DBMS: the gatekeeper between programs and data

The problem

One college database, three programs: results, fees and library. All three need students' names. Only one should ever change marks. And if the data moves to a new disk tomorrow, nobody wants to rewrite three programs.

The answer is a layer of software that stands between every program and the data: the database management system.

A DBMS is a collection of programs for storing and manipulating databases. It controls the database's structure and all access to the data. Its objectives are shareability, availability, evolvability and database integrity.

Key terms

Shareability

Different people and different processes must be able to use the same actual data at virtually the same time.

Evolvability

The ability of the DBMS to change in response to growing user needs and advancing technology.

Data independence

Application programs are not aware of how the data sets are physically stored. The DBMS sits between the programs and the data.

Sophisticated security

Each program's access intent (read, write, update, delete) can be set explicitly, and its view of records can be controlled down to single fields.

Short questions with model answers

1. Q1. Name the DBMS objective. (a) Many staff use one record at once. (b) Data is easy to reach when needed. (c) The system grows with new needs. (d) Shared data has integrity controls.

   • (a) shareability (b) availability
   • (c) evolvability (d) database integrity

   shareability · availability · evolvability · integrity

2. Q2. Name the advantage. (a) Data moves to a new disk; no program changes. (b) A clerk may read fees but not change them. (c) After a crash the database is rebuilt.

   • (a) data independence
   • (b) sophisticated security
   • (c) backup / recovery

   independence · security · backup/recovery

3. Q3. Why does a true DBMS keep a log as well as a backup copy?

   • Backup = the data at one moment
   • Log = every change since then

   Backup + log → full recovery

Common mistakes

• ✗ Using “database” and “DBMS” as the same thing.

  ✓ The database is the collection of related files. The DBMS is the software that stores, manipulates and controls access to it.

• ✗ Saying a backup copy alone is enough for recovery.

  ✓ Recovery uses the backup copy and the log of changes; without the log, work since the backup is lost.

• ✗ Copying the objective as “avaibility”.

  ✓ The book misprints it. The word is availability: both the data and the DBMS must be easily accessible to users.

MCQs

1. 1. DBMS stands for:

   1. (a) Data Backup Management Software
   2. (b) Database Management System
   3. (c) Data Base Monitoring Service
   4. (d) Digital Base Memory System
   Show answer

   (b) Database Management System: the programs that store and manipulate databases.

2. 2. Different people using the same actual data at virtually the same time is:

   1. (a) evolvability
   2. (b) shareability
   3. (c) availability
   4. (d) integrity
   Show answer

   (b) Shareability is the first objective listed in 1.4.1.

3. 3. The ability of a DBMS to change with growing user needs and new technology is:

   1. (a) evolvability
   2. (b) shareability
   3. (c) data integration
   4. (d) recovery
   Show answer

   (a) Evolvability means the system can evolve with needs and technology.

4. 4. Application programs not knowing how data is physically stored is:

   1. (a) data integrity
   2. (b) data independence
   3. (c) data dictionary
   4. (d) data redundancy
   Show answer

   (b) The DBMS sits between the programs and the data sets, hiding the physical implementation.

5. 5. To recover a damaged database, a DBMS uses:

   1. (a) only the log
   2. (b) only a backup copy
   3. (c) a backup copy and the log of changes
   4. (d) the data dictionary
   Show answer

   (c) The recovery utility rebuilds the database using the backup copy and the log of changes as input.

Quick revision

• A DBMS is the software that stores and manipulates databases and controls all access to them.
• Objectives: shareability, availability, evolvability and database integrity.
• Advantages: data independence, complex relationships, sophisticated security, backup/recovery and advanced reporting.