Client-server is the traditional method of communication between two computers over a network, or between two programs on the same computer. The client initiates contact to request a resource. The server holds that resource and delivers it to the client in response to the client's request. The concept of "two-tier" client-server architecture is usually discussed in the context of database applications.
Databases are formed and manipulated in a variety of plans. One example is a linked list; another is a table. The relational database is now the most common form of database structure found in computing today. This keeps data in tables that can be linked together and joined on a common field. Th relational table is queried by Boolean operands contained in Structured Query Language. In business environments, it is usual to centralize the database and grant access from other computers across the network. If the application accessing the database is resident on the same computer as the database, this is not considered a two-tiered system in networking terms, although the software behaves in a two- or three-tier manner because the application formatting the resulting data is a client to the application delivering the data from the database.
In a true two-tier client-server, the database resides on one computer, the server, and the user facing interface is resident on another -- this is the client. The application may reside on either computer. A three-tier architecture involves three computers. One holds the user interface, the other runs the application and the third holds the database. A single tier configuration (for network purposes) has all three resident on the same computer.
The main advantage of a two-tier architecture is that it allows third-party software to access the database. The displaying and process software does not need to be bundled with the database management system. This configuration is common where spreadsheets read in data from a central database, or where report-generating software accesses data through pre-organized views on the data. The two-tier system enables data from different sources to be pooled and merged, eradicating data redundancy and enabling different departments to benefit from each other's data.
The two-tier client server model is the basis of Enterprise Resource Planning systems. Initially, ERP systems were written for large companies to enable their accounts to be automatically generated by capturing all data related to sales and purchases in a central database. These systems are now accessible to small- and medium-sized companies. The ability to link database access to regular business software, like productivity suite programs greatly increases the affordability of two-tier client server systems.
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