ACID Properties in DBMS

ACID Properties in DBMS

ACID Properties in DBMS

Introduction

In the digital age, managing data with precision and reliability is crucial. A Database Management System (DBMS) plays a pivotal role in ensuring that the data remains intact even when multiple operations and transactions are performed. But what ensures that these transactions don’t leave the database in an unstable or inconsistent state?

The answer lies in four essential properties collectively known as ACID properties. These properties—Atomicity, Consistency, Isolation, and Durability—form the backbone of reliable database transactions. They guarantee that operations either complete fully and correctly, or leave the system untouched in case of a failure.

This blog from Updategadh will walk you through each ACID property in detail, explaining its importance with real-world examples.

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What is a Transaction in DBMS?

In database management systems, a transaction is a series of actions carried out as a single logical work unit. Either all of these operations must be completed successfully, or none at all.

Let’s consider an example. Imagine an online banking system where a user wants to pay a telephone bill of ₹1000 via their bank account to the telecom provider. The process involves:

  1. Debiting ₹1000 from the user’s bank account.
  2. Crediting ₹1000 to the telecom provider’s account.

From the user’s perspective, this appears to be a single action. However, in the background, it involves multiple steps. If the debit occurs but the credit fails, the user loses money while the bill remains unpaid—an unacceptable situation.

To prevent such issues, transactions in DBMS follow the ACID principles. If any step fails, the system should rollback all changes and restore the previous consistent state.

What Are ACID Properties?

The term ACID stands for:

  • Atomicity
  • Consistency
  • Isolation
  • Durability

Let’s dive into each one.

1. Atomicity

Atomicity ensures that a transaction is treated as a single indivisible unit. Either all operations in the transaction are executed, or none are. There is no in-between.

Example:

Remo wants to send $10 from Account A (balance: $30) to Account B (balance: $100). Two operations are required:

  • Debit $10 from Account A
  • Credit $10 to Account B

If the debit is successful but the credit fails due to a system error, Account A reflects $20, but Account B still shows $100. This is an incomplete and invalid state, violating atomicity.

To uphold atomicity, if one operation fails, the system should abort the transaction and rollback Account A to its original state.

2. Consistency

Consistency ensures that a database remains in a valid state before and after a transaction. All defined rules, constraints, and relationships must hold true after the transaction is complete.

Example:

Suppose Account A has $300, Account B has $100, and Account C has $50. If $50 is transferred to B and $20 to C, the balance in A should reflect $230 post-transaction. If other accounts still read A’s balance as $300, the database is inconsistent. This violates data integrity and is not allowed.

Thus, consistency ensures that the rules and constraints of the database are never violated during transactions.

3. Isolation

Isolation defines how the effects of one transaction are invisible to other transactions until it is committed. Even when transactions run concurrently, the final state of the database must be as if transactions were executed sequentially.

Example:

Imagine two transactions trying to withdraw money from the same account at the same time. If not handled properly, both could read the same balance and overdraw the account. With proper isolation, one transaction completes first, updates the balance, and only then does the second transaction proceed with the updated value.

This property prevents data conflicts and anomalies in concurrent executions.

4. Durability

Durability ensures that once a transaction is committed, the changes are permanently saved in the database—even if a system crash or power failure occurs immediately afterward.

Example:

After transferring ₹750 from Account A to Account B and the transaction is successfully committed, a sudden power outage shouldn’t erase this change. The updated balances must persist. Durability guarantees this permanence.

The recovery manager within DBMS ensures durability by using logs and backups to restore committed transactions after failures.

Complete ACID Example

Let’s see how all four properties apply in a single transaction.

Transaction T1: Transfer ₹750 from Account A to Account B

Read(A)
A := A - 750
Write(A)
Read(B)
B := B + 750
Write(B)

ACID Compliance:

  • Atomicity: Either both debit and credit occur, or none.
  • Consistency: Balance constraints are preserved.
  • Isolation: Other transactions wait or execute independently without affecting this one.
  • Durability: Changes persist even after a crash.

Merits of ACID Properties

Here’s why ACID properties are crucial in DBMS:

  • Data Reliability: Ensures operations don’t corrupt data.
  • Data Integrity: Guarantees logical correctness of data.
  • Concurrency Control: Safely manages multiple simultaneous transactions.
  • Robust Recovery: Supports automatic rollback and recovery from failures.

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Conclusion

ACID properties are the foundation of secure and reliable database systems. They ensure that transactions are processed accurately, consistently, and safely—even in high-load or failure-prone environments.

At Updategadh, we emphasize understanding these fundamentals because they not only make systems robust but also prepare developers and engineers to design scalable and error-free applications. Whether you’re building a banking platform or a shopping cart, respecting ACID principles is non-negotiable.


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