg your own implementation) also.

An example using Java DSL:

from("direct:start")
     // using our custom load balancer
     .loadBalance(new MyLoadBalancer())
     .to("mock:x", "mock:y", "mock:z");

And the same example using XML DSL:

<!-- this is the implementation of our custom load balancer -->
 <bean id="myBalancer" class="org.apache.camel.processor.CustomLoadBalanceTest$MyLoadBalancer"/>
 
 <camelContext xmlns="http://camel.apache.org/schema/spring">
   <route>
     <from uri="direct:start"/>
     <loadBalance>
       <!-- refer to my custom load balancer -->
       <custom ref="myBalancer"/>
       <!-- these are the endpoints to balancer -->
       <to uri="mock:x"/>
       <to uri="mock:y"/>
       <to uri="mock:z"/>
     </loadBalance>
   </route>
 </camelContext>

Notice in the XML DSL above we use <custom> which is only available in Camel 2.8 onwards. In older releases you would have to do as follows instead:

       <loadBalance ref="myBalancer">
         <!-- these are the endpoints to balancer -->
         <to uri="mock:x"/>
         <to uri="mock:y"/>
         <to uri="mock:z"/>
       </loadBalance>

To implement a custom load balancer you can extend some support classes such as LoadBalancerSupport and SimpleLoadBalancerSupport. The former supports the asynchronous routing engine, and the latter does not. Here is an example:

public static class MyLoadBalancer extends LoadBalancerSupport {
 
     public boolean process(Exchange exchange, AsyncCallback callback) {
         String body = exchange.getIn().getBody(String.class);
         try {
             if ("x".equals(body)) {
                 getProcessors().get(0).process(exchange);
             } else if ("y".equals(body)) {
                 getProcessors().get(1).process(exchange);
             } else {
                 getProcessors().get(2).process(exchange);
             }
         } catch (Throwable e) {
             exchange.setException(e);
         }
         callback.done(true);
         return true;
     }
 }