Application Architecture - Tiers or Layers?

While application architecture is complicated enough by itself, the ambiguity or multiple interpretations of words or concepts adds yet another wrinkle. One example of this that I've run into recently has to do with the separation of the logical and physical components within an application.

Most developers/architects will agree that there are (at least) 3 separations that are meaningful enough to warrant their own grouping:

• Client "stuff"
• Application "stuff"
• Data "stuff"

But are these "layers" or "tiers"? There are a lot of people that use these terms interchangeably, and until somewhat recently, I did too. That changed while I was doing some online research to pull together documentation for a recent project.

I stumbled upon a couple of articles that made a differentiation between the two that I think makes a lot of sense (thanks to Pranshu Jain's Blog and the Guidance Share Wiki for their insight).
Physical separation versus logical separation is the distinction. If the layers are physically separated (or have the potential to be physically separated), then they can be referred to as tiers. Logical separation (particularly for design concerns) is indicative of architecture layers.

So, as mentioned above, most web applications have 3 tiers:











I plan to continue to use this distinction moving forward, because I think there are some places where it can really add value (which I will discuss in more detail within future blog entries).