Well, In general, the user system interface client invokes services from the database management server. In many two tier designs, most of the application portion of processing is in the client environment.
The database management server usually provides the portion of the processing related to accessing data (often implemented in store procedures). Clients commonly communicate with the server through SQL statements or a call-level interface.
It should be noted that connectivity between tiers can be dynamically changed depending upon the user's request for data and services.
Webopedia states "Refers to client/server architectures in which the user interface runs on the client and the database is stored on the server. The actual application logic can run on either the client or the server."
One of the disadvantages of a two tier setup is performance.
For instance, suppose you get a sudden flood of requests coming in. Your web server might be able to handle the number of incoming connections, but now, since all of the non-DB processing is happening on the same machine, you become CPU-bound, so that, even though the web server can accept more connections, it can't get enough of the CPU to service all of the requests it has.
What is a 3 tier connection?
A 3-tier application is an application program that is organized into three major parts, each of which is distributed to a different place or places in a network. The three parts are:
- The workstation or presentation interface
- The business logic
- The database and programming related to managing it
In a typical 3-tier application, the application user's workstation contains the programming that provides the graphical user interface (GUI) and application-specific entry forms or interactive windows. (Some data that is local or unique for the workstation user is also kept on the local hard disk.)
Business logic is located on a local area network (LAN) server or other shared computer. The business logic acts as the server for client requests from workstations. In turn, it determines what data is needed (and where it is located) and acts as a client in relation to a third tier of programming that might be located on a mainframe computer.
The third tier includes the database and a program to manage read and write access to it. While the organization of an application can be more complicated than this, the 3-tier view is a convenient way to think about the parts in a large-scale program.
A 3-tier application uses the client/server computing model. With three tiers or parts, each part can be developed concurrently by different team of programmers coding in different languages from the other tier developers.
Because the programming for a tier can be changed or relocated without affecting the other tiers, the 3-tier model makes it easier for an enterprise or software packager to continually evolve an application as new needs and opportunities arise.
Existing applications or critical parts can be permanently or temporarily retained and encapsulated within the new tier of which it becomes a component.
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