今天在閱讀Martin Fowler的《Patterns of Enterprise Application Architecture》時，終于讓我``澄清"了tier和layer的關系。 Martin的原話是:
``When people discuss layering, there's often some confusion over the terms layer and tier. Often the two are used as synonyms, but most people see tier as implying a physical separation. Client-server systems are often described as two-tier systems, and the separation is physical: The client is a desktop and the server is a server. I use layer to stress that you don't have to run the layers on different machines. A distinct layer of domain logic often runs on either a desktop or the database server. In this situation you have two nodes but three distinct layers. With a local database I can run all three layers on a single laptop, but there will still be three distinct layers.
我們平時一直說``多層"結構，常?；煜褂胻ier或者layer這個詞( N-tier或者N-layer一般是摻和著用的)，Matin則非常清楚的區分了這兩個詞的用法: tier更多的是指物理上的層次關系，比如說Client/Server結構，是two-tier的，因為客戶端在桌面機上，服務器則在遠端的數據庫服務器上。 layer則更多的指邏輯上的層，我們通常說的Java EE，應該是N-layer的，因為從邏輯上，Java EE里面有表現層，業務邏輯層和數據永久層，在物理上，這三層可以在不同的tier上(表現層在PC上，業務邏輯層在應用服務器上，數據永久層則在數據庫服務器上)，也可以在一個tier上，比如Martin說的，如果把數據庫，應用服務器和瀏覽器都裝在一臺筆記本電腦上，那么，3-layer就是在1-tier上了。
因此，當我們以后談論Java EE的多層體系時，應該更精確的使用layer這個詞。
【關于Martin Fowler】
http://en.wikipedia.org/wiki/Martin_Fowler
http://martinfowler.com/