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Aggregator

This applies for Camel version 2.3 or newer. If you use an older version then use this Aggregator link instead.

The Aggregator from the EIP patterns allows you to combine a number of messages together into a single message.

A correlation _expression_ is used to determine the messages which should be aggregated together. If you want to aggregate all messages into a single message, just use a constant _expression_. An AggregationStrategy is used to combine all the message exchanges for a single correlation key into a single message exchange.

Aggregator options

The aggregator supports the following options:

Option	Default	Description
correlationExpression		Mandatory _expression_ which evaluates the correlation key to use for aggregation. The Exchange which has the same correlation key is aggregated together. If the correlation key could not be evaluated an Exception is thrown. You can disable this by using the `ignoreBadCorrelationKeys` option.
aggregationStrategy		Mandatory `AggregationStrategy` which is used to merge the incoming Exchange with the existing already merged exchanges. At first call the `oldExchange` parameter is `null`. On subsequent invocations the `oldExchange` contains the merged exchanges and `newExchange` is of course the new incoming Exchange.
strategyRef		A reference to lookup the `AggregationStrategy` in the Registry.
completionSize		Number of messages aggregated before the aggregation is complete. This option can be set as either a fixed value or using an _expression_ which allows you to evaluate a size dynamically - will use `Integer` as result. If both are set Camel will fallback to use the fixed value if the _expression_ result was `null` or `0`.
completionTimeout		Time in millis that an aggregated exchange should be inactive before its complete. This option can be set as either a fixed value or using an _expression_ which allows you to evaluate a timeout dynamically - will use `Long` as result. If both are set Camel will fallback to use the fixed value if the _expression_ result was `null` or `0`. You cannot use this option together with completionInterval, only one of the two can be used.
completionInterval		A repeating period in millis by which the aggregator will complete all current aggregated exchanges. Camel has a background task which is triggered every period. You cannot use this option together with completionTimeout, only one of them can be used.
completionPredicate		A Predicate to indicate when an aggregated exchange is complete.
completionFromBatchConsumer	`false`	This option is if the exchanges are coming from a Batch Consumer. Then when enabled the Aggregator2 will use the batch size determined by the Batch Consumer in the message header `CamelBatchSize`. See more details at Batch Consumer. This can be used to aggregate all files consumed from a File endpoint in that given poll.
eagerCheckCompletion	`false`	Whether or not to eager check for completion when a new incoming Exchange has been received. This option influences the behavior of the `completionPredicate` option as the Exchange being passed in changes accordingly. When `false` the Exchange passed in the Predicate is the aggregated Exchange which means any information you may store on the aggregated Exchange from the `AggregationStrategy` is available for the Predicate. When `true` the Exchange passed in the Predicate is the incoming Exchange, which means you can access data from the incoming Exchange.
groupExchanges	`false`	If enabled then Camel will group all aggregated Exchanges into a single combined `org.apache.camel.impl.GroupedExchange` holder class that holds all the aggregated Exchanges. And as a result only one Exchange is being sent out from the aggregator. Can be used to combine many incoming Exchanges into a single output Exchange without coding a custom `AggregationStrategy` yourself.
ignoreInvalidCorrelationKeys	`false`	Whether or not to ignore correlation keys which could not be evaluated to a value. By default Camel will throw an Exception, but you can enable this option and ignore the situation instead.
closeCorrelationKeyOnCompletion		Whether or not too late Exchanges should be accepted or not. You can enable this to indicate that if a correlation key has already been completed, then any new exchanges with the same correlation key be denied. Camel will then throw a `closedCorrelationKeyException` exception. When using this option you pass in a `integer` which is a number for a LRUCache which keeps that last X number of closed correlation keys. You can pass in 0 or a negative value to indicate a unbounded cache. By passing in a number you are ensured that cache won't grow too big if you use a log of different correlation keys.
discardOnCompletionTimeout	`false`	Camel 2.5: Whether or not exchanges which complete due to a timeout should be discarded. If enabled then when a timeout occurs the aggregated message will not be sent out but dropped (discarded).
aggregationRepository		Allows you to plugin you own implementation of `org.apache.camel.spi.AggregationRepository` which keeps track of the current inflight aggregated exchanges. Camel uses by default a memory based implementation.
aggregationRepositoryRef		Reference to lookup a `aggregationRepository` in the Registry.
parallelProcessing	`false`	When aggregated are completed they are being send out of the aggregator. This option indicates whether or not Camel should use a thread pool with multiple threads for concurrency. If no custom thread pool has been specified then Camel creates a default pool with 10 concurrent threads.
executorService		If using `parallelProcessing` you can specify a custom thread pool to be used. In fact also if you are not using `parallelProcessing` this custom thread pool is used to send out aggregated exchanges as well.
executorServiceRef		Reference to lookup a `executorService` in the Registry

Exchange Properties

The following properties are set on each aggregated Exchange:

header	type	description
`CamelAggregatedSize`	int	The total number of Exchanges aggregated into this combined Exchange.
`CamelAggregatedCompletedBy`	String	Indicator how the aggregation was completed as a value of either: `predicate`, `size`, `consumer`, `timeout` or `interval`.

About AggregationStrategy

The AggregationStrategy is used for aggregating the old (lookup by its correlation id) and the new exchanges together into a single exchange. Possible implementations include performing some kind of combining or delta processing, such as adding line items together into an invoice or just using the newest exchange and removing old exchanges such as for state tracking or market data prices; where old values are of little use.

Notice the aggregation strategy is a mandatory option and must be provided to the aggregator.

Here are a few example AggregationStrategy implementations that should help you create your own custom strategy.

//simply combines Exchange String body values using '+' as a delimiter
class StringAggregationStrategy implements AggregationStrategy {


    public Exchange aggregate(Exchange oldExchange, Exchange newExchange) {
        if (oldExchange == null) {
            return newExchange;
        }

        String oldBody = oldExchange.getIn().getBody(String.class);
        String newBody = newExchange.getIn().getBody(String.class);
        oldExchange.getIn().setBody(oldBody + "+" + newBody);
        return oldExchange;
    }
}

//simply combines Exchange body values into an ArrayList<Object>
class ArrayListAggregationStrategy implements AggregationStrategy {

    public Exchange aggregate(Exchange oldExchange, Exchange newExchange) {
	Object newBody = newExchange.getIn().getBody();
	ArrayList<Object> list = null;
        if (oldExchange == null) {
		list = new ArrayList<Object>();
		list.add(newBody);
		newExchange.getIn().setBody(list);
		return newExchange;
        } else {
	        list = oldExchange.getIn().getBody(ArrayList.class);
		list.add(newBody);
		return oldExchange;
	}
    }
}

About completion

When aggregation Exchanges at some point you need to indicate that the aggregated exchanges is complete, so they can be send out of the aggregator. Camel allows you to indicate completion in various ways as follows:

completionTimeout - Is an inactivity timeout in which is triggered if no new exchanges have been aggregated for that particular correlation key within the period.
completionInterval - Once every X period all the current aggregated exchanges are completed.
completionSize - Is a number indicating that after X aggregated exchanges it's complete.
completionPredicate - Runs a Predicate when a new exchange is aggregated to determine if we are complete or not
completionFromBatchConsumer - Special option for Batch Consumer which allows you to complete when all the messages from the batch has been aggregated. |

Notice that all the completion ways are per correlation key. And you can combine them in any way you like. It's basically the first which triggers that wins. So you can use a completion size together with a completion timeout. Only completionTimeout and completionInterval cannot be used at the same time.

Notice the completion is a mandatory option and must be provided to the aggregator. If not provided Camel will thrown an Exception on startup.

Persistent AggregationRepository

The aggregator provides a pluggable repository which you can implement your own org.apache.camel.spi.AggregationRepository.
If you need persistent repository then you can use either Camel HawtDB or SQL Component components.

Examples

See some examples from the old Aggregator which is somewhat similar to this new aggregator.

Setting options in Spring XML
Many of the options are configurable as attributes on the <aggregate> tag when using Spring XML.

Using completionTimeout

In this example we want to aggregate all incoming messages and after 3 seconds of inactivity we want the aggregation to complete. This is done using the completionTimeout option as shown:

from("direct:start")
    // aggregate all exchanges correlated by the id header.
    // Aggregate them using the BodyInAggregatingStrategy strategy which
    // and after 3 seconds of inactivity them timeout and complete the aggregation
    // and send it to mock:aggregated
    .aggregate(header("id"), new BodyInAggregatingStrategy()).completionTimeout(3000)
        .to("mock:aggregated");