Acid Properties | Database Management System (DBMS) - Software Development PDF Download

Introduction

Acid properties are an essential concept in the field of database management systems. They define the characteristics that a database must exhibit to ensure data integrity, consistency, and reliability. In this article, we will explore the three acid properties: Atomicity, Consistency, and Durability. We will also provide simple examples and code snippets to help you understand these concepts better.

Atomicity

Atomicity ensures that a transaction is treated as a single, indivisible unit of work. Either all the changes made by the transaction are committed to the database, or none of them are. Let's consider a simple example to illustrate atomicity.
Example: Suppose we have a bank transfer transaction that involves two accounts: Account A and Account B. The transaction deducts $100 from Account A and adds $100 to Account B. The atomicity property guarantees that if any part of the transaction fails, the entire transaction will be rolled back, and the database will be restored to its previous state.
Code Example: 

BEGIN TRANSACTION;

UPDATE Account SET balance = balance - 100 WHERE account_number = 'A';

UPDATE Account SET balance = balance + 100 WHERE account_number = 'B';

COMMIT;

Explanation: The code snippet above represents a transaction using SQL statements. The 'BEGIN TRANSACTION' command marks the start of a transaction, while the 'COMMIT' command commits the changes made by the transaction. If any part of the transaction encounters an error or failure, the transaction is rolled back, and no changes are applied to the database.

Consistency

Consistency ensures that a transaction brings the database from one valid state to another. It guarantees that the integrity constraints defined on the database are not violated during the execution of a transaction. Let's consider a simple example to understand consistency.
Example: Suppose we have a database table called "Students" with a constraint that the age of a student must be between 18 and 25. If a transaction attempts to insert a record with an age of 17, the consistency property ensures that the transaction is rolled back, and the database remains in a consistent state.

Code Example:

BEGIN TRANSACTION;

INSERT INTO Students (name, age) VALUES ('John', 17);

COMMIT;

Explanation: In the code snippet above, the transaction attempts to insert a record with an age of 17, which violates the consistency constraint. As a result, the transaction will be rolled back, and no changes will be made to the database.

Durability

Durability ensures that once a transaction is committed, its changes are permanently saved in the database, even in the event of a system failure. The changes made by a committed transaction should survive any subsequent system failures or crashes.
Example: Suppose a transaction updates a customer's address in a customer database. After the transaction is committed, the durability property ensures that the updated address remains intact even if there is a power outage or a server crash.
Code Example:

BEGIN TRANSACTION;

UPDATE Customers SET address = 'New Address' WHERE customer_id = 123;

COMMIT;

Explanation: In the code snippet above, the transaction updates the address of a customer with the ID 123. Once the transaction is committed, the updated address is durably stored in the database, making it resilient to system failures.

Sample Problems

Problem 1: Consider a banking application where a transaction transfers money from one account to another. Explain how the acid properties ensure data integrity and reliability in this scenario.

The atomicity property ensures that either the entire transaction of transferring money succeeds, or none of it happens. If any part of the transaction fails, the changes are rolled back, ensuring data consistency. The consistency property ensures that the accounts involved in the transaction remain in a valid state, adhering to constraints such as sufficient balance. The durability property guarantees that once the transaction is committed, the transferred amount is permanently saved in the database, surviving any system failures.

Problem 2: Explain why acid properties are crucial in e-commerce applications where inventory management is involved.

In e-commerce applications, atomicity ensures that if a customer purchases multiple items from the inventory, either all the items are successfully deducted from the stock, or none of them are. This prevents situations where some items are deducted, but others are not, leading to inconsistencies in inventory counts. Consistency ensures that the inventory remains in a valid state, preventing the sale of items that are out of stock. Durability ensures that once the inventory is updated, the changes are durable and survive system failures, ensuring accurate stock management.

Conclusion

Acid properties play a vital role in ensuring data integrity, consistency, and reliability in database management systems. Atomicity guarantees that transactions are treated as indivisible units, consistency ensures that transactions bring the database to a valid state, and durability ensures that committed changes survive system failures. By understanding and implementing these properties, developers can build robust and reliable applications.