This is not what is usually meant by a third tier. Personally I take database connection pooling for granted. What I mean when I talk about a 'data-tier' is th

This is not what is usually meant by a third tier. Personally I take database connection pooling for granted. What I mean when I talk about a 'data-tier' is the code that deals with accessing data without knowing in whatever datastore it is contained. If you properly hide for example Twitter#getRecentMessagesForUserById(userId) behind a (Java) interface then you can easily change from an implementation where you do direct database calls to a sharded solution or a cached solution. Other tiers that use this API will simply work because the interface has not changed. One day your app could be talking directly to a single MySQL database and the other day your app could be talking to a 60 node PostgreSQL cluster. It would never know since the details are hidden. This is totally against what you see in every Rails book or example app where the first thing that is done is direct ActiveRecord queries. Yes it is more work, but it pays off in the long term. It also greatly reduces local hacks for caching. All that is done in the right place. And automatically for all users of that tier. S.