18. The computer-implemented method of claim 17 , wherein the online system further maintains feature loops, each feature loop aggregating values of a feature expression across a plurality of actions and a response to each user action is determined based on the result of each feature loop evaluation.
17. A computer-implemented method comprising: maintaining, by a computer processor, in an online system, a plurality of feature expressions, wherein maintaining the plurality of feature expressions comprises evaluating the plurality of feature expressions for each user action, where at least one of the feature expressions comprises an expression composing one or more other feature expressions of the plurality of feature expressions using operators, the expression capable of being processed by a computer processor to map a set of input values to an output value, wherein the feature expression evaluates to a value describing one or more entities in the online system; receiving information describing a modification to the plurality of feature expressions, the modification comprising one or more of: addition of a new feature expression, deletion of an existing feature expression, or update of an existing feature expression, wherein the modification to the plurality of feature expressions is performed while the online system is executing; responsive to the plurality of feature expressions being modified, evaluating each feature expression of the modified plurality of feature expressions for a subsequent user action; and responding to the subsequent user action based on the values of the feature expressions from the modified plurality of feature expressions.