This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Integration 6.4.1!

Routing Slip

Starting with version 4.1, Spring Integration provides an implementation of the routing slip enterprise integration pattern. It is implemented as a routingSlip message header, which is used to determine the next channel in AbstractMessageProducingHandler instances, when an outputChannel is not specified for the endpoint. This pattern is useful in complex, dynamic cases, when it can become difficult to configure multiple routers to determine message flow. When a message arrives at an endpoint that has no output-channel, the routingSlip is consulted to determine the next channel to which the message is sent. When the routing slip is exhausted, normal replyChannel processing resumes.

Configuration for the routing slip is presented as a HeaderEnricher option — a semicolon-separated routing slip that contains path entries, as the following example shows:

<util:properties id="properties">
    <beans:prop key="myRoutePath1">channel1</beans:prop>
    <beans:prop key="myRoutePath2">request.headers[myRoutingSlipChannel]</beans:prop>
</util:properties>

<context:property-placeholder properties-ref="properties"/>

<header-enricher input-channel="input" output-channel="process">
    <routing-slip
        value="${myRoutePath1}; @routingSlipRoutingPojo.get(request, reply);
               routingSlipRoutingStrategy; ${myRoutePath2}; finishChannel"/>
</header-enricher>

The preceding example has:

A <context:property-placeholder> configuration to demonstrate that the entries in the routing slip path can be specified as resolvable keys.
The <header-enricher> <routing-slip> sub-element is used to populate the RoutingSlipHeaderValueMessageProcessor to the HeaderEnricher handler.
The RoutingSlipHeaderValueMessageProcessor accepts a String array of resolved routing slip path entries and returns (from processMessage()) a singletonMap with the path as key and 0 as initial routingSlipIndex.

Routing Slip path entries can contain MessageChannel bean names, RoutingSlipRouteStrategy bean names, and Spring expressions (SpEL). The RoutingSlipHeaderValueMessageProcessor checks each routing slip path entry against the BeanFactory on the first processMessage invocation. It converts entries (which are not bean names in the application context) to ExpressionEvaluatingRoutingSlipRouteStrategy instances. RoutingSlipRouteStrategy entries are invoked multiple times, until they return null or an empty String.

Since the routing slip is involved in the getOutputChannel process, we have a request-reply context. The RoutingSlipRouteStrategy has been introduced to determine the next outputChannel that uses the requestMessage and the reply object. An implementation of this strategy should be registered as a bean in the application context, and its bean name is used in the routing slip path. The ExpressionEvaluatingRoutingSlipRouteStrategy implementation is provided. It accepts a SpEL expression and an internal ExpressionEvaluatingRoutingSlipRouteStrategy.RequestAndReply object is used as the root object of the evaluation context. This is to avoid the overhead of EvaluationContext creation for each ExpressionEvaluatingRoutingSlipRouteStrategy.getNextPath() invocation. It is a simple Java bean with two properties: Message<?> request and Object reply. With this expression implementation, we can specify routing slip path entries by using SpEL (for example, @routingSlipRoutingPojo.get(request, reply) and request.headers[myRoutingSlipChannel]) and avoid defining a bean for the RoutingSlipRouteStrategy.

The requestMessage argument is always a Message<?>. Depending on context, the reply object may be a Message<?>, an AbstractIntegrationMessageBuilder, or an arbitrary application domain object (when, for example, it is returned by a POJO method invoked by a service activator). In the first two cases, the usual Message properties (payload and headers) are available when using SpEL (or a Java implementation). For an arbitrary domain object, these properties are not available. For this reason, be careful when you use routing slips in conjunction with POJO methods if the result is used to determine the next path.

If a routing slip is involved in a distributed environment, we recommend not using inline expressions for the Routing Slip path. This recommendation applies to distributed environments such as cross-JVM applications, using a request-reply through a message broker (such asAMQP Support or JMS Support), or using a persistent MessageStore (Message Store) in the integration flow. The framework uses RoutingSlipHeaderValueMessageProcessor to convert them to ExpressionEvaluatingRoutingSlipRouteStrategy objects, and they are used in the routingSlip message header. Since this class is not Serializable (it cannot be, because it depends on the BeanFactory), the entire Message becomes non-serializable and, in any distributed operation, we end up with a NotSerializableException. To overcome this limitation, register an ExpressionEvaluatingRoutingSlipRouteStrategy bean with the desired SpEL and use its bean name in the routing slip path configuration.

For Java configuration, you can add a RoutingSlipHeaderValueMessageProcessor instance to the HeaderEnricher bean definition, as the following example shows:

@Bean
@Transformer(inputChannel = "routingSlipHeaderChannel")
public HeaderEnricher headerEnricher() {
    return new HeaderEnricher(Collections.singletonMap(IntegrationMessageHeaderAccessor.ROUTING_SLIP,
            new RoutingSlipHeaderValueMessageProcessor("myRoutePath1",
                                                       "@routingSlipRoutingPojo.get(request, reply)",
                                                       "routingSlipRoutingStrategy",
                                                       "request.headers[myRoutingSlipChannel]",
                                                       "finishChannel")));
}

The routing slip algorithm works as follows when an endpoint produces a reply and no outputChannel has been defined:

The routingSlipIndex is used to get a value from the routing slip path list.
If the value from routingSlipIndex is String, it is used to get a bean from BeanFactory.
If a returned bean is an instance of MessageChannel, it is used as the next outputChannel and the routingSlipIndex is incremented in the reply message header (the routing slip path entries remain unchanged).
If a returned bean is an instance of RoutingSlipRouteStrategy and its getNextPath does not return an empty String, that result is used as a bean name for the next outputChannel. The routingSlipIndex remains unchanged.
If RoutingSlipRouteStrategy.getNextPath returns an empty String or null, the routingSlipIndex is incremented and the getOutputChannelFromRoutingSlip is invoked recursively for the next Routing Slip path item.
If the next routing slip path entry is not a String, it must be an instance of RoutingSlipRouteStrategy.
When the routingSlipIndex exceeds the size of the routing slip path list, the algorithm moves to the default behavior for the standard replyChannel header.