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 onwards and you should use this Aggregator2 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).
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 batchTimeout and 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.
If you were using JMS then you may wish to use the JMSDestination header as the correlation key; or some custom header for the stock symbol (using the above stock market example).
You can of course use many different Expression languages such as XPath, XQuery, SQL or various Scripting Languages.
Here is an example using XPath:
//aggregate based on the message content using an XPath expression
//example assumes an XML document starting with <stockQuote symbol='...'>
//aggregate messages based on their symbol attribute within the <stockQuote> element
//this example will aggregate all messages starting with <stockQuote symbol='APACHE'> into
//one exchange and all the other messages (different symbol or different root element) into another exchange.
For further examples of this pattern in use you could look at the junit test case
Using the Spring XML Extensions
The correlationExpression element is in Camel 2.0. For earliler versions of Camel you will need to specify your expression without the enclosing correlationExpression element.
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.
You can specify your own AggregationStrategy if you prefer as shown in the following example
Notice how the 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
batchSize to 0 (or negative). In Camel 1.6.1 or older you can set the
batchSize to a very large number to archive the same.
Camel 1.5: The out batch size. This is the number of exchanges currently aggregated in the
AggregationCollection. When this threshold is reached the batch is completed and send. By default this option is disabled. The difference to the
batchSize options is that this is for outgoing, so setting this size to e.g. 50 ensures that this batch will at maximum contain 50 exchanges when its sent.
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
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.
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
CamelBatchSize. See more details at Batch Consumer. This can be used to aggregate all files consumed from a File endpoint in that given poll.
Allows you to use a Predicate to signal when an aggregation is complete. See warning in top of this page.
AggregationCollection and AggregationStrategy
This aggregator uses a AggregationCollection to store the exchanges that is currently aggregated. The AggregationCollection uses a correlation Expression and an AggregationStrategy.
- 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:
The 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:
In this sample we use the predicate that we want at most 3 exchanges aggregated by the same correlation id, this is defined as:
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:
Then we setup the routing as follows:
And since this is based on an unit test we show the test code that send the bids and what is expected as the winners:
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:
Then we setup the routing as follows:
And since this is based on an unit test we show the test code that send the bids and what is expected as the expected reverse order:
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
In Camel 1.5.1 you will need to specify the aggregator as
<aggregator batchTimeout="500" collectionRef="aggregatorCollection">
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:
And the next part is part of an unit code that demonstrates this feature as we send in 5 exchanges each with a different value in the body.
And we will only get 1 exchange out of the aggregator, but we can access all the individual aggregated exchanges from the List which we can extract as a property from the Exchange using the key
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.
.aggregate(xpath("//order/@customerId"), new AggregateCustomerOrderStrategy()).batchConsumer().batchTimeout(60000).to("bean:processOrder");
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.