1. At a computer system including one or more processors and system memory, a method for composing a message processing pipeline for an application, the method comprising: an act of accessing a current pipeline specification for the application, the current pipeline specification specifying how to construct a message processing pipeline for the application; an act of accessing additional message processing requirements for the application; an act of identifying transforms that can be applied to the current pipeline specification to specify additional message processing functionality that satisfies the additional message processing requirements; an act of applying the identified transforms to the current pipeline specification to create a transformed pipeline specification for the application, the transformed pipeline specification specifying how to construct a message processing pipeline for the application that satisfies the additional message processing requirements, including the additional message processing requirements; and an act of interpreting the transformed pipeline specification to instantiate a message processing pipeline for the application, the instantiated message processing pipeline configured to satisfy the additional message processing requirements for the application.