13. A system comprising: one or more computers configured to perform a method comprising: obtaining a plurality of model representations of predictive models, wherein each model representation is associated with a respective user and expresses a respective predictive model; and selecting a model implementation for each of the model representations, each model implementation comprising one or more computer programs operable to be executed on one or more computing devices, each model implementation being selected for a corresponding model representation based on one or more system usage properties associated with the user associated with the corresponding model representation, wherein the one or more system usage properties comprise one or more of the following: (i) an amount of system resources that the user has consumed over a given period of time, (ii) a time of day during which system resources were previously used by the user, or (iii) an amount of money that was previously paid by the user for the system resources.
13. A system comprising: one or more computers configured to perform a method comprising: obtaining a plurality of model representations of predictive models, wherein each model representation is associated with a respective user and expresses a respective predictive model; and selecting a model implementation for each of the model representations, each model implementation comprising one or more computer programs operable to be executed on one or more computing devices, each model implementation being selected for a corresponding model representation based on one or more system usage properties associated with the user associated with the corresponding model representation, wherein the one or more system usage properties comprise one or more of the following: (i) an amount of system resources that the user has consumed over a given period of time, (ii) a time of day during which system resources were previously used by the user, or (iii) an amount of money that was previously paid by the user for the system resources. 14. The system of claim 13 , wherein the method further comprises: associating each of one or more of the selected model implementations with a node in a directed graph, wherein for one or more ordered pairs of nodes in the graph the prediction output of a model implementation associated with a tail node in the pair serves as input to a model implementation associated with a head node in the pair.