The Dynamic Router pattern, as shown in Figure 16.13, enables you to route a message consecutively through a series of processing steps, where the sequence of steps is not known at design time. The list of endpoints through which the message should pass is calculated dynamically at run time. Each time the message returns from an endpoint, the dynamic router calls back on a bean to discover the next endpoint in the route.

Figure 16.13. Dynamic Router Pattern

	Beware
	You must ensure the expression used for the dynamicRouter (such as a bean), returns null to indicate the end. Otherwise, the dynamicRouter will continue in an endless loop.

The dynamic router updates the exchange property, Exchange.SLIP_ENDPOINT, with the current endpoint as it advances through the route. This enables you to find out how far the exchange has progressed through the route.

In Java DSL you can use the dynamicRouter as follows:

from("direct:start")
    // use a bean as the dynamic router
    .dynamicRouter(bean(DynamicRouterTest.class, "slip"));

Which will leverage a Bean in Component Reference to compute the slip on-the-fly, which could be implemented as follows:

// Java
/**
 * Use this method to compute dynamic where we should route next.
 *
 * @param body the message body
 * @return endpoints to go, or <tt>null</tt> to indicate the end
 */
public String slip(String body) {
    bodies.add(body);
    invoked++;

    if (invoked == 1) {
        return "mock:a";
    } else if (invoked == 2) {
        return "mock:b,mock:c";
    } else if (invoked == 3) {
        return "direct:foo";
    } else if (invoked == 4) {
        return "mock:result";
    }

    // no more so return null
    return null;
}

The same example in XML would be:

<bean id="mySlip" class="org.apache.camel.processor.DynamicRouterTest"/>

<camelContext xmlns="http://camel.apache.org/schema/spring">
    <route>
        <from uri="direct:start"/>
        <dynamicRouter>
            <!-- use a method call on a bean as dynamic router -->
            <method ref="mySlip" method="slip"/>
        </dynamicRouter>
    </route>

    <route>
        <from uri="direct:foo"/>
        <transform><constant>Bye World</constant></transform>
        <to uri="mock:foo"/>
    </route>

</camelContext>

You can also use the @DynamicRouter annotation. For example:

// Java
public class MyDynamicRouter {

    @Consume(uri = "activemq:foo")
    @DynamicRouter
    public String route(@XPath("/customer/id") String customerId, @Header("Location") String location, Document body) {
        // query a database to find the best match of the endpoint based on the input parameteres
        // return the next endpoint uri, where to go. Return null to indicate the end.
    }
}

The route method is invoked repeatedly as the message progresses through the slip. The idea is to return the endpoint URI of the next destination. Return null to indicate the end. You can return multiple endpoints if you like, just as the Routing Slip, where each endpoint is separated by a delimiter.