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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. The default strategy just chooses the latest message; so its ideal for throttling messages.

For example, imagine a stock market data system; you are receiving 30,000 messages per second; you may want to throttle down the updates as, say, a GUI cannot cope with such massive update rates. So you may want to aggregate these messages together so that within a window (defined by a maximum number of messages or a timeout), messages for the same stock are aggregated together; by just choosing the latest message and discarding the older prices. (You could apply a delta processing algorithm if you prefer to capture some of the history).

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 oldExchang parameter is null. On subsequent invocations the oldExchnage contains the merged exchanges and newExchange is of course the new incoming Exchange.

aggregationStrategyRef

 

A reference to lookup the AggregationStrategy in the Registry.

completionSize

 

Number of messages aggregated before the aggregation is complete.

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.

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 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.

ignoreBadCorrelationKeys

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.

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

Exchange Properties

The following properties is set on each Exchange that are aggregated:

header

type

description

CamelAggregatedSize

int

The total number of Exchanges aggregated into this combined Exchange.

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.

Examples

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.

See also

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