13. A system comprising: one or more computers configured to perform a method comprising: obtaining a model representation of a predictive model, wherein the model representation is associated with a respective user and expresses a respective predictive model; and selecting a model implementation for the model representation, the model implementation comprising one or more computer programs operable to be executed on one or more computing devices, and the model implementation being selected for the corresponding model representation based on one or more system usage properties associated with the user associated with the corresponding model representation; and causing execution of the selected model implementation.
13. A system comprising: one or more computers configured to perform a method comprising: obtaining a model representation of a predictive model, wherein the model representation is associated with a respective user and expresses a respective predictive model; and selecting a model implementation for the model representation, the model implementation comprising one or more computer programs operable to be executed on one or more computing devices, and the model implementation being selected for the corresponding model representation based on one or more system usage properties associated with the user associated with the corresponding model representation; and causing execution of the selected model implementation. 16. The system of claim 13 , wherein the method further comprises: assigning a system resources quota to the user; determining an amount of system resources that the user has consumed over a given period of time; and selecting the model implementation based on an amount of unconsumed system resources according to the system resource quota, wherein an amount of system resources that can be consumed by the selected model implementation does not exceed the amount of unconsumed system resources.