Recipient List

Camel supports the Recipient List from the EIP patterns.

How do we route a message to a list of dynamically specified recipients?

image

Define a channel for each recipient. Then use a Recipient List to inspect an incoming message, determine the list of desired recipients, and forward the message to all channels associated with the recipients in the list.

Options

The Recipient List eip supports 2 options, which are listed below.

Name Description Default Type

description

Sets the description of this node.

String

disabled

Whether to disable this EIP from the route during build time. Once an EIP has been disabled then it cannot be enabled later at runtime.

false

Boolean

expression

Required Expression that returns which endpoints (url) to send the message to (the recipients). If the expression return an empty value then the message is not sent to any recipients.

ExpressionDefinition

delimiter

Delimiter used if the Expression returned multiple endpoints. Can be turned off using the value false. The default value is ,.

,

String

aggregationStrategy

Sets the AggregationStrategy to be used to assemble the replies from the recipients, into a single outgoing message from the RecipientList. By default Camel will use the last reply as the outgoing message. You can also use a POJO as the AggregationStrategy.

AggregationStrategy

aggregationStrategyMethodName

This option can be used to explicit declare the method name to use, when using POJOs as the AggregationStrategy.

String

aggregationStrategyMethodAllowNull

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.

false

Boolean

parallelAggregate

Deprecated If enabled then the aggregate method on AggregationStrategy can be called concurrently. Notice that this would require the implementation of AggregationStrategy to be implemented as thread-safe. By default this is false meaning that Camel synchronizes the call to the aggregate method. Though in some use-cases this can be used to archive higher performance when the AggregationStrategy is implemented as thread-safe.

false

Boolean

parallelProcessing

If enabled then sending messages to the recipients occurs concurrently. Note the caller thread will still wait until all messages has been fully processed, before it continues. Its only the sending and processing the replies from the recipients which happens concurrently. When parallel processing is enabled, then the Camel routing engin will continue processing using last used thread from the parallel thread pool. However, if you want to use the original thread that called the recipient list, then make sure to enable the synchronous option as well. In parallel processing mode, you may want to also synchronous = true to force this EIP to process the sub-tasks using the upper bounds of the thread-pool. If using synchronous = false then Camel will allow its reactive routing engine to use as many threads as possible, which may be available due to sub-tasks using other thread-pools such as CompletableFuture.runAsync or others.

false

Boolean

synchronous

Sets whether synchronous processing should be strictly used. When enabled then the same thread is used to continue routing after the recipient list is complete, even if parallel processing is enabled.

false

Boolean

timeout

Sets a total timeout specified in millis, when using parallel processing. If the Recipient List hasn’t been able to send and process all replies within the given timeframe, then the timeout triggers and the Recipient List breaks out and continues. Notice if you provide a TimeoutAwareAggregationStrategy then the timeout method is invoked before breaking out. 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.

0

String

executorService

To use a custom Thread Pool to be used for parallel processing. Notice if you set this option, then parallel processing is automatic implied, and you do not have to enable that option as well.

ExecutorService

stopOnException

Will now stop further processing if an exception or failure occurred during processing of an org.apache.camel.Exchange and the caused exception will be thrown. Will also stop if processing the exchange failed (has a fault message) or an exception was thrown and handled by the error handler (such as using onException). In all situations the recipient list will stop further processing. This is the same behavior as in pipeline, which is used by the routing engine. The default behavior is to not stop but continue processing till the end.

false

Boolean

ignoreInvalidEndpoints

Ignore the invalidate endpoint exception when try to create a producer with that endpoint.

false

Boolean

streaming

If enabled then Camel will process replies out-of-order, eg in the order they come back. If disabled, Camel will process replies in the same order as defined by the recipient list.

false

Boolean

onPrepare

Uses the Processor when preparing the org.apache.camel.Exchange to be used send. This can be used to deep-clone messages that should be send, or any custom logic needed before the exchange is send.

Processor

cacheSize

Sets the maximum size used by the org.apache.camel.spi.ProducerCache which is used to cache and reuse producers when using this recipient list, when uris are reused. Beware that when using dynamic endpoints then it affects how well the cache can be utilized. If each dynamic endpoint is unique then its best to turn off caching by setting this to -1, which allows Camel to not cache both the producers and endpoints; they are regarded as prototype scoped and will be stopped and discarded after use. This reduces memory usage as otherwise producers/endpoints are stored in memory in the caches. However if there are a high degree of dynamic endpoints that have been used before, then it can benefit to use the cache to reuse both producers and endpoints and therefore the cache size can be set accordingly or rely on the default size (1000). If there is a mix of unique and used before dynamic endpoints, then setting a reasonable cache size can help reduce memory usage to avoid storing too many non frequent used producers.

Integer

shareUnitOfWork

Shares the org.apache.camel.spi.UnitOfWork with the parent and each of the sub messages. Recipient List will by default not share unit of work between the parent exchange and each recipient exchange. This means each sub exchange has its own individual unit of work.

false

Boolean
See the cacheSize option for more details on how much cache to use depending on how many or few unique endpoints are used.

Exchange properties

The Recipient List eip supports 2 exchange properties, which are listed below.

The exchange properties are set on the Exchange by the EIP, unless otherwise specified in the description. This means those properties are available after this EIP has completed processing the Exchange.

Name Description Default Type

CamelRecipientListEndpoint

The endpoint uri of this recipient list.

String

CamelToEndpoint

Endpoint URI where this Exchange is being sent to.

String

Using Recipient List

The Recipient List EIP allows routing the same message to a number of endpoints and process them in a different way.

There can be one or more destinations, and Camel will execute them sequentially (by default). However, a parallel mode exists which allows processing messages concurrently.

The Recipient List EIP has many features and is based on the Multicast EIP. For example, the Recipient List EIP is capable of aggregating each message into a single response message as the result after the Recipient List EIP.

Using 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 constant:

Java
from("jms:queue:a")
    .recipientList(constant("seda:x,seda:y,seda:z"));
XML
<route>
    <from uri="jms:queue:a"/>
    <recipientList>
        <constant>seda:x,seda:y,seda:z</constant>
    </recipientList>
</route>

Using 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 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 (separated by comma).

Java
from("jms:queue:a")
    .recipientList(header("foo"));
XML
<route>
    <from uri="jms:queue:a"/>
    <recipientList>
        <header>foo</constant>
    </recipientList>
</route>

How is dynamic destinations evaluated

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

  • java.util.Collection

  • java.util.Iterator

  • arrays

  • org.w3c.dom.NodeList

  • a single String with values separated by comma (the delimiter configured)

  • any other type will be regarded as a single value

Configuring delimiter for dynamic destinations

In XML 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 custom delimiter to use instead.

<route>
  <from uri="direct:a"/>
  <!-- use semicolon as a delimiter for String-based values -->
  <recipientList delimiter=";">
    <header>myHeader</header>
  </recipientList>
</route>

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 three endpoints to send to. You can use spaces between the endpoints as Camel will trim the value when it looks up the endpoint to send to.

And in Java DSL, you specify the delimiter as second parameter as shown below:

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

Using parallel processing

The Recipient List supports parallelProcessing similar to what Multicast and Split EIPs have as well. When using parallel processing, then a thread pool is used to have concurrent tasks sending the Exchange to multiple recipients concurrently.

You can enable parallel mode using parallelProcessing as shown:

Java
from("direct:a")
    .recipientList(header("myHeader")).parallelProcessing();
XML
<route>
    <from uri="direct:a"/>
    <recipientList parallelProcessing="true">
        <header>myHeader</header>
    </recipientList>
</route>

When parallel processing is enabled, then the Camel routing engin will continue processing using last used thread from the parallel thread pool. However, if you want to use the original thread that called the recipient list, then make sure to enable the synchronous option as well.

Using custom thread pool

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 future versions).

The Recipient List EIP will by default continue to process the entire exchange even in case one of the sub messages will throw an exception during routing.

For example, if you want to route to three destinations and the second destination fails by an exception. What Camel does by default is to process the remainder destinations. You have the chance to deal with the exception when aggregating using an AggregationStrategy.

But sometimes you want the Camel to stop and let the exception be propagated back, and let the Camel Error Handler handle it. You can do this by specifying that it should stop in case of an exception occurred. This is done by the stopOnException option as shown below:

Java
from("direct:start")
    .recipientList(header("whereTo")).stopOnException()
    .to("mock:result");

    from("direct:foo").to("mock:foo");

    from("direct:bar").process(new MyProcessor()).to("mock:bar");

    from("direct:baz").to("mock:baz");
XML
<routes>
    <route>
        <from uri="direct:start"/>
        <recipientList stopOnException="true">
            <header>whereTo</header>
        </recipientList>
        <to uri="mock:result"/>
    </route>

    <route>
        <from uri="direct:foo"/>
        <to uri="mock:foo"/>
    </route>

    <route>
        <from uri="direct:bar"/>
        <process ref="myProcessor"/>
        <to uri="mock:bar"/>
    </route>

    <route>
        <from uri="direct:baz"/>
        <to uri="mock:baz"/>
    </route>
</routes>

In this example suppose a message is sent with the header whereTo=direct:foo,direct:bar,direct:baz that means the recipient list sends messages to those three endpoints.

Now suppose that the MyProcessor is causing a failure and throws an exception. This means the Recipient List EIP will stop after this, and not the last route (direct:baz).

Ignore invalid endpoints

The Recipient List supports ignoreInvalidEndpoints (like Routing Slip EIP). You can use it to skip endpoints which are invalid.

Java
from("direct:a")
    .recipientList(header("myHeader")).ignoreInvalidEndpoints();
XML
<route>
    <from uri="direct:a"/>
    <recipientList ignoreInvalidEndpoints="true">
        <header>myHeader</header>
    </recipientList>
</route>

Then let us say the myHeader contains the following two endpoints direct:foo,xxx:bar. The first endpoint is valid and works. However, the second one is invalid and will just be ignored. Camel logs at DEBUG level about it, so you can see why the endpoint was invalid.

Using timeout

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 consumer 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 challenging for Camel to shut down in a graceful manner may continue to run. So use this option with a bit of care.

For example, in the unit test below, you can see that we multicast the message to three 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".

from("direct:start")
    .multicast(new AggregationStrategy() {
            public Exchange aggregate(Exchange oldExchange, Exchange newExchange) {
                if (oldExchange == null) {
                    return newExchange;
                }

                String body = oldExchange.getIn().getBody(String.class);
                oldExchange.getIn().setBody(body + newExchange.getIn().getBody(String.class));
                return oldExchange;
            }
        })
        .parallelProcessing().timeout(250).to("direct:a", "direct:b", "direct:c")
    // use end to indicate end of multicast route
    .end()
    .to("mock:result");

from("direct:a").delay(1000).to("mock:A").setBody(constant("A"));

from("direct:b").to("mock:B").setBody(constant("B"));

from("direct:c").to("mock:C").setBody(constant("C"));

By default, if a timeout occurs the AggregationStrategy is not invoked. However, you can implement the timeout method: 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 have completed within the timeframe. The remainder will be canceled. Camel will also only invoke the timeout method in the TimeoutAwareAggregationStrategy once, for the first index which caused the timeout.

Using ExchangePattern in recipients

The recipient list will by default use the current Exchange Pattern. Though one can imagine use-cases where one wants to send a message to a recipient using a different exchange pattern. For example, you may have a route that initiates as an InOnly route, but want to use InOut exchange pattern with a recipient list. You can configure the exchange pattern directly in the recipient endpoints.

For example, in the route below we pick up new files (which will be started as InOnly) and then route to a recipient list. As we want to use InOut with the ActiveMQ (JMS) endpoint we can now specify this using the exchangePattern=InOut option. Then the response from the JMS request/reply will then be continued routed, and thus the response is what will be stored in as a file in the outbox directory.

from("file:inbox")
    // the exchange pattern is InOnly initially when using a file route
    .recipientList().constant("activemq:queue:inbox?exchangePattern=InOut")
    .to("file:outbox");

The recipient list will not alter the original exchange pattern. So in the example above the exchange pattern will still be InOnly when the message is routed to the file:outbox endpoint. If you want to alter the exchange pattern permanently then use .setExchangePattern in the route.

See more details at Event Message and Request Reply EIPs.

See Also

Because Recipient List EIP is based on the Multicast, then you can find more information in Multicast EIP about features that are also available with Recipient List EIP.