Desirable Properties of Transactions (ACID Properties)

 

Desirable Properties of Transactions (ACID Properties)

In database systems, transactions must satisfy a set of important properties to ensure correct, reliable, and consistent database operations.

These properties are called the ACID properties.

ACID =

  • A – Atomicity

  • C – Consistency

  • I – Isolation

  • D – Durability

These properties are enforced by concurrency control and recovery mechanisms in the DBMS.

1. Atomicity

Definition

Atomicity means “all or nothing.”

A transaction must be executed completely, or not executed at all.

If any part of the transaction fails, the DBMS must undo all its operations.

Example

Bank Transfer Transaction:

1. Deduct ₹500 from Account A
2. Add ₹500 to Account B

Two possible outcomes:

✔ Correct execution

A = A - 500
B = B + 500

✖ Incorrect partial execution

A = A - 500
B not updated

This would cause data inconsistency.

Atomicity ensures that:

Either both operations occur
OR
Neither occurs

Who Enforces Atomicity?

The Recovery Subsystem of the DBMS ensures atomicity.

If a failure occurs:

  • All partial updates are rolled back

  • The database returns to the previous state

2. Consistency Preservation

Definition

Consistency means:

A transaction must take the database from one consistent state to another consistent state.

A consistent state is a state where all integrity constraints are satisfied.

Example

Suppose a banking rule:

Account balance must never be negative

If a withdrawal transaction attempts:

Balance = ₹1000
Withdraw = ₹2000

The transaction must not complete, because it would violate the constraint.

Thus the database remains consistent.

Responsibility for Consistency

Consistency is ensured by:

  1. Database programmers

  2. Integrity constraint enforcement in the DBMS

The DBMS checks rules such as:

  • Primary key constraints

  • Foreign key constraints

  • Domain constraints

  • Business rules

3. Isolation

Definition

Isolation means that:

Each transaction should execute as if it is the only transaction running in the system.

Even though multiple transactions run concurrently, their execution should appear independent.

Why Isolation Is Needed

Without isolation, transactions may interfere with each other, causing problems such as:

  • Lost updates

  • Dirty reads

  • Incorrect summaries

  • Unrepeatable reads

Isolation prevents such interference.

Example

Two transactions updating the same bank account:

Transaction T1:

Deposit ₹500

Transaction T2:

Withdraw ₹200

Without isolation, their operations may overlap and produce incorrect results.

With isolation, the system ensures that the final result is equivalent to serial execution.

Who Enforces Isolation?

The Concurrency Control Subsystem of the DBMS enforces isolation.

Techniques include:

  • Locking

  • Timestamp ordering

  • Multiversion concurrency control

4. Durability (Permanency)

Definition

Durability means:

Once a transaction is committed, its changes must permanently remain in the database, even if failures occur.

After commit, the updates cannot be lost.

Example

Suppose a transaction inserts a new customer record.

COMMIT

Even if the system crashes immediately afterward:

  • The new record must still exist in the database.

How Durability Is Achieved

Durability is ensured by the recovery subsystem using:

  • System logs

  • Database backups

  • Redo operations

The system records updates in the log file, so they can be restored after a crash.

Summary of ACID Properties

PropertyMeaning            Responsible Component
Atomicity        Transaction executes completely or not at all       Recovery system
Consistency        Database moves from one consistent state to
            another		       Programmers + DBMS
        constraints
Isolation        Transactions execute independently      Concurrency control
Durability        Committed changes remain permanent      Recovery system

Levels of Isolation

Different database systems provide different isolation levels.

Isolation LevelGuarantees
Level 0            Prevents overwriting dirty data
Level 1            Prevents lost updates
Level 2            Prevents lost updates and dirty reads
Level 3 (True Isolation)            Prevents lost updates, dirty reads, and unrepeatable reads

Another commonly used approach is Snapshot Isolation, where transactions work with a snapshot of the database.

Summary 

ACID properties ensure that:

  • Transactions execute correctly

  • Database remains consistent

  • Concurrent execution is safe

  • Committed data is permanent

Without ACID properties, database systems would produce unreliable and inconsistent results.


ACID properties (Atomicity, Consistency, Isolation, Durability) guarantee reliable and correct transaction execution in a DBMS.

Comments

Post a Comment

Popular posts from this blog

Database Management Systems DBMS PCCST402 Semester 4 KTU CS 2024 Scheme

 About Me Database Management Systems DBMS PCCST402 Scheme and Syllabus Model Question Paper - KTU BTech DBMS PCCST402 2024 Scheme Module-1 Introduction to DBMS Introduction to DBMS Example  Database Database System Vs Traditional File Processing Advantages of using DBMS Database System Concepts and Architecture Data Models, Schemas and Instances   Three schema architecture and Data Independence Database Languages and Interfaces Centralized and Client Server Architectures in DBMS Classification of Database Management Systems Conceptual Data Modeling and Database Design Conceptual Modeling and the Role of the ER Model in Database Design Entities and Attributes  Company Database Application - Example Entity Types, Entity Sets, Keys, and Value Sets in the ER Model Initial Conceptual Design of the COMPANY Database Relationship Types in the ER Model Relationship Degree, Role Names, and Recursive Relationships Constraints on Binary Relationship Types Attributes of Relation...
Read more

Data Models, Schemas and Instances

  Data Models, Schemas and Instances  1. Data Abstraction In databases, data abstraction means hiding the complex details of how data is stored and focusing only on what is important. Different users can view data at different levels of detail depending on their needs. 2. Data Models A data model is a collection of concepts used to describe the structure of a database. It defines: Data types Relationships among data Constraints on data Operations for accessing and updating data Data models help us understand and design databases without worrying about low-level storage details. Categories of Data Models a) High-level / Conceptual Data Models Close to how users think about data. Use concepts like entities , attributes , and relationships . Example: ER (Entity–Relationship) Model . b) Representational (Implementation) Data Models In between conceptual and physical models. Easy for users to understand but still close to how data is st...
Read more

Introduction to Database Management System -DBMS

Image
In today’s digital world, almost every application—from mobile banking and e-commerce to university portals and hospital systems—depends on the efficient storage, organization, and retrieval of data. As the amount of information generated by individuals and organizations continues to grow, managing this data in a structured and meaningful way becomes essential. This is where database systems play a central role. A database can be thought of as a well-organized collection of related data, designed so that it can be easily accessed, managed, and updated. But the database itself is only one component. To manage it effectively, we rely on a specialized software package known as a Database Management System (DBMS) . A DBMS provides a systematic and automated way to store large volumes of data, enforce rules for consistency and correctness, protect data from unauthorized access, and allow multiple users to work with the data simultaneously without conflicts. A database typically has three...
Read more