GENERAL DISCUSSIONS

In order to claim a complete 3-Tier architecture, all three layers should be able to run in separate computers.

Diagram 1: 3-Tier Architecture

These three layers are briefly described as below:

Presentation layer: a layer that users can access directly, such as desktop UI, web page and etc. Also called client. 

Application layer: this layer encapsulates the business logic (such as business rules and data validation), domain concept, data access logic and etc. Also called middle layer.  

Data layer: the external data source to store the application data, such as database server, CRM system, ERP system, mainframe or other legacy systems and etc. The one we meet often today is database server. For N-Tier architecture, we need to use the non-embedded database server, such as SQL server, Oracle, DB2, MySQL or PostgreSQL. The non-embedded database server can be run in an individual computer. Whereas, the embedded type databases, such as Microsoft access, dbase and etc, cannot run in an individual computer, and then cannot be used as the data layer of the 3-Tier architecture. 

1, 2, 3 or More Tier Architecture 

1-Tier: all above layers can only run in one computer. In order to achieve 1-Tier, we need to use the embedded database system, which cannot run in an individual process. Otherwise, there will be at least 2-Tier because non-embedded databases usually can run in an individual computer (tier).   

2-Tier: either presentation layer and application layer can only run in one computer, or application layer and data layer can only run in one computer. The whole application cannot run in more than 2 computers. 

3-Tier: the simplest case of N-Tier architecture; all above three layers are able to run in three separate computers. Practically, these three layers can also be deployed in one computer (3-Tier architecture, but deployed as 1-Tier). 

N-Tier: 3 or more tiers architecture. Diagram 2 below depicts a typical N-Tier architecture. Some layers in 3-Tier can be broken further into more layers. These broken layers may be able to run in more tiers. For example, application layer can be broken into business layer, persistence layer or more. Presentation layer can be broken into client layer and client presenter layer. In diagram 2, in order to claim a complete N-Tier architecture, client presenter layer, business layer and data layer should be able to run in three separate computers (tiers). Practically, all these layers can also be deployed in one compute (tier).  

Diagram 2: N-Tier Architecture

Below are brief summaries on all layers in Diagram 2: 

Client layer: this layer is involved with users directly. There may be several different types of clients coexisting, such as WPF, Window form, HTML web page and etc.  

Client presenter layer: contains the presentation logic needed by clients, such as ASP .NET MVC in IIS web server. Also it adapts different clients to the business layer.

Business layer: handles and encapsulates all of business domains and logics; also called domain layer. 

Persistence layer: handles the read/write of the business data to the data layer, also called data access layer (DAL).  

Data layer: the external data source, such as a database. 

Sometimes, the number of tiers is able to be equal or more than 3, but client presenter layer, business layer and data layer cannot run in three separate computers (tiers). Is this a N-Tier architecture? we categorize this N-Tier as an incomplete N-Tier architecture because its client presenter layer, business layer and data layer cannot run in three separate computers (tiers).