This applies for Camel version 2.2 or older. If you use a newer version then the Aggregator has been rewritten from Camel 2.3 on and you should use this Aggregator2 link instead.
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).
Using the aggregator correctly
Torsten Mielke wrote a nice blog entry with his thoughts and experience on using the aggreagator. Its a well worth read.
AggregationStrategy changed in Camel 2.0
In Camel 2.0 the
AggregationStrategy callback have been changed to also be invoked on the very first Exchange.
On the first invocation of the
aggregate method the
oldExchange parameter is
null. The reason is that we have not aggregated anything yet.
So its only the
newExchange that has a value. Usually you just return the
newExchange in this situation. But you still have the power to decide what to do, for example you can do some alternation on the exchange or remove some headers. And a more common use case is for instance to count some values from the body payload. That could be to sum up a total amount etc.
BatchTimeout and CompletionPredicate
You cannot use both
completionPredicate to trigger a completion based on either on reaching its goal first. The batch timeout will always trigger first, at that given interval.
Using the Fluent Builders
The following example shows how to aggregate messages so that only the latest message for a specific value of the
cheese header are sent.
JMSDestination header as the correlation key; or some custom header for the stock symbol (using the above stock market example).
For further examples of this pattern in use you could look at the junit test case
Using the Spring XML Extensions
correlationExpression element is in Camel 2.0. For earlier versions of Camel you will need to specify your expression without the enclosing
The following example shows how to create a simple aggregator using the XML notation; using an Expression for the correlation value used to aggregate messages together.
AggregationStrategy if you prefer as shown in the following example
strategyRef attribute is used on the
<aggregator> element to refer to the custom strategy in Spring.
The following properties is set on each Exchange that are aggregated:
Camel 1.x: The total number of Exchanges aggregated in this combined Exchange.
Camel 2.0: The total number of Exchanges aggregated into this combined Exchange.
Camel 2.0: The current index of this Exchange in the batch.
The aggregator supports the following batch options:
The in batch size. This is the number of incoming exchanges that is processed by the aggregator and when this threshold is reached the batch is completed and send.
Camel 1.6.2/2.0: You can disable the batch size so the Aggregator is only triggered by timeout by setting the
In Camel 1.6.1 or older you can set the
Camel 1.5: The out batch size. This is the number of exchanges currently aggregated in the
Timeout in millis. How long should the aggregator wait before its completed and sends whatever it has currently aggregated.
Camel 2.0: If enabled then Camel will group all aggregated Exchanges into a single combined
Camel 2.0: This option is if the exchanges is coming from a Batch Consumer. Then when enabled the Aggregator will use the batch size determined by the Batch Consumer in the message header
Allows you to use a Predicate to signal when an aggregation is complete. See warning in top of this page.
This aggregator uses a
AggregationCollection to store the exchanges that is currently aggregated. The
AggregationCollection uses a correlation Expression and an
- The correlation Expression is used to correlate the incoming exchanges. The default implementation will group messages based on the correlation expression. Other implementations could for instance just add all exchanges as a batch.
- The strategy 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.
Camel provides these implementations:
By default Camel uses
UseLatestAggregationStrategy, so this simple example will just keep the latest received exchange for the given correlation Expression:
PredicateAggregationCollection is an extension to
DefaultAggregationCollection that uses a Predicate as well to determine the completion. For instance the Predicate can test for a special header value, a number of maximum aggregated so far etc. To use this the routing is a bit more complex as we need to create our
AggregationCollection object as follows:
Using this the aggregator will complete if we receive 3 exchanges with the same correlation id or when the specified timeout of 500 msecs has elapsed (whichever criteria is met first).
Using Custom Aggregation Strategy
In this example we will aggregate incoming bids and want to aggregate the highest bid. So we provide our own strategy where we implement the code logic:
Using Custom Aggregation Collection
In this example we will aggregate incoming bids and want to aggregate the bids in reverse order (this is just an example). So we provide our own collection where we implement the code logic:
Custom aggregation collection in Spring DSL
You can also specify a custom aggregation collection in the Spring DSL. Here is an example for Camel 2.0
Using Grouped Exchanges
Available as of Camel 2.0
You can enable grouped exchanges to combine all aggregated exchanges into a single
org.apache.camel.impl.GroupedExchange holder class that contains all the individual aggregated exchanges. This allows you to process a single Exchange containing all the aggregated exchange. Lets start with how to configure this in the router:
Using Batch Consumer
Available as of Camel 2.0
The Aggregator can work together with the Batch Consumer to aggregate the total number of messages that the Batch Consumer have reported. This allows you for instance to aggregate all files polled using the File consumer.
batchConsumer Camel will automatic adjust the
batchSize according to reported by the Batch Consumer in this case the file consumer. So if we poll in 7 files then the aggregator will aggregate all 7 files before it completes. As the timeout is still in play we set it to 60 seconds.
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.