[three tier architecture]

1. What is the third floor ？

UI( The presentation layer ): It mainly refers to the interface interacting with users . It is used to receive data input by users and display data needed by users after processing .

BLL:( Business logic layer ): UI Layer and the DAL The bridge between layers . Implement business logic . The business logic specifically includes ： verification 、 Calculation 、 Business rules and so on .

DAL:( Data access layer ): Dealing with databases . The main purpose is to increase data 、 Delete 、 Change 、 check . Submit the data stored in the database to the business layer , At the same time, the data processed by the business layer is saved to the database .（ Of course, these operations are based on UI Layer of . The user's needs are reflected in the interface （UI）,UI Reflect to BLL,BLL Reflect to DAL,DAL Data manipulation , After operation, return one by one , Until the user needs data feedback to the user ）

Each layer has its own responsibility , So how to connect the three layers ？

1、 Single reference （ See the picture below ）

2、 At this time, the entity layer （Entity） coming .（ notes ： Of course , The role of the entity layer is more than that ）

**Entity( Physical layer )：** It doesn't belong to any of the three layers , But it is an essential layer .

**Entity Role in three-tier architecture ：**

- 1、 Implement the in object-oriented thinking " encapsulation ";
- 2、 Through the third floor , Transfer data between the three layers ;（ notes ： To be exact, the entity layer runs through the three layers , To connect the three layers ）
- 3、 For beginners , It can be understood in this way ： Each data table corresponds to an entity , That is, the fields in each data table correspond to the attributes in the entity （ notes ： Of course , In fact, it's not . Why? ？1＞, Maybe the entity we need does not exist in the entity corresponding to the data table ;2＞, We can put all fields in all data tables in one entity ）
- 4、 Each layer （UI—>BLL—>DAL） Data transfer between （ A one-way ） Is passed by variables or entities as parameters , In this way, the connection between the three layers is constructed , Completed the implementation of the function .

But for a lot of data , Using variables as parameters is a little complicated , Because there are too many parameters , Easy to confuse . such as ： I want to pass the employee information to the lower level , The information includes ： Employee number 、 full name 、 Age 、 Gender 、 Wages ．．．． Using variables as parameters , Then there will be many parameters in our method , Most likely when used , Confuse parameter matching . Now , If you use entities as parameters , It will be very convenient , Don't consider the problem of parameter matching , You can use which attribute in the entity directly , Very convenient . This also improves efficiency .

> （** notes ：** Why can we temporarily understand that each data table corresponds to an entity ？？ answer ： Everybody knows , The purpose of our system , Is to provide services for users , Users don't care how your system background works , Users only care about whether the software is easy to use , Whether the interface is in line with your own mind . The user interface is easy to add 、 Delete 、 Change 、 check , Then there should be corresponding additions in the database 、 Delete 、 Change 、 check , The specific operation object of adding, deleting, modifying and querying is the data in the database , To put it bluntly, the fields in the table . therefore , Treat each data table as an entity class , The attributes encapsulated by the entity class correspond to the fields in the table , In this case , When the entity runs through three layers , You can add, delete, modify and query data ）

in summary : Dependencies between the three layers and the entity layer :

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2. Why use three layers

The purpose of using a three-tier architecture ： decoupling ！！！

Also take the example of the hotel above ：

（1） The waiter （UI layer ） Ask for leave —— Find another waiter ; The cook （BLL layer ） Resignation —— Recruit another chef ; buyer （DAL） Resignation —— Recruit another buyer ; （2） Customer feedback ：

• 1、 The service attitude of your store is not good —— The waiter's question . Fire the waiter ;

• 2、 There are insects in your food —— The chef's problem . Change the cook ;

Any change in one layer will not affect the other ！！！

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3. Difference from two layers

Two layers of ：

（ When any place changes , Need to redevelop the whole system ." Multi-storey " On the first floor , Unclear division of labor and high coupling —— Difficult to adapt to changes in demand , Low maintainability 、 Low scalability ）

Three layers ：

（ At what level does the change take place , Just change the layer , There is no need to change the entire system . A clear hierarchy , A clear division of responsibilities , Low coupling between each layer —— Improved efficiency , Adapt to changing needs , High maintainability , High scalability ）

Sum up , The advantages of three-tier architecture ：

• 1, The structure is clear 、 Low coupling

• 2, High maintainability , High scalability

• 3, It is conducive to synchronous development tasks , Easy to adapt to changing needs

Advantages and disadvantages of three-tier architecture ：

• 1、 It reduces the performance of the system . This is self-evident . If you don't use a layered structure , Many businesses can visit the database directly , In order to obtain the corresponding data , Now it has to be done through the middle tier .

• 2、 Sometimes it leads to cascading changes . This change is especially reflected in the top-down direction . If you need to add a function in the presentation layer , In order to ensure that its design conforms to the layered structure , You may need to add code in the corresponding business logic layer and data access layer

• 3、 Increased code , Increased workload