Database-three-tier architecture

Data layer (DAL), Business Logic Layer (BLL), Presentation layer (UI)

three-layer structure principle:
In 3 levels, the main functions and business logic of the system are processed in the business logic layer.
The so-called three-tier architecture is the addition of a "middle tier", also called a component layer, between the client and the database. Here the three-layer system, not refers to the physical three layer, not simply placed three machines is three-tier architecture, also not only B/s application is the three-tier architecture, three layer refers to the logic of the three layer, even if the three layers placed on a machine.
The three-tier application puts business rules, data access, legitimacy checks, etc. into the middle tier for processing. Typically, the client does not interact directly with the database, but instead establishes a connection with the middle tier through com/dcom communication, and then interacts with the database through the middle tier.
Presentation Layer
At the outermost (topmost), closest to the user. Used to display data and receive input from users, providing an interactive interface for users
Business Logic Layer
The business logic layer is undoubtedly the part of the system architecture that embodies the core value. Its focus is mainly on the development of business rules, the implementation of business processes and other business requirements related to the system design, that is, it is related to the domain of the system (domain) logic, and many times, the business logic layer is also called the domain layer. For example, Martin Fowler, in the book Patterns of Enterprise application Architecture, divides the entire architecture into three main layers: the presentation layer, the domain layer, and the data source layer. As a pioneer in domain-driven design, Eric Evans has made a more detailed division of the business Logic layer, subdivided into application and domain layers, and further separates domain logic from domain logic solutions through layering.
The position of the business logic layer in the architecture is critical, it is in the middle of the data access layer and the presentation layer, which plays the role of connecting link in the data exchange. Because layers and layers are a weakly coupled structure, the dependencies between layers and layers are downward, and the bottom layer is "ignorant" to the upper layer, and changing the upper-level design has no effect on the bottom of its invocation. If you follow the idea of interface-oriented design in a layered design, this downward dependency should also be a weak dependency. Therefore, without changing the definition of the interface, the ideal layered architecture should be a "drawer" architecture which can be extracted and replaceable. Because of this, the design of the business logic layer is especially critical for a scalable architecture, because it plays two different roles. For the data access layer, it is the caller and, for the presentation layer, it is the callee. Dependency and dependent relationships are entangled in the business logic layer, and how to implement the decoupling of dependencies is a task left to the designer in addition to implementing business logic.
Data Layer
Data Access layer: sometimes referred to as the persistence layer, its function is mainly responsible for database access, access to the database system, binary files, text documents or XML documents.
The simple argument is to implement the select,insert,update,delete of the data table. If you want to add an ORM element, it will include the mapping between the object and the data table, and the persistence of the object entity.

Advantages of the three-layer structure:
What are the advantages of layered structure? Martin Fowler the answer in the book Patterns of Enterprise application Architecture:
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 layer and layer of dependence, (high cohesion, low coupling);
4, in favor of standardization;
5, facilitate the reuse of the logic of each layer.
In summary, layered design can achieve the following objectives: decentralized attention, loose coupling, logical multiplexing, and standard definitions .

Disadvantages of the three-layer structure:

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.
Component-based three-layer B/S structure overview
In the design of software architecture, layered structure is the most common and most important structure.

Microsoft's recommended layered structure is generally divided into three tiers, from bottom to top: The data access layer, the business logic layer (or become the domain layer), the presentation layer.

Database-three-tier architecture