18. A system, comprising: a plurality of processing devices in data communication, wherein the processing devices are operable to: for each representation of a plurality of representations of predictive models, selecting a model implementation from a plurality of model implementation for the representation, wherein each representation comprises a description of a respective predictive model; associating each of the model implementations with a respective 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 first node in the pair serves as input to a model implementation associated with a second node in the pair, wherein: for at least a first node in the directed graph the system realizes the node by special purpose circuitry, and a model implementation associated with the first node and configured to be executed by the special purpose circuitry is selected for the first node based on its configuration to be executed by the special purpose circuitry; and for at least a second node in the directed graph the system realizes the node by a general purpose microprocessor, and a model implementation associated with the second node and configured to be executed by the general purpose microprocessor is selected for the second node based on its configuration to be executed by the general purpose microprocessor.
18. A system, comprising: a plurality of processing devices in data communication, wherein the processing devices are operable to: for each representation of a plurality of representations of predictive models, selecting a model implementation from a plurality of model implementation for the representation, wherein each representation comprises a description of a respective predictive model; associating each of the model implementations with a respective 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 first node in the pair serves as input to a model implementation associated with a second node in the pair, wherein: for at least a first node in the directed graph the system realizes the node by special purpose circuitry, and a model implementation associated with the first node and configured to be executed by the special purpose circuitry is selected for the first node based on its configuration to be executed by the special purpose circuitry; and for at least a second node in the directed graph the system realizes the node by a general purpose microprocessor, and a model implementation associated with the second node and configured to be executed by the general purpose microprocessor is selected for the second node based on its configuration to be executed by the general purpose microprocessor. 21. The system of claim 18 , further comprising: assigning a system resource quota to a user associated with a representation; determining an amount of system resources that model implementations for the respective user have 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.