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The round robin load balancer is stateful as it keeps state of which endpoint to use next time.

Using the Fluent Builders

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{snippet:id=example|lang=java|url=camel/trunk/camel-core/src/test/java/org/apache/camel/processor/RoundRobinLoadBalanceTest.java}
Using the Spring configuration

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Option

Type

Default

Description

inheritErrorHandler

boolean

true

Camel 2.3: Whether or not the Error Handler configured on the route should be used. Disable this if you want failover to transfer immediately to the next endpoint. On the other hand, if you have this option enabled, then Camel will first let the Error Handler try to process the message. The Error Handler may have been configured to redeliver and use delays between attempts. If you have enabled a number of redeliveries then Camel will try to redeliver to the same endpoint, and only fail over to the next endpoint, when the Error Handler is exhausted.

maximumFailoverAttempts

int

-1

Camel 2.3: A value to indicate after X failover attempts we should exhaust (give up). Use -1 to indicate never give up and continuously try to failover. Use 0 to never failover. And use e.g. 3 to failover at most 3 times before giving up. This option can be used whether or not roundRobin is enabled or not.

roundRobin

boolean

false

Camel 2.3: Whether or not the failover load balancer should operate in round robin mode or not. If not, then it will always start from the first endpoint when a new message is to be processed. In other words it restart from the top for every message. If round robin is enabled, then it keeps state and will continue with the next endpoint in a round robin fashion. When using round robin it will not stick to last known good endpoint, it will always pick the next endpoint to use. You can also enable sticky mode together with round robin, if so then it will pick the last known good endpoint to use when starting the load balancing (instead of using the next when starting).

stickybooleanfalseCamel 2.16: Whether or not the failover load balancer should operate in sticky mode or not. If not, then it will always start from the first endpoint when a new message is to be processed. In other words it restart from the top for every message. If sticky is enabled, then it keeps state and will continue with the last known good endpoint. You can also enable sticky mode together with round robin, if so then it will pick the last known good endpoint to use when starting the load balancing (instead of using the next when starting).

Camel 2.2 or older behavior
The current implementation of failover load balancer uses 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.

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

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{snippet:id=e1|lang=java|url=camel/trunk/camel-core/src/test/java/org/apache/camel/processor/FailOverNotCatchedExceptionTest.java}
You can specify multiple exceptions to failover as the option is varargs, for instance:

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An example using Java DSL:

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{snippet:id=e1|lang=java|url=camel/trunk/camel-core/src/test/java/org/apache/camel/processor/FailoverRoundRobinTest.java}
And the same example using Spring XML:
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{snippet:id=e1|lang=xml|url=camel/trunk/components/camel-spring/src/test/resources/org/apache/camel/spring/processor/FailoverRoundRobinTest.xml}

Tip
titleDisabled inheritErrorHandler

You can configure inheritErrorHandler=false if you want to failover to the next endpoint as fast as possible. By disabling the Error Handler you ensure it does not intervene which allows the failover load balancer to handle failover asap. By also enabling roundRobin mode, then it will keep retrying until it success. You can then configure the maximumFailoverAttempts option to a high value to let it eventually exhaust (give up) and fail.

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An example using Java DSL:

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{snippet:id=e1|lang=java|url=camel/trunk/camel-core/src/test/java/org/apache/camel/processor/CustomLoadBalanceTest.java}
And the same example using XML DSL:
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{snippet:id=e1|lang=xml|url=camel/trunk/components/camel-spring/src/test/resources/org/apache/camel/spring/processor/SpringCustomRefLoadBalanceTest.xml}
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:

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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:

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{snippet:id=e2|title=Custom load balancer implementation|lang=java|url=camel/trunk/camel-core/src/test/java/org/apache/camel/processor/CustomLoadBalanceTest.java}

Circuit Breaker

The Circuit Breaker load balancer is a stateful pattern that monitors all calls for certain exceptions. Initially the Circuit Breaker is in closed state and passes all messages. If there are failures and the threshold is reached, it moves to open state and rejects all calls until halfOpenAfter timeout is reached. After this timeout is reached, if there is a new call, it will pass and if the result is success the Circuit Breaker will move to closed state, or to open state if there was an error.

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