You can update the dynamic lookup cache values with the results of an expression.
For example, a product table target has a numeric column that contains an order count. Each time the Lookup transformation receives an order for the product, it updates the dynamic cache order_count with the results of the following expression:
order_count = order_count + 1
The Lookup transformation returns the order_count.
You can configure how the Integration Service handles the case where the expression evaluates to null.