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OCR H446 · Component 1 · 1.3.2

Database Management
Systems (DBMS)

OCR A Level Computer Science · cszone.co.uk
H446 SpecA Level
Learning Objectives

By the end of this topic you will be able to:

Define a DBMS and explain its role
Describe the functions of a DBMS: concurrent access, integrity, security, backup and recovery
Explain transaction processing and ACID properties
Understand the role of the Database Administrator (DBA)
What is a DBMS?

Database Management System

A DBMS is software that manages the creation, maintenance, and use of databases. It provides an interface between the physical data storage and the applications and users that access it. Examples: MySQL, PostgreSQL, Oracle, Microsoft SQL Server, SQLite.
Abstraction: applications interact with the DBMS through a query language (SQL), not with the physical storage directly. The DBMS handles how data is physically stored and retrieved.
Multi-user access: a DBMS allows many users and applications to access the same database simultaneously, managing conflicts and ensuring data consistency.
DBMS Functions

Key DBMS Functions

Data Access & Security
Access control: user accounts with permissions (read-only, read-write, admin) ensure users only see and modify data they are authorised to.
Encryption: data can be stored and transmitted encrypted to protect sensitive information.
Integrity & Recovery
Integrity constraints: enforces data rules (primary keys, foreign keys, NOT NULL) to prevent invalid data being stored.
Backup and recovery: regular automated backups; transaction logs allow rollback to a consistent state after failure.
ACID

Transaction Processing and ACID

A transaction is a sequence of operations treated as a single logical unit of work. If any step fails, the entire transaction is rolled back. The ACID properties guarantee reliability:
Atomicity: all operations in a transaction complete successfully, or none take effect (all or nothing). No partial updates.
Consistency: a transaction brings the database from one valid state to another. All integrity constraints remain satisfied.
Isolation: concurrent transactions execute as if they were serial (one after another). Intermediate states are not visible to other transactions.
Durability: once a transaction is committed, the changes are permanent even if the system fails immediately afterwards (stored to non-volatile storage).
Exam Practice
OCR H446 Style · 4 marks
A bank transfers £500 from Account A to Account B. Explain how the ACID properties ensure this transaction is handled correctly even if the system fails mid-transaction.
[4 marks]
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Atomicity: if the system crashes after deducting from A but before crediting B, the entire transaction is rolled back — no money is lost or created.
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Consistency: the total money in the system remains unchanged before and after — integrity constraints prevent an invalid state (e.g. negative balance if applicable).
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Isolation: if two transfers occur simultaneously, they execute as if sequential so they don't interfere with each other's reads and writes.
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Durability: once the transaction is committed, the new balances are permanently stored even if the system immediately loses power.
Common Mistakes

Don’t Lose Marks

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Confusing a database with a DBMS — a database is the organised collection of data; a DBMS is the software that manages it. MySQL is a DBMS; the data it manages is the database.
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Saying Atomicity means transactions are fast — atomicity means transactions are indivisible (all or nothing), not that they are quick. An atomic transaction can take a long time but still succeeds or fails as a whole.
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Forgetting to apply ACID to the given scenario — exam questions give a specific context (e.g. bank transfer, booking system). You must relate each ACID property to that specific scenario to gain marks, not just define it in the abstract.
1.3.2d Complete
Well done! ✓
Database Management Systems (DBMS)
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