Failover Load Balancer

This EIP allows using fail-over (in case of failures, the exchange will be tried on the next endpoint) with the Load Balancer EIP.

Options

The Failover Load Balancer eip supports 0 options, which are listed below.

Name Description Default Type

exception

A list of class names for specific exceptions to monitor. If no exceptions are configured then all exceptions are monitored.

List

roundRobin

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. 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).

String

sticky

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).

String

maximumFailoverAttempts

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 roundRobin is enabled or not.

-1

String

Exchange properties

The Failover Load Balancer eip has no exchange properties.

Example

In the example below, calling the three http services is done with the load balancer:

  • Java

  • XML

from("direct:start")
    .loadBalance().failover()
        .to("http:service1")
        .to("http:service2")
        .to("http:service3")
    .end();
<route>
    <from uri="direct:start"/>
    <loadBalance>
        <failoverLoadBalancer/>
        <to uri="http:service1"/>
        <to uri="http:service2"/>
        <to uri="http:service3"/>
    </loadBalance>
</route>

In the default mode, the fail-over load balancer will always start with the first processor (i.e., "http:service1"). And in case this fails, then try the next, until either it succeeded or all of them failed. If all failed, then Camel will throw the caused exception which means the Exchange is failed.

Using round-robin mode

You can use the roundRobin mode to start again from the beginning, which then will keep trying until one succeed. To prevent endless retries, then it’s recommended to set a maximum fail-over value.

Setting this in Java DSL is not pretty as there are three parameters:

from("direct:start")
    .loadBalance().failover(10, false, true)
        .to("http:service1")
        .to("http:service2")
        .to("http:service3")
    .end();
.failover(10, false, true)

Where 10 is the maximum fail over attempts, And false is a special feature related to inheriting error handler. The last parameter true is to use round-robin mode.

In XML, it is straightforward as shown:

<route>
    <from uri="direct:start"/>
    <loadBalance>
        <failoverLoadBalancer roundRobin="true" maximumFailoverAttempts="10"/>
        <to uri="http:service1"/>
        <to uri="http:service2"/>
        <to uri="http:service3"/>
    </loadBalance>
</route>

Using sticky mode

The sticky mode is used for remember the last known good endpoint, so the next exchange will start from there, instead from the beginning.

For example, support that http:service1 is down, and that service2 is up. With sticky mode enabled, then Camel will keep starting from service2 until it fails, and then try service3.

If sticky mode is not enabled (it’s disabled by default), then Camel will always start from the beginning, which means calling service1.

  • Java

  • XML

Setting sticky mode in Java DSL is not pretty as there are four parameters.

from("direct:start")
    .loadBalance().failover(10, false, true, true)
        .to("http:service1")
        .to("http:service2")
        .to("http:service3")
    .end();
The last true argument is to enable sticky mode. 
<route>
    <from uri="direct:start"/>
    <loadBalance>
        <failoverLoadBalancer roundRobin="true" maximumFailoverAttempts="10" stickyMode="true"/>
        <to uri="http:service1"/>
        <to uri="http:service2"/>
        <to uri="http:service3"/>
    </loadBalance>
</route>

Fail-over on specific exceptions

The fail-over load balancer can be configured to only apply for a specific set of exceptions. Suppose you only want to fail-over in case of java.io.Exception or HttpOperationFailedException then you can do:

from("direct:start")
    .loadBalance().failover(IOException.class, HttpOperationFailedException.class)
        .to("http:service1")
        .to("http:service2")
        .to("http:service3")
    .end();

And in XML DSL:

<route>
    <from uri="direct:start"/>
    <loadBalance>
        <failoverLoadBalancer>
            <exception>java.io.IOException</exception>
            <exception>org.apache.camel.http.base.HttpOperationFailedException</exception>
        </failoverLoadBalancer>
        <to uri="http:service1"/>
        <to uri="http:service2"/>
        <to uri="http:service3"/>
    </loadBalance>
</route>
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