At the moment, the correlation expression in the aggregator module only takes into consideration the incoming message rather than the whole existing collected messages so far + the incoming message.
It would be great to gain access to the whole collection apart from the incoming message so messages could be correlated in a way such as:
Collect 1 (and only one) message of each type, if we have fruits, for example we may want to get one pear, and one banana aggregated per basket, and we want to release a basket once it's ready. If we receive 100 bananas, we want to prepare 100 baskets and wait for pears to arrive so our fruity baskets can be released.
There is no way to do such a thing if the correlation key only takes into consideration the incoming message.