1. A computer-readable medium comprising computer-program instructions executable by a processor for enforcing policy in a multi-computer service application having a plurality of software modules that execute on a plurality of computers, the multi-computer service application further having access to a communications medium that allows data communications between different ones of the computers, the computer-program instructions comprising instructions for: configuring logical output ports and logical input ports on different modules in accordance with a logical model of the multi-computer service application, wherein each logical input and output port is defined by port software; configuring logical data connections between the logical output and input ports in accordance with the logical model; configuring each port to communicate through different numbers of logical data connections without modifying the port software; sending a notification from a particular module to a policy module; and responding, by the policy module, to the notification by: determining a request for one or more destination modules; providing the request to an output port of the policy module; and forwarding, by the output port, the request to input ports of a plurality of the modules in accordance with the configured logical data connections.