Systems and methods to enforce policy in a multi-computer service application are described. The application includes multiple software modules that execute on multiple computers. The multi-computer service application has access to a communications medium that allows data communications between different ones of the computers. The software modules have logical input and output ports and logical data connections between modules. Each logical port is defined by port software. In one aspect, a particular module sends a notification to a policy module. Responsive to the notification, the policy module (a) determines a request for one or more destination modules, and (b) provides the request to an output port of the policy module. The output port forwards the request to input ports of a plurality of the modules in accordance with the logical data connections.