A method of supplying Segmentation as a Service, abbreviated SegAAS, from a SegAAS server, including: causing delivery, either by the SegAAS server or by a website operator server, of instrumentation code, segment code processing code and a callback function configured to run on a user's machine wherein the instrumentation code is configured to generate instrument readings that track a user session during a visit to a website and wherein the segment code processing code is configured to receive and process segment codes during the visit by invoking the callback function code that is configured to run on the user's machine and to perform actions on the user's machine specified by an operator of the website, as triggered by delivery of a list of the segment codes; receiving at the SegAAS server a series of the instrument readings from the user session during the visit to the website; analyzing the instrument readings received at the SegAAS server to track progress of the user session and to generate and update a list of segment codes that characterize results of the analyzing according to parameters or rules specified by the operator of the website being visited; repeatedly causing delivery to a SegAAS library running on the user's machine of updated lists of the segment codes that identify user segments based on the analyzing of the progress, wherein the SegAAS library invokes the callback function code by publishing each updated list and the callback function code conditionally invokes a service that corresponds to a particular segment code in the updated list; and whereby the operator of the website can, by invoking the callback function code to deliver the updated segment codes, time performance of unsolicited actions directed to retaining the user and extending the user session with the website. causing delivery, either by the SegAAS server or by a website operator server, of instrumentation code, segment code processing code and a callback function configured to run on a user's machine wherein the instrumentation code is configured to generate instrument readings that track a user session during a visit to a website and wherein the segment code processing code is configured to receive and process segment codes during the visit by invoking the callback function code that is configured to run on the user's machine and to perform actions on the user's machine specified by an operator of the website, as triggered by delivery of a list of the segment codes; wherein the instrumentation code is configured to generate instrument readings that track a user session during a visit to a website and wherein the segment code processing code is configured to receive and process segment codes during the visit by invoking the callback function code that is configured to run on the user's machine and to perform actions on the user's machine specified by an operator of the website, as triggered by delivery of a list of the segment codes; receiving at the SegAAS server a series of the instrument readings from the user session during the visit to the website; analyzing the instrument readings received at the SegAAS server to track progress of the user session and to generate and update a list of segment codes that characterize results of the analyzing according to parameters or rules specified by the operator of the website being visited; repeatedly causing delivery to a SegAAS library running on the user's machine of updated lists of the segment codes that identify user segments based on the analyzing of the progress, wherein the SegAAS library invokes the callback function code by publishing each updated list and the callback function code conditionally invokes a service that corresponds to a particular segment code in the updated list; and whereby the operator of the website can, by invoking the callback function code to deliver the updated segment codes, time performance of unsolicited actions directed to retaining the user and extending the user session with the website.