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2.3 or newer. If you use an older version then use this [Aggregator] link instead.

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The [Aggregator|http://www.enterpriseintegrationpatterns.com/Aggregator.html] from the [EIP patterns|Enterprise Integration Patterns] allows you to combine a number of messages together into a single message. 

!http://www.enterpriseintegrationpatterns.com/img/Aggregator.gif!

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. 

h3. Aggregator options
The aggregator supports the following options:
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|| 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 {{oldExchang}} parameter is {{null}}. On subsequent invocations the {{oldExchnage}} 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. Camel has a background task that runs once a minute to check for inactive aggregated exchanges. 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 can be used. |
| completionInterval | | A repeating period in millis by which the aggregator will complete all current aggregated exchanges. Camel has a background tasks which is trigger every period. You cannot use this option together with completionTimeout, only one of the can be used. | 
| completionPredicate | | A [Predicate] to indicate when an aggregated exchange is complete. |
| completionFromBatchConsumer | {{false}} | This option is if the exchanges is 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|File2] 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 avail 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 incomming 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 thrown an Exception, but you can enable this option and ignore the situation instead. |
| closeCorrelationKeyOnCompletion |  | Whether or not too _late_ Exchange 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 wont grown too big if you use a log of different correlation keys. |
| 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.

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| 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 not 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] |
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h3. Exchange Properties
The following properties is set on each Exchange that are aggregated:
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|| 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}}. |
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h3. About AggregationStrategy
The {{AggregationStrategy}} is used for aggregate 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.


h3. About completion
When aggregation [Exchange]s 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 has been aggregated for that particular correlation key within the period.
- completionInterval - Once every X period all the current aggregated exchanges is completed.
- completionSize - Is a number indicating that after X aggregated exchanges its 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 your like. Its 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.


h3. 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 the Camel [HawtDB] component.

In this example we want to aggregate all incoming messages and when we have 3 messages aggregated (in the same correlation group) we want the aggregation to complete. This is done using the {{completionSize}} option as shown:

In this example we want to aggregate all incoming messages and after a period of inactivity we want the aggregation to complete. The period should be computed at runtime based on the {{timeout}} header in the incoming messages. This is done using the {{completionTimeout}} option as shown:

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:Using This

 Pattern}

h3. See also
- The [Loan Broker Example] which uses an aggregator
- [Blog post by Torsten Mielke|http://tmielke.blogspot.com/2009/01/using-camel-aggregator-correctly.html] about using the aggregator correctly.
- The old [Aggregator] 
- [HawtDB] for persistence support
- [Aggregate Example] for an example application