1. A method of controlling a distributed application executable on a distributed computer, said method comprising operating said distributed computer to: intercept a message passing between computers within said distributed computer performing said distributed application; read said message and in dependence upon the contents of said message identify a corresponding run-time-updatable message-handling policy indicating, for predetermined characteristics of the content of a message, lists of actions to be performed on said message; read action-to-software module mapping data which indicates for each of the actions in said list a software module to carry out the action; and perform said list of actions by triggering the execution of the corresponding software modules found in said mapping data.