Three-tier database architecture

Three-tier Architecture and Data independence

Section 1.3 lists four important features of database methods, including the following three features: (1) Separation of procedures and data (procedures-Data independence and procedures-operational independence ); (2) multi-user views are supported; (3) database description (mode) is stored using cataloguing ). In this section, we will specify an architecture for the database system, which is called the three-tier schema architecture (three-schema architecture) architecture, the purpose of proposing this architecture is to help achieve these features and make them more intuitive. The concept of Data independence will be further discussed later.

2.2.1 three-tier architecture

As shown in 2.2, the purpose of the three-tier architecture is to separate user applications from physical databases. In this architecture, the pattern can be defined in the following three layers:

1. The internal layer (internal level) has an internal schema, which describes the physical storage structure of the database. The internal mode uses a physical data model to fully describe all the details and access paths of data storage in the database.

2. The concept layer (conceptual level) has a conceptual schema, which describes the structure of the entire database for the user group. The concept mode hides the details of the physical storage structure and focuses on the entity, data type, relationship, user operations, and constraints. After a database system is implemented, a data model is used to describe the concept mode. This implementation concept mode is usually based on a conceptual pattern design in the advanced data model.

3. The external level or view level includes many external modes or user views ). Each external mode describes the database that a specific user group is interested in, and other parts of the database are hidden from the user group. As described in, each external mode is usually implemented using a data model, and may be based on an external mode design of an advanced data model.

Figure 2.2 three-tier architecture

The three-tier mode architecture is a very convenient tool. With this structure, you can intuitively understand the mode layers in the database system. Most DBMS do not completely separate the three-layer mode, but only support the three-layer mode architecture to a certain extent. Some DBMS may include some physical layer details in the conceptual mode. In most DBMS that support user views, the external mode is also specified by the same data model that describes the concept layer information. Of course, some DBMS also allow different data models at the conceptual and external layers.

Note that the three modes are only data descriptions, and the actual data is in the physical layer. In a three-tier Schema-based DBMS, each user group only references its own external schema. Therefore, the DBMS must convert the requests specified to the external mode into a request oriented to the conceptual mode, and then convert the request to an internal mode to process the storage database. If a request requires database retrieval, the data extracted from the stored database must be reformatted to match the user's external view. The process of converting requests and results between layers is called mapping ). Ing may take a long time, so some DBMS (especially those used to support small databases) do not support external views. However, even in such a system, some mappings are required to convert requests between the concept layer and the internal layer.

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