Database System Concepts and Architecture

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Database System Concepts and Architecture

• From Fundamentals of Database Systems by
  Elmasri/Navathe

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Evolution of DBMS Packages

• Early monolithic systems
• Tightly integrated systems
• Modular design, with a client/server system
  architecture
• Client module typically handles UI
• Server module typically handles data storage,
  access, search etc.
• Mirrors trends in computing

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Data Models

• Data Abstraction
• In this context, also refers to different users
  perceiving data at their preferred level
• Data Model
• Collection of concepts that can be used to
  describe the structure of the database
• Structure of the database
• Data types, relationships, and constraints

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Data Models (contd)

• Most models include basic operations for
  specifying retrievals and updates
• Specify a set of valid user-defined operations
  that are allowed on the database objects
• Dynamic aspect or behavior

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Categories of Data Models

• High-level or Conceptual Data Models
• Provide concepts that are close to the way many
  users perceive data (end users)
• Use concepts such as entities, attributes and
  relationships
• Representational or Implementation Data Models
• Concepts may be understood by end users
• Hide some details of data storage
• These models include relational, network and
  hierarchical models
• Sometimes called record-based data models since
  data is represented using record structures

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Categories of Data Models (contd)

• Low-level or physical data models
• Provide concepts that describe the details of how
  data is stored as files
• Record formats, record orderings, and access
  paths
• Access Path is a structure that makes the search
  for particular database records efficiently
• Index is an example of an access path that allows
  direct access to data using an index term or a
  keyword

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Schemas, Instances, and Database State

• Database Schema
• The description of the database
• Specified during database design
• Expected to change when requirements change (some
  operations for schema evolution while database is
  in operation are available)
• Schema Diagram
• Most data models have certain conventions for
  displaying schemas as diagrams
• Displays only some aspects of a schema, such as
  names of record types and data items
• Some constraints are difficult to represent
• Schema Construct
• Each object in the schema (e.g., student, course)

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Database State or Snapshot

• Actual data in the database may change frequently
• The data in the database at a particular moment
  in time is called a database state or snapshot
• Also called the current set of occurrences or
  instances in the database

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Database Schema vs. Database State

• The distinction is very important
• When we define a database, we specify its
  database schema only to the DBMS
• Database state is the empty state (w/ no data)
• Initial state when the database is first
  populated or loaded with initial data
• Current State State at any point in time. Every
  time an update operation is applied to the
  database, database state changes

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Database Schema vs. Database State (contd)

• Valid State A state that satisfies the structure
  and the constraints specified in the schema
• The schema is sometimes called the intension, and
  a database state is called an extension of the
  schema

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Three-Schema Architecture

• The goal is to separate user applications and
  physical database
• Schemas defined at
• External (or View) Level
• includes external schemas or user views
• Conceptual Level
• Schema describes database structure
• Internal Level
• Schema describes physical storage of database
• Mapping Process of transforming requests and
  results between levels

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Data Independence

• Definition Capacity to change the schema at one
  level without having to change the schema at the
  next higher level (mappings may change)
• Logical Data Independence
• Conceptual Schema/External Schema
• Physical Data Independence
• Internal Schema/Conceptual Schema

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Database Languages

• Data Definition Language (DDL)
• If DBMS does not clearly distinguish between
  levels, DDL is used to define conceptual and
  internal schemas
• If it does used to define conceptual schema only
• Storage Definition Language (SDL)
• Used to define internal schema
• View Definition Language (VDL)
• Specify user views
• Data Manipulation Language (DML)

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DML Types

• High-level or nonprocedural DML
• Used on its own (e.g., entered interactively from
  a display monitor) to specify complex operations
• or embedded in a general-purpose programming
  language (statements will be precompiled)
• Set-at-a-time (or set oriented) DML (e.g., SQL)
• Low-level or procedural DML
• Must be embedded in a programming language
• Also called record-at-a-time (e.g., DL1 for
  hierarchical model)
• Typically retrieves and processes each record
  from a set
• Needs programming concepts such as looping

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DML (contd)

• Query in high-level (declarative) DML often
  specifies what data to retrieve not how to
  retrieve it
• Query Language
• Embedded
• Host language
• Data Sublanguage high-level DML

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DBMS Interfaces

• Menu-Based interfaces for Web Clients or Browsing
• Forms-Based Interfaces (e.g., Oracle Forms)
• Graphical User Interfaces
• Natural Language Interfaces
• Speech Input and Output
• Interfaces for Parametric Users (e.g., tellers)
• Interfaces for the DBA

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Database System Environment

• DBMS Component Modules
• At a higher level (than the OS) Stored Data
  Manager Module controls access to the DBMS info
  (both database and catalog)

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Database System Utilities

• Some common utilities for the DBA
• Loading utility to load existing files
• conversion tools existing source internal
  schemas
• Backup
• Database Storage Reorganization
• Performance Monitoring
• Interfacing with the Network

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Tools, Application Environments, Communication
Facilities

• Tools for database designers, users
• Expanded Data Dictionary (data repository) system
• In addition to schemas and constraints, stores
  design decisions, application program
  descriptions, user info.
• Application Development Environments
• Communications Software
• DB/DC integrated DBMS and data communications
  system

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DBMS Architectures

• Centralized DBMS Architecture
• Client/Server Architecture
• Two-Tier
• Client Query Server (or Transaction Server)
• ODBC, JDBC
• Client Data Server
• Three-Tier and n-Tier for Web applications
• Introduces middle-tier (application server or web
  server)
• Business rules

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Classification of DBMS

• Several Criteria
• Data model
• Relational, Object, Hierarchical, Network
• Number of Users
• Single/Multi User Systems
• Number of Sites
• Centralized/Distributed (Homogeneous/Heterogeneous
  )
• Cost
• Access Path Options (inverted file structures)
• General/Special Purpose (e.g., OLTP)

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