Database technology-database system structure (three-level mode and second-level image)

Database system structure

• Structural classification :

  • Single user structure : the entire database system is installed on one computer, and data cannot be shared between different machines
  • Master-slave structure :One host with multiple terminalsThe multi-user structure of the database system (applications, DBMS, data) are all concentrated on the host, all processing tasks are completed by the host, and each user accesses the database concurrently through the terminal of the host
  • Distributed structure : the data in the database is logically a whole, butPhysically distributed in different nodes of the computer networkon.
  • Client / server structure : Separate DBMS functions and applications. The network is specifically used to perform DBMS functions, called database servers, referred to as servers; computers on other nodes install peripheral applications, called clients
  • Multilayer structure of browser / application server / database server :
    • Client: browser software, user interface
    • Server: Web server, application server, database server

• Concept of pattern

  • Mode : a description of the type, reflecting the structure of the data and its connections,Can be understood as a table
  • Instance : a specific value of the pattern, there can be multiple instances of the same pattern,Can be understood as the value in the table
  • Type : A description of the structure and attributes of a class of data
  • Value : is a specific assignment of type

• Three-level model structure
  

  • Mode (logical mode) :

    • DatabaseLogical structure and characteristics of all datadescription of
    • There is only one mode for a database,
    • Is the structure of the database system modelmiddle layer

  • External mode (sub mode or user mode) :

    • in useLogical structure and characteristics of local datadescription of
    • Between mode and application
    • The relationship between mode and external mode:One to many, A database has multiple external modes, reflecting different user needs
    • The relationship between external models and applications:One to many, The same external mode can also be used for multiple applications of a certain user

  • Internal mode (storage mode)

    • Data physical structure and storage methoddescription of
    • Whether compressed storage, whether encrypted, etc.
    • A database has only one internal mode
      

• Secondary image

  • External mode / mode image
    • Mode : describes the global logical structure of the data
    • External mode : describes the local logical structure of the data
    • One mode has multiple external modes
    • Maintain logical independence of data
  • Mode / Intra Mode Mapping
    • Defines the correspondence between the global logical structure of the data and the storage structure
    • The mode / internal mode image of the database is unique
    • Maintain physical independence of data
      

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