A computer implemented method, apparatus, and computer usable program code for controlling when to send messages in a stream processing system. A policy is determined by utilizing probability statistics and a cost function prior to stream processing. The policy specifies under which conditions a message is sent eagerly and under which other conditions the message is delayed. A filter is operated during stream processing that selects which of the messages to send from a sending transform based upon a threshold. A controller is operated during stream processing that observes a current state of a receiving transform and that applies the policy based on the current state to determine whether to change the threshold.