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What is the difference between layers and tiers?

1.What is the difference between layers and tiers?
Layers refer to logical seperation of code. Logical layers help you organise your code better. For example an application can have the following layers.
1)Presentation Layer or UI Layer
2)Business Layer or Business Logic Layer
3)Data Access Layer or Data Layer
The aboove three layers reside in their own projects, may be 3 projects or even more. When we compile the projects we get the respective layer DLL. So we have 3 DLL's now.
Depending upon how we deploy our application, we may have 1 to 3 tiers. As we now have 3 DLL's, if we deploy all the DLL's on the same machine, then we have only 1 physical tier but 3 logical layers.
If we choose to deploy each DLL on a seperate machine, then we have 3 tiers and 3 layers.
So, Layers are a logical separation and Tiers are a physical separation. We can also say that, tiers are the physical deployment of layers.
Tiers:
1) Presenation Tier or UI Tier (Hosts the Presentation Layer or UI Layer). This can be considered as web server in case of an ASP.NET web application.
2) Application Tier or Business Tier (Hosts Business Layer or Business Logic Layer).
3) Data Access Tier or Data Tier (Hosts Data Access Layer or Data Layer).
4) Database Tier - SQL Server or Oracle (or any other database) which has tables, stored procedures and other database objects.
In general the following are the responsibilities of each layer or tier:
1)Presentation Layer or Tier is usually responsible for interacting with the user.
2)Business Layer or Tier is responsible for implementing the business logic of the application.
3)Data Access Layer or Tier is responsible for encapsulating the code that accesses the persistent data stores such as a relational database.
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