Database System Structure

I learned database (7)-the structure of the database system is in the software industry, and the software is structured. The database management system is a software after all, so we also talk about the structure. The structure of the database system can be viewed from different angles or layers. In terms of the system itself, the database system usually adopts a three-level structure, that is, the internal structure of the system. If the slave number

I learned database (7)-the structure of the database system is in the software industry, and the software is structured. The database management system is a software after all, so we also talk about the structure. The structure of the database system can be viewed from different angles or layers. In terms of the system itself, the database system usually adopts a three-level structure, that is, the internal structure of the system. If the slave number

I learned database (7) -- Database System Structure

In the software industry, the software is structured, and the database management system is, after all, a software. The structure of the database system can be viewed from different angles or layers.

In terms of the system itself, the database system usually adopts a three-level structure, that is, the internal structure of the system. From the end user's perspective, its structure is divided into single-user structure, master-slave structure, distributed structure, C/S structure, B/S structure, and Browser/Application Server/database server structure, that is, the external architecture of the data system.

Follow these steps to learn more about Level 3 models and various external architectures one by one. Before learning the three-level mode, first understand the concept of the database system mode.

Data Models have the concepts of "type" and "value. A type refers to the description of the structure and attributes of a type of data. A value is a specific value of a type. As mentioned in the previous article, such as the concept of entity and entity, and object set. The schema is the logical structure and feature description of all data in the database, that is, the type description. A specific value of the mode becomes an instance of the mode. One mode can have multiple instances, such as the student information database. If school A and school B use the same database model, the database of school A and the database of school B must be different, because the student information is different. The mode is relatively stable, and the instance is changed because the data is constantly updated. The schema is the structure and connection of the data, while the instance reflects the status of the database at a certain time point. There are many database management systems based on different data models. on different operating systems, the data storage structure is also different, but the architecture usually has the same characteristics, it adopts a three-level structure and provides two-level image functions.

The three-level mode structure of the database refers to the three-level structure of the database system: external mode, mode, and internal mode. It is the three-level mode structure of the database system:

The external mode is also called the submode or user mode. It is the logical structure and feature description of local data that can be viewed and used by application programmers and end users, and is the data view of database users, as shown in the figure, the external mode is the logical representation of data related to an application, and a database can have many sub-modes, the same external mode can correspond to multiple application modes. It can be concluded that other applications can be added in the same external mode to meet other requirements. However, for different applications, even the same data used can have different structures, types, and security levels. We can also see that none of the Hong Kong servers and applications use two or more external models. Another useful information is that an application can only retrieve data from the corresponding external mode, but not from other external modes, this provides a very favorable protection measure to ensure database security.

A schema is also called a logical schema. It describes the logical structure and features of data and is a public data view for all users. It is an intermediate layer of the database system schema structure. It is not related to the physical details of data and the hardware environment, nor to specific applications and advanced programming languages. A database has only one mode. It is based on a data model, considers the needs of all users in a unified manner, and combines the two into a logical whole. When defining a schema, You need to define the logical structure of the data, as well as the relationships between the data and requirements related to security and integrity.

The internal mode is also called the storage mode. A database has only one internal mode, which is used to describe the physical structure and storage mode. It is the internal representation of data. For example, using Heap Storage records or CLUSTER Storage, the index is B + tree index or hash index.

This three-level mode is a three-level abstraction of data, allowing users to logically abstract and process data without having to care about the specific representation and storage methods of the data. To achieve the connection and conversion at these three layers, the database management system provides two layers of images: external mode/mode image and mode/internal mode image.

In the previous article, we mentioned that the database system is a little bit more independent than manual data management and file systems. One of them is that data independence is high, and the two-layer image function ensures high data independence, data independence is divided into logical and physical independence.

External mode/mode image. The previous writing mode is the global logical structure of the data, while external mode is the local logical structure of the data. One mode can have multiple external modes. When the mode changes, such as adding new relationships and attributes, the Database Administrator then adjusts the external mode/mode images without having to adjust the external mode. The application is written according to the external mode, so that the application does not need to be changed, which ensures the logical independence of data and the program, that is, the logical independence of data.

The database has only one mode and only one internal mode, so there is only one mode/internal mode image, which defines the correspondence between the Global Logical Structure and the storage structure. After the storage structure of a single data changes, the database administrator can change the mode/internal mode image without changing the mode, so the application does not need to change. This ensures the physical independence of data.

A database system generally consists of a database, a database management system, a development tool, an application system, and a database administrator. Because the data size of the database system is large and the DBMS function is very powerful, the hardware resource selection requirements of the database system are high, such as the requirement for sufficient memory and disk space. Database System Software mainly includes DBMS, operating system, advanced language and its compilation system, and database application systems developed with DBMS-centered application development tools as specific application environments. Mainly include database administrators, system analysts and database designers, application programmers and users. (I will forward two articles about DBA employment later as the database administrator DBA. These two articles are boring and I think they are good. I will share them with you after this article .)

Then I will give a brief introduction to the external architecture of various database systems.

Single-user structure: the entire database system (application, DBMS, and data) is mounted on one computer and exclusive to one user. data cannot be shared between different machines.

Master-slave structure: a multi-user structure with multiple terminals on a host. The database system is stored on the host and Hong Kong Space. All processing tasks are completed by the host, each user accesses the database concurrently through the terminal of the host to share data resources.

Distributed structure: the data in the database is logically integrated, but physically distributed on different nodes in the computer network. each node in the network can independently process data in the local database, local applications can simultaneously access and process data in multiple remote databases to execute global applications.

C/S structure: separates DBMS functions from applications. A computer on a certain node in the network is used to execute DBMS functions. It is called a database server or server for short; the computer on other nodes installs the DBMS peripheral application development tool. The user's application system is called a client. One is a centralized server structure, one database server and multiple clients, and the other is a distributed server structure with multiple database servers on the network, the distributed server structure combines the customer/server with the distributed database.