19. A computer program product comprising a nontransitory computer-readable storage medium containing computer program code for: accessing a cumulative feature store storing feature values determined from user actions performed before a time point; accessing an incremental feature store storing feature values determined from user actions performed after the time point; calculating, by a computer processor of an online system, a weighted combination of feature values from the incremental feature store and corresponding feature values from the cumulative feature store; updating the cumulative feature store by replacing the corresponding feature values in the cumulative feature store with the weighted combination of feature values; receiving an update to one or more of the feature values stored in the incremental feature store, the update comprising a new value for one or more of the feature values; and updating the incremental feature store based on the received update by replacing the corresponding feature values in the incremental feature store with feature values in the received update.
19. A computer program product comprising a nontransitory computer-readable storage medium containing computer program code for: accessing a cumulative feature store storing feature values determined from user actions performed before a time point; accessing an incremental feature store storing feature values determined from user actions performed after the time point; calculating, by a computer processor of an online system, a weighted combination of feature values from the incremental feature store and corresponding feature values from the cumulative feature store; updating the cumulative feature store by replacing the corresponding feature values in the cumulative feature store with the weighted combination of feature values; receiving an update to one or more of the feature values stored in the incremental feature store, the update comprising a new value for one or more of the feature values; and updating the incremental feature store based on the received update by replacing the corresponding feature values in the incremental feature store with feature values in the received update. 20. The computer program product of claim 19 , wherein a feature is represented as an expression based on one or more of values describing user actions or other feature values.