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Recipient List

The Recipient List from the EIP patterns allows you to route messages to a number of dynamically specified recipients.

The recipients will receive a copy of the same Exchange, and Camel will execute them sequentially.

Options

Name	Default Value	Description
`delimiter`	`,`	Delimiter used if the _expression_ returned multiple endpoints.
`strategyRef`		An AggregationStrategy that will assemble the replies from recipients into a single outgoing message from the Recipient List. By default Camel will use the last reply as the outgoing message. From Camel 2.12 onwards you can also use a POJO as the `AggregationStrategy`, see the Aggregate page for more details.
`strategyMethodName`		Camel 2.12: This option can be used to explicit declare the method name to use, when using POJOs as the `AggregationStrategy`. See the Aggregate page for more details.
`strategyMethodAllowNull`	`false`	Camel 2.12: If this option is `false` then the aggregate method is not used if there was no data to enrich. If this option is `true` then `null` values is used as the `oldExchange` (when no data to enrich), when using POJOs as the `AggregationStrategy`. See the Aggregate page for more details.
`parallelProcessing`	`false`	Camel 2.2: If enabled, messages are sent to the recipients concurrently. Note that the calling thread will still wait until all messages have been fully processed before it continues; it's the sending and processing of replies from recipients which happens in parallel.
`executorServiceRef`		Camel 2.2: A custom Thread Pool to use for parallel processing. Note that enabling this option implies parallel processing, so you need not enable that option as well.
`stopOnException`	`false`	Camel 2.2: Whether to immediately stop processing when an exception occurs. If disabled, Camel will send the message to all recipients regardless of any individual failures. You can process exceptions in an AggregationStrategy implementation, which supports full control of error handling.
`ignoreInvalidEndpoints`	`false`	Camel 2.3: Whether to ignore an endpoint URI that could not be resolved. If disabled, Camel will throw an exception identifying the invalid endpoint URI.
`streaming`	`false`	Camel 2.5: If enabled, Camel will process replies out-of-order - that is, in the order received in reply from each recipient. If disabled, Camel will process replies in the same order as specified by the _expression_.
`timeout`		Camel 2.5: Specifies a processing timeout milliseconds. If the Recipient List hasn't been able to send and process all replies within this timeframe, then the timeout triggers and the Recipient List breaks out, with message flow continuing to the next element. Note that if you provide a TimeoutAwareAggregationStrategy, its `timeout` method is invoked before breaking out. Beware: If the timeout is reached with running tasks still remaining, certain tasks for which it is difficult for Camel to shut down in a graceful manner may continue to run. So use this option with a bit of care. We may be able to improve this functionality in future Camel releases.
`onPrepareRef`		Camel 2.8: A custom Processor to prepare the copy of the Exchange each recipient will receive. This allows you to perform arbitrary transformations, such as deep-cloning the message payload (or any other custom logic).
`shareUnitOfWork`	`false`	Camel 2.8: Whether the unit of work should be shared. See the same option on Splitter for more details.

Static Recipient List

The following example shows how to route a request from an input queue:a endpoint to a static list of destinations

Using Annotations
You can use the RecipientList Annotation on a POJO to create a Dynamic Recipient List. For more details see the Bean Integration.

Using the Fluent Builders

Using the Spring XML Extensions

Dynamic Recipient List

Usually one of the main reasons for using the Recipient List pattern is that the list of recipients is dynamic and calculated at runtime. The following example demonstrates how to create a dynamic recipient list using an _expression_ (which in this case it extracts a named header value dynamically) to calculate the list of endpoints which are either of type Endpoint or are converted to a String and then resolved using the endpoint URIs.

Using the Fluent Builders

The above assumes that the header contains a list of endpoint URIs. The following takes a single string header and tokenizes it

Iteratable value

The dynamic list of recipients that are defined in the header must be iteratable such as:

java.util.Collection
java.util.Iterator
arrays
org.w3c.dom.NodeList
a single String with values separated with comma
any other type will be regarded as a single value

Using the Spring XML Extensions

For further examples of this pattern in use you could look at one of the junit test case

Using delimiter in Spring XML

In Spring DSL you can set the delimiter attribute for setting a delimiter to be used if the header value is a single String with multiple separated endpoints. By default Camel uses comma as delimiter, but this option lets you specify a customer delimiter to use instead.

So if myHeader contains a String with the value "activemq:queue:foo, activemq:topic:hello , log:bar" then Camel will split the String using the delimiter given in the XML that was comma, resulting into 3 endpoints to send to. You can use spaces between the endpoints as Camel will trim the value when it lookup the endpoint to send to.

Note: In Java DSL you use the tokenizer to archive the same. The route above in Java DSL:

    from("direct:a").recipientList(header("myHeader").tokenize(","));


In Camel 2.1 its a bit easier as you can pass in the delimiter as 2nd parameter:

    from("direct:a").recipientList(header("myHeader"), "#");



Sending to multiple recipients in parallel
Available as of Camel 2.2

The Recipient List now supports parallelProcessing that for example Splitter also supports. You can use it to use a thread pool to have concurrent tasks sending the Exchange to multiple recipients concurrently.


    from("direct:a").recipientList(header("myHeader")).parallelProcessing();



And in Spring XML its an attribute on the recipient list tag.


   <route>
       <from uri="direct:a"/>
       <recipientList parallelProcessing="true">
           <header>myHeader</header>
       </recipientList>
   </route>



Stop continuing in case one recipient failed
Available as of Camel 2.2

The Recipient List now supports stopOnException that for example Splitter also supports. You can use it to stop sending to any further recipients in case any recipient failed. 


    from("direct:a").recipientList(header("myHeader")).stopOnException();



And in Spring XML its an attribute on the recipient list tag.


   <route>
       <from uri="direct:a"/>
       <recipientList stopOnException="true">
           <header>myHeader</header>
       </recipientList>
   </route>



Note: You can combine parallelProcessing and stopOnException and have them both true.

Ignore invalid endpoints
Available as of Camel 2.3

The Recipient List now supports ignoreInvalidEndpoints which the Routing Slip also supports. You can use it to skip endpoints which is invalid.


    from("direct:a").recipientList(header("myHeader")).ignoreInvalidEndpoints();



And in Spring XML its an attribute on the recipient list tag.


   <route>
       <from uri="direct:a"/>
       <recipientList ignoreInvalidEndpoints="true">
           <header>myHeader</header>
       </recipientList>
   </route>



Then lets say the myHeader contains the following two endpoints direct:foo,xxx:bar. The first endpoint is valid and works. However the 2nd is invalid and will just be ignored. Camel logs at INFO level about, so you can see why the endpoint was invalid.


Using custom AggregationStrategy
Available as of Camel 2.2

You can now use you own AggregationStrategy with the Recipient List. However its not that often you need that. What its good for is that in case you are using Request Reply messaging then the replies from the recipient can be aggregated. By default Camel uses UseLatestAggregationStrategy which just keeps that last received reply. What if you must remember all the bodies that all the recipients send back, then you can use your own custom aggregator that keeps those. Its the same principle as with the Aggregator EIP so check it out for details.


    from("direct:a")
        .recipientList(header("myHeader")).aggregationStrategy(new MyOwnAggregationStrategy())
        .to("direct:b");




And in Spring XML its an attribute on the recipient list tag.


   <route>
       <from uri="direct:a"/>
       <recipientList strategyRef="myStrategy">
           <header>myHeader</header>
       </recipientList>
       <to uri="direct:b"/>
   </route>

   <bean id="myStrategy" class="com.mycompany.MyOwnAggregationStrategy"/>



Knowing which endpoint when using custom AggregationStrategy
Available as of Camel 2.12

When using a custom AggregationStrategy then the aggregate method is always invoked in the sequential order (also if parallel processing is enabled) of the endpoints the Recipient List is using. However from Camel 2.12 this is easier to know as the newExchange Exchange has a property stored (key is Exchange.RECIPIENT_LIST_ENDPOINT with the uri of the Endpoint.


    @Override
    public Exchange aggregate(Exchange oldExchange, Exchange newExchange) {
        String uri = newExchange.getProperty(Exchange.RECIPIENT_LIST_ENDPOINT, String.class);
        ...
    }





Using custom thread pool
Available as of Camel 2.2

A thread pool is only used for parallelProcessing. You supply your own custom thread pool via the ExecutorServiceStrategy (see Camel's Threading Model), the same way you would do it for the aggregationStrategy. By default Camel uses a thread pool with 10 threads (subject to change in a future version).

Using method call as recipient list
You can use a Bean to provide the recipients, for example:

from("activemq:queue:test").recipientList().method(MessageRouter.class, "routeTo");



And then MessageRouter:

public class MessageRouter {

    public String routeTo() {
        String queueName = "activemq:queue:test2";
        return queueName;
    }
}



When you use a Bean then do not also use the @RecipientList annotation as this will in fact add yet another recipient list, so you end up having two. Do not do like this.


public class MessageRouter {

    @RecipientList
    public String routeTo() {
        String queueName = "activemq:queue:test2";
        return queueName;
    }
}



Well you should only do like that above (using @RecipientList) if you route just route to a Bean which you then want to act as a recipient list.

So the original route can be changed to:

from("activemq:queue:test").bean(MessageRouter.class, "routeTo");


Which then would invoke the routeTo method and detect its annotated with @RecipientList and then act accordingly as if it was a recipient list EIP.


Using timeout
Available as of Camel 2.5

If you use parallelProcessing then you can configure a total timeout value in millis. Camel will then process the messages in parallel until the timeout is hit. This allows you to continue processing if one message is slow. For example you can set a timeout value of 20 sec.

Tasks may keep running
If the timeout is reached with running tasks still remaining, certain tasks for which it is difficult for Camel to shut down in a graceful manner may continue to run.  So use this option with a bit of care.  We may be able to improve this functionality in future Camel releases.

For example in the unit test below you can see we multicast the message to 3 destinations. We have a timeout of 2 seconds, which means only the last two messages can be completed within the timeframe. This means we will only aggregate the last two which yields a result aggregation which outputs "BC".




Timeout in other EIPs
This timeout feature is also supported by Splitter and both multicast and recipientList.

By default if a timeout occurs the AggregationStrategy is not invoked. However you can implement a specialized version

public interface TimeoutAwareAggregationStrategy extends AggregationStrategy {

    /**
     * A timeout occurred
     *
     * @param oldExchange  the oldest exchange (is <tt>null</tt> on first aggregation as we only have the new exchange)
     * @param index        the index
     * @param total        the total
     * @param timeout      the timeout value in millis
     */
    void timeout(Exchange oldExchange, int index, int total, long timeout);



This allows you to deal with the timeout in the AggregationStrategy if you really need to. 

Timeout is total
The timeout is total, which means that after X time, Camel will aggregate the messages which has completed within the timeframe. The remainders will be cancelled. Camel will also only invoke the timeout method in the TimeoutAwareAggregationStrategy once, for the first index which caused the timeout.


Using onPrepare to execute custom logic when preparing messages
Available as of Camel 2.8

See details at Multicast


Using This Pattern

If you would like to use this EIP Pattern then please read the Getting Started, you may also find the Architecture useful particularly the description of Endpoint and URIs. Then you could try out some of the Examples first before trying this pattern out.

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[CONF] Apache Camel > Recipient List

Recipient List

Page edited by Claus Ibsen

Changes (2)

Full Content

Recipient List

Options

Static Recipient List

Dynamic Recipient List

Iteratable value

Using delimiter in Spring XML

Sending to multiple recipients in parallel

Stop continuing in case one recipient failed

Ignore invalid endpoints

Using custom `AggregationStrategy`

Knowing which endpoint when using custom `AggregationStrategy`

Using custom thread pool

Using method call as recipient list

Using timeout

Using onPrepare to execute custom logic when preparing messages

Using This Pattern

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