What are the main differences between two-tier and three-tier system architectures?

Architectures with two and three levels of system structuring differ in the number of layers and the distribution of responsibilities among them.

Two-tier architecture:

• Usually consists of a client application and a server (or database).
• The client is responsible for the interface and logic, the server for data storage and business logic.
• Example: a thick client that directly accesses the database.

Three-tier architecture:

• Includes three layers: presentation (UI), business logic, and data.
• Each layer is isolated and interacts with neighboring layers through well-defined interfaces.
• Allows better scaling, maintenance, and development of the system.

Main differences:

• In three-tier architecture, business logic is separated into its own layer, improving modularity.
• Two-tier is simpler but less flexible and scalable.

Example:

• Two-tier: client application directly accesses the database.
• Three-tier: client communicates with an application server that processes logic and interacts with the database.