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; storing the user-provided logic at a location accessible to a business transaction framework; 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, the business transaction framework performing the steps of: (a) determining, for the particular column, at least one value from the set consisting of a current value, a projected value, and a confirmed value; (b) causing the user-provided logic to generate an indication of whether the update should be allowed by invoking the user-provided logic, and passing to the user-provided logic said at least one value; (c) determining whether to allow the update based, at least in part, on the indication.