Aggregator2

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.

Aggregator options

The aggregator supports the following options:

Exchange Properties

The following properties are set on each aggregated Exchange:

About AggregationStrategy

The AggregationStrategy is used for aggregating 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.

Notice the aggregation strategy is a mandatory option and must be provided to the aggregator.

Here are a few example AggregationStrategy implementations that should help you create your own custom strategy.

//simply combines Exchange String body values using '+' as a delimiter
class StringAggregationStrategy implements AggregationStrategy {

    public Exchange aggregate(Exchange oldExchange, Exchange newExchange) {
        if (oldExchange == null) {
            return newExchange;
        }

        String oldBody = oldExchange.getIn().getBody(String.class);
        String newBody = newExchange.getIn().getBody(String.class);
        oldExchange.getIn().setBody(oldBody + "+" + newBody);
        return oldExchange;
    }
}

//simply combines Exchange body values into an ArrayList<Object>
class ArrayListAggregationStrategy implements AggregationStrategy {

    public Exchange aggregate(Exchange oldExchange, Exchange newExchange) {
	Object newBody = newExchange.getIn().getBody();
	ArrayList<Object> list = null;
        if (oldExchange == null) {
		list = new ArrayList<Object>();
		list.add(newBody);
		newExchange.getIn().setBody(list);
		return newExchange;
        } else {
	        list = oldExchange.getIn().getBody(ArrayList.class);
		list.add(newBody);
		return oldExchange;
	}
    }
}

About completion

When aggregation Exchanges at some point you need to indicate that the aggregated exchanges is complete, so they can be send out of the aggregator. Camel allows you to indicate completion in various ways as follows:

• completionTimeout - Is an inactivity timeout in which is triggered if no new exchanges have been aggregated for that particular correlation key within the period.
• completionInterval - Once every X period all the current aggregated exchanges are completed.
• completionSize - Is a number indicating that after X aggregated exchanges it's complete.
• completionPredicate - Runs a Predicate when a new exchange is aggregated to determine if we are complete or not
• completionFromBatchConsumer - Special option for Batch Consumer which allows you to complete when all the messages from the batch has been aggregated. |

Notice that all the completion ways are per correlation key. And you can combine them in any way you like. It's basically the first which triggers that wins. So you can use a completion size together with a completion timeout. Only completionTimeout and completionInterval cannot be used at the same time.

Notice the completion is a mandatory option and must be provided to the aggregator. If not provided Camel will thrown an Exception on startup.

Persistent AggregationRepository

The aggregator provides a pluggable repository which you can implement your own org.apache.camel.spi.AggregationRepository.
If you need persistent repository then you can use either Camel HawtDB or SQL Component components.

Examples

See some examples from the old Aggregator which is somewhat similar to this new aggregator.

Using completionTimeout

In this example we want to aggregate all incoming messages and after 3 seconds of inactivity we want the aggregation to complete. This is done using the completionTimeout option as shown:

And the same example using Spring XML:

Using completionSize

In this example we want to aggregate all incoming messages and when we have 3 messages aggregated (in the same correlation group) we want the aggregation to complete. This is done using the completionSize option as shown:

And the same example using Spring XML:

Using completionPredicate

In this example we want to aggregate all incoming messages and use a Predicate to determine when we are complete. The Predicate can be evaluated using either the aggregated exchange (default) or the incoming exchange. We will so both situations as examples. We start with the default situation as shown:

And the same example using Spring XML:

And the other situation where we use the eagerCheckCompletion option to tell Camel to use the incoming Exchange. Notice how we can just test in the completion predicate that the incoming message is the END message:

And the same example using Spring XML:

Using dynamic completionTimeout

In this example we want to aggregate all incoming messages and after a period of inactivity we want the aggregation to complete. The period should be computed at runtime based on the timeout header in the incoming messages. This is done using the completionTimeout option as shown:

And the same example using Spring XML:

Note: You can also add a fixed timeout value and Camel will fallback to use this value if the dynamic value was null or 0.

Using dynamic completionSize

In this example we want to aggregate all incoming messages based on a dynamic size per correlation key. The size is computed at runtime based on the mySize header in the incoming messages. This is done using the completionSize option as shown:

And the same example using Spring XML:

Note: You can also add a fixed size value and Camel will fallback to use this value if the dynamic value was null or 0.

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
• Blog post by Torsten Mielke about using the aggregator correctly.
• The old Aggregator
• HawtDB or SQL Component for persistence support
• Aggregate Example for an example application