Methods, systems, and apparatus, including computer programs encoded on computer storage media, for routing events of an event stream in a stream processing system. One of the methods includes receiving, by a router, an event stream of events; identifying, for each event, by the router, a respective partition of context data that includes context data related to the event and providing the event to a respective local modeler that stores the partition of context data identified for the event in operational memory of the local modeler; processing, by each local modeler, events received from the router and aggregating information associated with each event to generate aggregated information; providing, by one or more of the local modelers, to a central modeler, the respective aggregated information; and determining, by the central modeler, a plurality of parameters of a machine learning model using the received aggregated information.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A method comprising: receiving, by a router, an event stream of events, wherein the router executes on a system of one or more computers; identifying, for each event, by the router, a partition of first context data that includes first context data related to the event; providing each event, by the router, to a local modeler of a first plurality of local modelers of a stream processing system that is identified as a local modeler that stores the partition of first context data identified for the event in operational memory of the local modeler, and wherein the local modelers execute on the system of one or more computers; processing, by each local modeler in parallel, events received from the router using the first context data in the respective partition of first context data by performing a first plurality of actions, the actions including aggregating information associated with each event received by the local modeler to generate aggregated information; providing, by one or more of the local modelers, to a first central modeler executing on the system of one or more computers, the respective aggregated information generated by the one or more of the local modelers; and determining, by the first central modeler, a plurality of parameters of a first machine learning model using the aggregated information received by the first central modeler. 2. The method of claim 1, wherein a first and a second local modeler of the first plurality of local modelers each store a same partition of first context data in operational memory. 3. The method of claim 1, further comprising: before identifying a partition of first context data, determining that an action of the first plurality of actions requests first context data. 4. The method of claim 1, wherein: for each local modeler, instructions implementing the first plurality of actions and the respective partition of first context data are stored in operational memory in a respective operating system process; and each local modeler processes, in the respective operating system process, events received by the local modeler according by performing the first plurality of actions. 5. The method of claim 1, further comprising: providing, by the first central modeler, the parameters to one or more of the first plurality of local modelers for storage. 6. The method of claim 5, further comprising: processing, by each of the one or more of the first plurality of local modelers applying the first machine learning model, the events received by the local modeler to perform scoring of the event stream; and aggregating, by each of the one or more of the first plurality of local modelers, information associated with each event received by the local modeler to generate updated aggregated information. 7. The method of claim 1, wherein identifying a partition of context data that includes context data related to an event comprises: obtaining information identifying a first information type included in the event; obtaining data included in the event that is associated with the first information type; and performing a hashing process using the obtained data to identify the partition. 8. The method of claim 1, wherein the router is a first router, the method further comprising: providing, by the first plurality of local modelers, processed events to a second router different from the first router; identifying, by the second router, for each processed event, a partition of second context data that includes context data related to the processed event; and providing each processed event, by the second router, to a respective local modeler of a second plurality of local modelers of the stream processing system, wherein the respective local modeler is a local modeler that stores the partition of second context data identified for the processed event in operational memory of the local modeler, and wherein the local modelers in the second plurality of local modelers execute on the system of one or more computers. 9. The method of claim 8, further comprising: processing, by each local modeler in the second plurality of local modelers, events received from the second router using the second context data in the respective partition of second context data by performing a second plurality of actions. 10. The method of claim 1, wherein the router is a first router, the method further comprising: providing, by the first plurality of local modelers, processed events to a second router different from the first router, wherein the processed events include further information added by local modelers in the first plurality of local modelers; and identifying, by the second router, for each processed event, using the further information added to the processed event, a respective local modeler of a second plurality of local modelers of the stream processing system, wherein the local modelers in the second plurality of local modelers execute on the system of one or more computers. 11. The method of claim 10, wherein the further information is a label classifying the respective event. 12. The method of claim 1, comprising: providing, by the one or more local modelers of the first plurality of local modelers, to a second central modeler executing on the system, aggregated information generated by the one or more local modelers of the first plurality of local modelers; and determining, by the second central modeler, parameters of a second machine learning model using the aggregated information received by the second central modeler. 13. The method of claim 12, comprising: receiving, by the first central modeler, data specifying a first learning program for determining parameters of the first machine learning model from aggregated information received by the first central modeler; and receiving, by the second central modeler, data specifying a third learning program for determining parameters of a third machine learning model from aggregated information received by the second central modeler. 14. The method of claim 1, wherein: context data is pre-existing data related to an event that does not occur within data attributes of the event itself. 15. The method of claim 1, wherein: for each local modeler, instructions implementing the first plurality of actions and the respective partition of first context data are stored in operational memory in a respective operating system process; for each local modeler, common context data is stored in operational memory in each respective operating system process; and each local modeler processes, in the respective operating system process, events received by the local modeler by performing the first plurality of actions using the respective partition of first context data and the common context data. 16. A method comprising: receiving, by a router, an event stream of events, wherein the router executes on a system of one or more computers; providing each event, by the router, to a respective local modeler of a first plurality of local modelers of a stream processing system according to a routing key included in each event that identifies a local modeler, wherein the local modelers execute on the system of one or more computers; processing, by each local modeler in parallel, events received from the router by performing a first plurality of actions including aggregating information associated with each event received by the local modeler to generate aggregated information; providing, by one or more of the local modelers, to a first central modeler executing on the system of one or more computers, the respective aggregated information generated by the one or more of the local modelers; and determining, by the first central modeler, a plurality of parameters of a first machine learning model using the aggregated information received by the first central modeler. 17. The method of claim 16, wherein: the routing key for each event identifies the respective local modeler. 18. The method of claim 16, further comprising: accessing, by the router, mappings between ranges of routing keys and respective partitions of context data that store context data related to routing keys included in a range; and identifying as the respective local modeler for each event a local modeler of the first plurality of local modelers that stores a partition of context data identified by the mappings for the routing key of the event. 19. The method of claim 16, further comprising, for each event: hashing the routing key included in the event to obtain an identifier of a partition of context data; and identifying as the respective local modeler a local modeler of the first plurality of local modelers that stores the partition of context data identified by the identifier. 20. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving, by a router, an event stream of events, wherein the router executes on a system of one or more computers; identifying, for each event, by the router, a partition of first context data that includes first context data related to the event; providing each event, by the router, to a local modeler of a first plurality of local modelers of a stream processing system that is identified as a local modeler that stores the partition of first context data identified for the event in operational memory of the local modeler, and wherein the local modelers execute on the system of one or more computers; processing, by each local modeler in parallel, events received from the router using the first context data in the respective partition of first context data by performing a first plurality of actions including aggregating information associated with each event received by the local modeler to generate aggregated information; providing, by one or more of the local modelers, to a first central modeler executing on the system of one or more computers, the respective aggregated information generated by the one or more of the local modelers; and determining, by the first central modeler, a plurality of parameters of a first machine learning model using the aggregated information received by the first central modeler.