hey all, just getting started on hadoop and curious what the best way in mapreduce would be to count unique visitors if your logfiles looked like this...

DATE       siteID  action   username
05-05-2010 siteA   pageview jim
05-05-2010 siteB   pageview tom
05-05-2010 siteA   pageview jim
05-05-2010 siteB   pageview bob
05-05-2010 siteA   pageview mike

and for each site you wanted to find out the unique visitors for each site?

I was thinking the mapper would emit siteID \t username and the reducer would keep a set() of the unique usersnames per key and then emit the length of that set. However that would be potentially storing millions of usernames in memory which doesn't seem right. Anyone have a better way?

I'm using python streaming by the way

thanks