Three-tier architecture and its pros and cons

The three-tier architecture (3-tier application) typically has a three-tier architecture that divides the entire business application into: The presentation layer (UI), the Business Logic layer (BLL), the data Access Layer (DAL). The purpose of distinguishing hierarchy is to "high cohesion, low coupling" thought.
1, the presentation layer (UI): Popular speaking is to show the user interface, that is, users in the use of a system when he saw the gains.
2, Business Logic Layer (BLL): The operation of the specific problem, it can be said that the operation of the data layer, the data business logic processing.
3, data access Layer (DAL): This layer of transactions directly manipulate the database, for data additions, deletions, modifications, updates, lookups and so on.
Note: (Cohesion: the degree to which each element in a module is tightly coupled; coupling: a measure of the degree of interconnection between different modules within a software structure)

Advantages and Disadvantages
Advantages:
1, the developer can only focus on the entire structure of one of the layers;
2, can easily use the new implementation to replace the original level of implementation;
3. Can reduce the dependence between layer and layer;
4, in favor of standardization;
5, facilitate the reuse of the logic of each layer.
6, strong extensibility. Different layers are responsible for different levels, such as PetShop can be easily configured to achieve the conversion between SQL Server and Oracle, of course, it can also realize the conversion between B/S and C/s
7, high security. The client can access the data layer only through the logical layer, reducing the entry point and shielding many dangerous system functions.
8, the project structure is clearer, the division of labor is more clear, conducive to later maintenance and upgrade
Disadvantages:
1, reduce the performance of the system. This is self-evident. Without a tiered structure, many businesses can access the database directly to get the data, and now they have to do it through the middle tier.
2. Sometimes cascade changes are caused. This kind of modification is especially reflected in the top-down direction. If you need to add a feature in the presentation layer to ensure that the design conforms to the layered structure, you may need to add code to the appropriate business logic layer and the data access layer
3, increase the amount of code, increase the workload

Three-tier architecture and its pros and cons