Described herein are systems and methods in which key values associated with events are aggregated as those events are processed by an event processing engine. In some embodiments, the system maintains a number of key-value mappings associated with a plurality of key values to be updated. Each key value may be associated with a key log that can be accessed to quickly generate useful information while minimizing the number of operations needed to generate that information. The key log may be updated each time that the associated key value is updated in order to maintain a running history of key-value updates.