1. A computer-implemented method comprising: maintaining, in an online system, a set of feature loops, wherein a feature loop comprises an expression capable of being processed by a computer processor to map a set of input values to an output value, the expression evaluating to a value describing one or more entities in the online system, wherein evaluating the feature loop comprises aggregating values of the expression across a plurality of user actions; receiving information describing a modification to the set of feature loops, wherein the modification to the set of feature loops is performed while the online system is executing; responsive to the set of feature loops being modified, evaluating feature loops of the set of modified feature loops for a subsequent user action; and responding to the subsequent user action based on the values of feature loops from the set of modified feature loops.