Overview
N-Tier architecture is an industry-proved software architecture model, suitable to support enterprise-level client/server applications by resolving issues like scalability, security, fault tolerance and etc. .NET has many tools and features, but .NET doesn’t have pre-defined ways to guard how to implement N-Tier architecture.N-Tier Architecture Introduction.
Tier and Layer
Firstly we need to clarify the difference between two terms in N-Tier architecture: tier and layer. Tier usually means the physical deployment computer. Usually an individual running server is one tier.By contrast, layer usually means logic software component group mainly by functionality; layer is used for software development purpose.
Each layer may run in an individual tier. However, multiple layers may also be able to run in one tier.
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Tier and Process
If a layer can run in an individual process, usually it will also be able to run in an individual computer (tier), hence it can be considered capable for an individual tier in N-Tier architecture. However, this isn’t always true, if there are two layers which are implemented to run in two individual processes but they communicate in a such way that their IPC (inter-process communication) is solely based on a non-distributed way, such as the local shared memory, then these two layers can run in two different processes only in the same computer, not in two different computers.Layer and Process
A layer may run in an individual process; several layer may also run in an individual process; a layer may run in several processes too. If you read above section “Tier and Layer Relationship”, you can understand here easily.3-Tier Architecture
The simplest of N-Tier architecture is 3-Tier which typically contains the following software component layers: presentation layer, application layer and data layer, which are depicted below.
A layer can access directly only the public components of its directly-below layer. For example, presentation layer can only access the public components in application layer, but not in data layer. Doing so can minimize the dependencies of one layer on other layers. This dependency minimization will bring benefits for layer development/maintenance, upgrading, scaling and etc. Doing so also makes the tier security enforcement possible. Finally, doing so can also avoid cyclic dependencies among software components.
In order to claim a complete 3-Tier architecture, all three layers should be able to run in separate computers.
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).
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.
Advantages and Disadvantages of Different Tier Architectures
1 or 2-Tier
Advantages: simple and fast for a lower number of users due to fewer processes and fewer tiers; low cost for hardware, network, maintenance and deployment due to less hardware and network bandwidth needed.Disadvantages: will have issues when the number of users gets big; as limitation to solve issues like security, scalability, fault tolerance and etc because it can be deployed in only 1 or 2 computes.
N-Tier Architecture
Advantages: the following are general advantages:loads of dis/advantages and tips can be found on the excellent article this is based on:
http://www.codeproject.com/Articles/430014/N-Tier-Architecture-and-Tips
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