The central place in the data processing system is assigned to data organizations on computer media. Initially, the file organization of the information database, designed to solve problem complexes (Figure 10.20), for which programs were developed in the algorithmic programming language (PL/1, Pascal, Fortran and t) .P.). In the programs, the files of the local information database of the task (LIB) were managed: creation, search, deletion of file entries, modification of the values of their fields. As the number of tasks increases, data duplication in various LIBs inevitably occurs, therefore, it is impossible to guarantee the relevance, completeness and integrity of the data of all LIBs. This reduces the quality of information for making management decisions, increases the cost of creating and maintaining an automated management system.
Fig. 10.20. File Organization of the Information Base
The concept of databases emerged at the turn of the 60's. XX century. as an alternative to the file organization of the information base. On the one hand, the need to integrate data, provide single input and reuse data with various tasks, improve the quality of information (relevance, reliability and timeliness of receipt), and the emergence of large-scale direct access devices for data storage (magnetic drums and magnetic disks), development of methods and the programming tools, on the other hand, together contributed to the development of the concept of databases.
In Fig. 10.21 is the proposed by the American National Standards Institute (ANSI/SPARC) Subcommittee on Standardization of Requirements and Planning in 1975, based on the model approach, the allocation of logical and the multilevel presentation of the database physical data presentation layers iodine management DBMS - database management system.
Fig. 10.21. Model database view
Boolean defines the content and logic of data usage by applications. To represent an integrated data structure, use the conceptual model ( model or data schema ). To represent a particular data structure for an individual task, use the external model ( submodel or data schema ), which is a subset of the conceptual model. The external model defines the composition and structure of the data, as well as the allowed methods for working with data - reading, writing, modifying and deleting. Due to external models, authorized access to the database of various applications can be controlled.
The physical data layer takes into account the features of the storage environment: the organization method and data access method, physical organization parameters (data formats, record lengths, physical block sizes, overflow management methods data in the physical space, etc.), the location of files, etc.
Typical properties of databases:
• multilevel representation, model description of domain data;
• data integration (instead of the LIB set, a single database is used);
• the independence of data processing logic from their physical representation;
• application of a specialized program - database management system (DBMS) for centralized management of database data;
• guaranteed level of data quality.
To key concepts of database organization are:
• Information and technological architecture of the database;
• the structure and models of the database data;
• The physical organization of database data on a computer medium.
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