Database Management Systems

File System Approach

• No centralized storage of data

• Each department controls and stores its own data

• Collection of apps that perform services to the end users

DBMS Approach

• Centralized database/repository that can be viewed by any number of users based on their interests

1. Self-describing Nature of a Database System

2. Insulation Between Programs and Data, and Data Abstraction

3. Support of Multiple Views of the Data

4. Sharing of Data and Multiuser Transaction Processing

Characteristics of DBMS Approach

Self-describing Nature of a Database System

• DB System - DB + Meta-data (DB Definition)

• Stored in DBMS Catalog

• Used by DBMS Software & DB Users

Insulation Between Programs and Data, and Data Abstraction

• DBMS software must work equally well with any number of database applications

• In traditional file processing, data-definition → part of application programs → work with only one specific DB

Support of Multiple Views of the Data

Different views of the DB for different users

View

subset of database, contains virtual data derived from database (not explicitly stored)

Sharing of Data and Multiuser Transaction Processing

For multiple users to be able to access a database at the same time, a multiuser database must include concurrency control

Concurrency Control

prevents 2 or more users from editing the same data at the same time

OLTP (Online Transaction Processing)

• Applications that require concurrency control

• Major part of database application

• DBMS must enforce several transaction properties

1. Isolation

2. Atomicity

Transaction Properties

Isolation

Data is blocked/isolated from other users to prevent them from editing

Atomicity

DBMS must ensure that the transaction is executed completely or not at all

THREE-SCHEMA ARCHITECTURE

• Separate the user applications from the physical database, which is the foundation for database independence

1. Internal Level

2. Conceptual Level

3. External Level

The database description is organized into three distinct levels

Internal Level

• Schema that describes the physical storage structure of the database

• Details how the data is stored on disk and defines the access paths for efficient retrieval

Conceptual Level

• Provides a unified, high-level description of the entire database

• Focuses on describing the entities, relationships, and constraints

• Hides physical storage details

External Level

• Consists of multiple external schemas/user views

• Each describes the specific part of the DB that a user group is interested in, hiding the rest

DATA INDEPENDENCE

enabled by the layered architecture (three-schema architecture)

Logical Data Independence

• Ability to change the conceptual schema without having to change the external schema

• Ex. adding a new entity or relationship

Physical Data Independence

• Ability to change the internal schema without having to change the conceptual schema

• Ex. modifying file organization or storage structures