今天在閱讀Martin Fowler的《Patterns of Enterprise Application Architecture》時(shí)，終于讓我``澄清"了tier和layer的關(guān)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(shí)一直說(shuō)``多層"結(jié)構(gòu)，常常混淆著使用tier或者layer這個(gè)詞( N-tier或者N-layer一般是摻和著用的)，Matin則非常清楚的區(qū)分了這兩個(gè)詞的用法: tier更多的是指物理上的層次關(guān)系，比如說(shuō)Client/Server結(jié)構(gòu)，是two-tier的，因?yàn)榭蛻舳嗽谧烂鏅C(jī)上，服務(wù)器則在遠(yuǎn)端的數(shù)據(jù)庫(kù)服務(wù)器上。 layer則更多的指邏輯上的層，我們通常說(shuō)的Java EE，應(yīng)該是N-layer的，因?yàn)閺倪壿嬌希琂ava EE里面有表現(xiàn)層，業(yè)務(wù)邏輯層和數(shù)據(jù)永久層，在物理上，這三層可以在不同的tier上(表現(xiàn)層在PC上，業(yè)務(wù)邏輯層在應(yīng)用服務(wù)器上，數(shù)據(jù)永久層則在數(shù)據(jù)庫(kù)服務(wù)器上)，也可以在一個(gè)tier上，比如Martin說(shuō)的，如果把數(shù)據(jù)庫(kù)，應(yīng)用服務(wù)器和瀏覽器都裝在一臺(tái)筆記本電腦上，那么，3-layer就是在1-tier上了。
因此，當(dāng)我們以后談?wù)揓ava EE的多層體系時(shí)，應(yīng)該更精確的使用layer這個(gè)詞。
【關(guān)于Martin Fowler】
http://en.wikipedia.org/wiki/Martin_Fowler
http://martinfowler.com/