Layering

Breaking down a system into layers has a number of important benefits

1. You can understand a single layer as a coherent whole without knowing much about the other layers. You can understand how to build an FTP service on top of TCP without knowing the details of how ethernet works
2. You can substitute layers with alternative implementations of the same basic services. An FTP service can run over ethernet, PPP, or whatever a cable company uses without change.
3. You minimize dependencies between the layers. If the cable company changes its physical transmission system, providing they make IP work we don't have to alter our FTP service.
4. Layers make good places for standardization. TCP and IP are standards because they define how their layers should operate.
5. Once you have a layer built you can use it for many higher level services. So TCP/IP is used by FTP, telnet, SSH, http. Otherwise all of these higher level protocols would have to write their own lower level protocols

Layering is an important technique, but there are downsides.

1. Layers encapsulate some things well, but not all. As a result you sometimes get cascading changes. The classic example of this in a layered enterprise application is adding a field that needs to display on the UI, be in the database, and thus be added to every layer in between.
2. Extra layers can harm performance. At every layer things typically need to be transformed from one representation to another. However the encapsulation of underlying function often gives you efficiency gains that more than compensate. A layer that controls transactions can be optimized and will then make everything faster.

But the hardest part of a layered architecture is deciding what layers to have and what the responsibility of each layer should be.

 

转载于:https://www.cnblogs.com/ellipse-gao/archive/2007/05/25/759161.html