13 November 2007

I always encourage people to go with 2-tier deployments if possible, because more tiers increases complexity and cost.

But it is important to recognize when the value of 3-tier outweighs that complexity/cost. The primary three motivators are:

1. Security – using an app server allows you to shift the database credentials from clients to the server, adding security because a would-be hacker (or savvy end-user) would need to hack into the app server to get those credentials.
2. Network infrastructure limitations – sometimes a 3-tier solution can help offload processing from CPU-bound or memory-bound machines, or it can help minimize network traffic over slow or congested links. Direct database communication is a chatty dialog, and using an app server allows a single blob of data to go across the slow link, and the chatty dialog to occur across a fast link, so 3-tier can result in a net win for performance if the client-to-server link is slow or congested.
3. Scalability – using an app server provides for database connection pooling. The trick here, is that if you don’t need it, you’ll harm performance by having an app server, so this is only useful if database connections are taxing your database server. Given the state of modern database hardware/software, scalability isn’t a big deal for the vast majority of applications anymore.

The CSLA .NET data portal is of great value here, because it allows you to switch from 2-tier to 3-tier with a configuration change - no code changes required*. You can deploy in the cheaper and simpler 2-tier model to start with, and if one of these motivators later justifies the complexity of moving to 3-tier, you can make that move with relative ease.

* disclaimer: if you plan to make such a move, I strongly suggest testing in 3-tier mode during development. While the data portal makes this 2-tier to 3-tier switch seamless, it only works if the code in the business objects follows all the rules around serialization and layer seperation. Testing in a 3-tier mode helps ensure that none of those rules get accidentally broken during development.