# Aggregate Mediator

The aggregate mediator implements the Message Aggregator EIP and will aggregate the messages or responses for splitted messages using either the clone or iterate mediators. At the same time it can aggregate messages on the presence of matching elements specified by the correlateOn XPATH expression. Aggregate will collect the messages coming into it until the messages collected on the aggregation satisfies the complete condition. The completion condition can specify a minimum or maximum number of messages to be collected, or a timeout value in seconds, after which the aggregation terminates. On completion of the aggregation it will merge all of the collected messages and invoke the onComplete sequence on it. The merged message would be created using the XPath expression specified by the attribute 'expression' on the 'onComplete' element.

### Adding an Aggregate mediator

As shown in figure 1,

1. Click Add Child
2. Go to Advanced and click Aggregate

Figure 1: Adding an Aggregate mediator

### Configuring aggregator specific options

After adding an aggregate mediator, clicking on it will open up the aggregator specific options below the sequence editor. Figure 2 shows all the aggregator specific options.

Figure 2: Aggregator specific options

Aggregate mediator field descriptions,

Field name | Description
Aggregate ID | This optional attribute can be used to aggregate only responses for splitted messages that are created by a specific clone/iterate mediator. Aggregate ID should be same as the ID of the corresponding clone/iterate mediator that creates splitted messages. This is particularly useful when aggregating responses of messages that are created using nested clone/iterate mediators.
Aggregation Expression | An XPath expression specifying based on which elements to aggregate. Set of messages that's taken for aggregation is determined through Correlation Expression.
Completion Timeout | Time that the aggregator waits for messages.
Completion Max-messages | Maximum number of messages that can exist in an aggregation.
Completion Min-messages | Minimum number of messages required for the aggregation to complete.
Correlation Expression | An XPath expression specifying the pattern messages will be collected for aggregation. Specific aggregation condition will be set through Aggregation Expression.