1. A method comprising: receiving, at a data center including one or more processing modules and providing a native processing environment, an untrusted application that includes a data parallel pipeline, wherein the data parallel pipeline specifies multiple parallel data objects that contain multiple elements and multiple parallel operations that are associated with untrusted functions that operate on the elements; instantiating a first secured processing environment in the native processing environment and on one or more of the processing modules; executing the untrusted application in the first secured processing environment, wherein executing the application generates a dataflow graph of deferred parallel data objects and deferred parallel operations corresponding to the data parallel pipeline; communicating information representing the data flow graph outside of the first secured processing environment; applying, outside of the first secured processing environment and in the native processing environment, one or more graph transformations to the information representing the dataflow graph to generate a revised dataflow graph that includes one or more of the deferred parallel data objects and deferred, combined parallel data operations that are associated with one or more of the untrusted functions; and executing the deferred, combined parallel operations to produce materialized parallel data objects corresponding to the deferred parallel data objects, wherein executing the deferred, combined parallel operations comprises: instantiating one or more second secured processing environments in the native processing environment and on one or more of the processing modules; executing the untrusted functions associated with the deferred, combined parallel operations in the one or more second secured processing environments.