Chapter 2: Database Systems Architecture

Database Schema and Instance

The collection of information stored at a specific time is called an instance of the database. The overall design of the database is called the database schema and the schema is changed infrequently. The logical schema is the most important as the programmer uses it to construct application programs. We say application programs have data independence when changes to the physical schema doesn’t require changes in the application program.

Levels of DBS Architecture

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Database Architecture refers to the structural design and methodology of a database system. The most commercial DBMS available today is based on ANSI-SPARC.

Internal Level: It is also known as the physical level. It describes how the data is actually stored in storage devices. It deals with low level data structures, file structures and access methods in detail.
Conceptual Level: It is also known as the logical level. It describes the type of data and their relationships in the database. This is where application developers work.
External Level: It is also known as the view level. It describes only part of the entire database that is relevant to the end user.

Advantages
Provides data abstraction
The end user is not concerned about the physical level
Upper levels are immune to changes in lower levels
Same database can be accessed by different users using different views

Multi-user Database Architectures

Teleprocessing
File-server
Client-server

Teleprocessing

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Teleprocessing is also known as Mainframe Architecture. It includes a single mainframe and multiple terminals. This system places tremendous burden on the central mainframe.

File-server

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File-server is a computer that is connected to several workstations in a network. The database resides on the file-server and the DMBS and application reside on the workstations. Some disadvantages include heavy traffic and high cost.