Aggregator
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 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).
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 correlationExpression element is in Camel 2.0. For earliler versions of Camel you will need to specify your expression without the enclosing correlationExpression element.
<aggregator> <simple>header.cheese</simple> <to uri="mock:result"/> </aggregator>
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.
Message Headers
The following headers is set on each Exchange that are aggregated:
header |
type |
description |
---|---|---|
|
int |
Camel 1.x: The total number of Exchanges aggregated in this combined Exchange. |
|
int |
Camel 2.0: The total number of Exchanges aggregated into this combined Exchange. |
Batch options
The aggregator supports the following batch options:
Option |
Default |
Description |
---|---|---|
batchSize |
100 |
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. |
outBatchSize |
0 |
Available as of Camel 1.5. The out batch size. This is the number of exchanges currently aggregated in the |
batchTimeout |
1000L |
Timeout in millis. How long should the aggregator wait before its completed and sends whatever it has currently aggregated. |
groupExchanges |
false |
Camel 2.-0: If enabled then Camel will group all aggregated Exchanges into a single combined |
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:
- DefaultAggregationCollection
- PredicateAggregationCollection
- UseLatestAggregationStrategy
Examples
Default example
By default Camel uses DefaultAggregationCollection
and UseLatestAggregationStrategy
, so this simple example will just keep the latest received exchange for the given correlation Expression:
Using PredicateAggregationCollection
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:
header(Exchange.AGGREGATED_COUNT).isEqualTo(3)
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"> <expression/> <to uri="mock:result"/> </aggregator>
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 using the GroupedExchange#get
method.
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
- The Loan Broker Example which uses an aggregator