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.
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).
Torsten Mielke wrote a nice blog entry with his thoughts and experience on using the aggreagator. Its a well worth read.
In Camel 2.0 the
On the first invocation of the
You cannot use both
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.
JMSDestinationheader as the correlation key; or some custom header for the stock symbol (using the above stock market example).
from("activemq:someReallyFastTopic") .aggregator(header("JMSDestination")) .to("activemq:someSlowTopicForGuis");
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 from("seda:start").aggregate().xpath("/stockQuote/@symbol", String.class).batchSize(5).to("mock:result"); //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. from("seda:start").aggregate().xpath("name(/stockQuote[@symbol='APACHE'])", String.class).batchSize(5).to("mock:result");
For further examples of this pattern in use you could look at the junit test case
Using the Spring XML Extensions
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.
AggregationStrategyif you prefer as shown in the following example
strategyRefattribute 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
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).
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:
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:
You can also specify a custom aggregation collection in the Spring DSL. Here is an example for Camel 2.0
<aggregator batchTimeout="500" collectionRef="aggregatorCollection"> <expression/> <to uri="mock:result"/> </aggregator>
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:
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.
from("file://inbox") .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.