Three Tier and N-tier Architecture - Architectures

       Three-tier (layer) is a client-server architecture in which the user interface, business process (business rules) and data storage and data access are developed and maintained as independent modules or most often on separate platforms. Basically, there are 3 layers, tier 1 (presentation tier, GUI tier), tier 2 (business objects, business logic tier) and tier 3 (data access tier). These tiers can be developed and tested separately.

       Separation of the user interface from business logic and database access has many advantages. Some of the advantages are as follows:

• Reusability of the business logic component results in quick development. Let's say we have a module that handles adding, updating, deleting and finding customers in the system. As this component is developed and tested, we can use it in any other project that might involve maintaining customers.
• Transformation of the system is easy. Since the business logic is separate from the data access layer, changing the data access layer won’t affect the business logic module much. Let's say if we are moving from SQL Server data storage to Oracle there shouldn’t be any changes required in the business layer component and in the GUI component.
• Change management of the system is easy. Let's say if there is a minor change in the business logic, we don’t have to install the entire system in individual user’s PCs. E.g. if GST (TAX) is changed from 10% to 15% we only need to update the business logic component without affecting the users and without any downtime.
• Having separate functionality servers allows for parallel development of individual tiers by application specialists.
• Provides more flexible resource allocation. Can reduce the network traffic by having the functionality servers strip data to the precise structure needed before sending it to the clients.

As more users access the system a three-tier solution is more scalable than the other solutions because you can add as many middle tiers (running on each own server) as needed to ensure good performance (N-tier or multiple-tier).

Security is also the best in the three-tier architecture because the middle layer protects the database tier.There is one major drawback to the N-tier architecture and that is that the additional tiers increase the complexity and cost of the installation.