Advantages and Disadvantages of the architectures

1-teir Architecture

Advantages	Disadvantages
Easy to implement and optimize performance.	Do not support remote/       distributed access for data resources.
Do not have compatibility or Context switching issues.	The cost of deployment is less.eg-development and management cost.
The cost of deployment is less.eg-development and management cost.	Monolithic manner of the code causes higher maintenance.
	The cost of the central mainframe is high.

2-tier Architecture

Advantages	Disadvantages
Applications can be easily developed due to simplicity. (www.unmedu.com)	Heterogeneous environments/Business environments with rapidly changing rules and regulations are not suitable since the database server has to handle the business logic which slows down database performance. (www.unmedu.com)
Maximum user satisfaction is gained with accurate and fast prototyping of applications through robust tools. (www.unmedu.com)	Since client beholds most of the application logic, problems arise in controlling the software version and re-distributing of new versions. (www.unmedu.com)
Since this contains static business rules it’s more applicable for homogenous environments. (www.unmedu.com)	Security wise this is complicated as users need to have separate login information for every SQL server. (www.unmedu.com)
Database server and business logic is physically close, which offers higher performance. (www.unmedu.com)	Client tools and SQL middleware implemented in 2-tier environment is proprietary which remains cautious on long term feasibility. (www.unmedu.com)
	Scalability: The 2-tier model lacks scalability as it   supports only a limited number of users. When simultaneous client requests increases application performance degrades rapidly due to the fact that clients necessitate separate connections and CPU memory to proceed. (Thota   2010)
	Minimal Logic Sharing: Since the application logic is coupled with the client it’s difficult to re-use logic dispersed among applications and tools. (Thota 2010)
	Dispersion of Applications: Any change in an application should reflect all clients. If higher number of users exists in the system, it entails substantial administrative overhead. (Thota 2010)
	Change of Database Structure: Most applications used for interaction is dependent on the database structure creating an issue when re-designing, as they are intimate with the prevailing structure. (Thota 2010)

3-tier Architecture

Advantages	Disadvantages
Improved Scalability: Due to the distributed deployment of application servers, scalability of the system is enhanced since a separate connection from each client is not required whereas connections from few application servers are sufficient. Moreover TP monitors or ORBs is used for load balancing and in synchronizing severs.(Sarma 2010)	Complexity of Communication:  Usually more effort should be enforced when creating 3-tier applications as the communication points are increased (client to middle tier to server, instead of directly client to server) and the performance increased by tools like Visual Basic, PowerBuilder, Delphi will be reduced. (Sarma 2010)
Enhanced Re-usage: A similar logic can be sustained in many clients or applications. In appliance of object standards like COM/DCOM or CORBA, the language in the business-logic tier can be made transparent. .(Sarma 2010)	Fewer Tools: Additional effort is required due to increase of performance whereas the 2-tier model can handle the particular function using an automated tool. It also has productive tools like reporting tools. Sarma 2010)
Improved Data Integrity: Data corruption through client applications   can be eliminated as the data passed in the middle tier for database updations ensures its validity. .(Woodger Computing Inc 2010)
Enhanced Security: Through the implementation of several layers, enhances the data security on a service-by-service basis. As clients do not interact with the database directly, it provides less risk and confliction with unauthorized data. The placement of the business logic on a centralized server makes the data more secure. .(Woodger Computing Inc 2010)
Reduced Distribution: The layered architecture enables to update only the application servers, not all distributed clients in case of a modification in the business logic. .(Woodger Computing Inc 2010)
Redundant Sever Availability: mission-critical applications seek the   use of superfluous application servers and database servers as its possible to create an application which will recover the system from network or server failures, with redundant servers. .(Woodger Computing Inc 2010)
Hidden Database Structure: The actual structure of the database often remains hidden from requesters enabling any change of the database to be transparent. Thus a process in the middle tier which exchanges data with other applications can sustain its current interface while a modification of the underlying database structure.(Woodger Computing Inc 2010)

n-tier Architecture

Advantages	Disadvantages
n tier architecture includes all advantages of the 3-teir model since its an expansion of the 3-teir methodology. Mainly the performance is increased due to off-load from the   database tier and the client tier, enabling it to suit medium to high volume industries (Kambalyal 2010).	Due to the componentization of the tiers, the complex structure is difficult to implement or maintain. (Kambalyal 2010)

Bibliography

·         www.unmedu.com. (2011). 2-Tier Architecture’s Disadvantages. [Online]. Available from: http://www.unm.edu/~network/presentations/course/appendix/appendix_k/tsld052.htm. [Accessed: 29th January 2011]

·         Thota. (2010). Advantages and disadvantages of 2-Tier Architecture. [Online]. Available from: http://www.dotnetspider.com/forum/32148-advantages-disadvantages-two-tier-architec.aspx. [Accessed: 29th January 2011]

·         Woodger Computing Inc. (2010). Multi-Tier Architectures. [Online]. Available from: http://www.woodger.ca/archmult.htm. [Accessed: 29th January 2011]

·         Sarma. (2010). 3-Tier Architecture. [Online]. Available from: http://www.dotnetspider.com/forum/32148-advantages-disadvantages-two-tier-architec.aspx. [Accessed: 29th January 2011]