I'm looking at the scenario of how to keep track of the number of unique visitors within a given time period. Inserting user ids into a wide row would allow me to have a list of every user within the time period that the row represented. My experience in the past was that using get_count on a row to get the column count got slow pretty quick but that might still be the easiest way to get the count of unique users with some sort of caching of the count so that it's not expensive subsequently. Using Hadoop is overkill for this scenario. Any other approaches?
Ed
