Functions of a Database

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Chapter 7

• Functions of a Database
• Management System

Database Management
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Objectives

• Discuss the following nine functions, or
  services, that should be provided by a DBMS, and
  the manner in which these services are typically
  provided
• data storage, retrieval, and update
• a user-accessible catalog
• shared update support
• backup and recovery services

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Objectives

• security services
• integrity services
• data independence support
• replication support and
• utility services.

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Data, Storage, Retrieval, and Update

• Provides users with the ability to store,
  retrieve, and update the data that are in the
  database.
• Defines the fundamental capability of a DBMS.

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Catalog

• Contains descriptions of database components and
  are accessible to users.
• Provides crucial information for those who are in
  charge of a database, or for those who write
  programs to access the database.

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Shared Update

• Ensures accuracy when several users are updating
  the database at the same time.

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Figure 7.1 Ryan Updates the Database
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Figure 7.2 Elena Updates the Database
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Figure 7.3 Inconsistent Data
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Avoiding the Problem

• Prohibit shared update.
• Use batch processing
• Lock other users from accessing records that are
  being updated by another.

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Figure 7.4 Using a Locking Scheme
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Figure 7.4 Using a Locking Scheme
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Avoiding the Problem

• Two-Phase Locking
• Locks are held until all the required updates
  have been completed.
• Deadlock
• Occurs when two users hold more than one lock at
  a time.
• Locking on PC-Based DBMSs
• Usually much more limited than locking facilities
  provided by mainframe DBMSs.

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Avoiding the Problem

• Timestamping
• The DBMS assigns to each database update the
  unique time when the update started, called a
  timestamp.
• Avoids the need to lock rows and eliminates the
  processing time needed to apply and release
  locks also detects and resolves deadlocks.

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Backup and Recovery

• Provides a mechanism for recovering the database
  in the event that the database is damaged in any
  way.
• The process of returning the database to a
  correct state is called recovery.
• Periodically making a copy of the database is
  called backup.

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Security

• The protection of the database against
  unauthorized access.
• Passwords
• Encryption
• Views

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Integrity

• Integrity constraints, or conditions, fall into
  four categories
• Data type
• Legal values
• Format
• Key constraints

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Figure 7.5 Integrity Constraints in a DBMS
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Data Independence

• Facilities that allow programs to be independent
  of the structure of the database.
• Addition of a field
• Changing the length of a field
• Creating a new index
• Adding or changing a relationship

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Figure 7.6Replication

• A way to manage copies of the same data at
  multiple locations.

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Utilities

• Assist in the general maintenance of the
  database.
• Permit changes to be made in the database
  structure.
• Permit the addition of new indexes and the
  deletion of ones no longer needed.
• Access to DOS, Windows, Linus, and other
  operating system services from within the DBMS.