1. A method for performing shared updates, comprising: receiving user-provided logic for determining whether updates to a particular column are valid in the presence of concurrent updates to the same value; receiving a request, associated with a particular business transaction, to perform an update to a value in the particular column; and in response to the request: (a) determining, for the particular column, at least one of a projected value and a confirmed value; (a) causing the user-provided logic to generate an indication of whether the update should be allowed, in the presence of concurrent updates to the same value, by: invoking the user-provided logic, and passing, to the user-provided logic, the at least one of the projected value and the confirmed value to enable the user-provided logic to determine whether the update should be allowed based, at least in part, on one or more constraints on the value of the particular column and the at least one of the projected value and the confirmed value; (c) determining whether to allow the update based, at least in part, on the indication.