Transaction Management in DBMS | Database Management System (DBMS) - Software Development PDF Download

Introduction

In the world of databases, managing transactions is a crucial aspect to ensure data consistency and reliability. Transactions are sets of operations performed on a database that should be executed as a single unit, either succeeding completely or failing entirely. This article will provide a beginner-friendly introduction to transaction management in a Database Management System (DBMS). We will explore the concepts of ACID properties, transaction states, and isolation levels, along with practical examples and code snippets for better understanding.

What is a Transaction?

A transaction is a logical unit of work in a DBMS. It represents a series of database operations, such as inserting, updating, or deleting records, that are treated as a single indivisible unit. The primary objective of a transaction is to maintain the integrity and consistency of the database. If any operation within a transaction fails, all the changes made so far need to be rolled back, ensuring that the database remains in a consistent state.

ACID Properties

Transactions must adhere to the ACID properties, which stand for:

• Atomicity: Atomicity ensures that a transaction is treated as an all-or-nothing operation. Either all the changes within a transaction are successfully committed to the database, or none of them are. If a failure occurs, the transaction is rolled back to its original state.
• Consistency: Consistency ensures that a transaction brings the database from one valid state to another valid state. It ensures that any constraints or rules defined on the database are not violated during the execution of a transaction.
• Isolation: Isolation ensures that concurrent transactions do not interfere with each other. Each transaction should execute in isolation, as if it were the only transaction executing on the database. This property prevents dirty reads, non-repeatable reads, and other data inconsistencies.
• Durability: Durability guarantees that once a transaction is committed, its changes are permanent and will survive any subsequent failures, such as power outages or system crashes. The committed data is stored in a non-volatile memory, ensuring its durability.

Transaction States

A transaction goes through different states during its lifecycle:

• Active: The initial state of a transaction when it starts executing.
• Partially Committed: The state reached when all the operations within the transaction have been executed successfully, but the changes are not yet made permanent.
• Committed: The state reached when the changes made by a transaction have been successfully persisted in the database.
• Failed: The state reached when an error occurs during the execution of a transaction, making it impossible to proceed further. The transaction is rolled back to its initial state.
• Aborted: The state reached when a transaction fails or is explicitly rolled back. The changes made by the transaction are undone, and the database is restored to its state before the transaction started.

Isolation Levels

Isolation levels determine how concurrent transactions interact with each other. Different isolation levels provide varying degrees of data consistency and performance.
The four standard isolation levels are:

• Read Uncommitted: The lowest isolation level where transactions can read uncommitted changes made by other transactions. This level offers the highest performance but compromises data consistency.
• Read Committed: Transactions can only read committed changes made by other transactions. This level prevents dirty reads, but non-repeatable reads and phantom reads may still occur.
• Repeatable Read: Transactions can read consistent snapshots of the data, even if other transactions make changes. This level prevents dirty reads and non-repeatable reads, but phantom reads can still occur.
• Serializable: The highest isolation level that provides the strongest data consistency. Transactions are executed in a serial manner, one after another. It prevents dirty reads, non-repeatable reads, and phantom reads, but it can lead to decreased concurrency and performance.

Example: Performing Transactions in SQL

Let's explore an example of how transactions are performed in SQL. Consider a table named "employees" with columns "id," "name," and "salary."

-- Example table creation

CREATE TABLE employees (

  id INT PRIMARY KEY,

  name VARCHAR(50),

  salary DECIMAL(10, 2)

);

Suppose we want to update an employee's salary and insert a new employee into the table as a single transaction. Here's how it can be done in SQL:

BEGIN TRANSACTION;

UPDATE employees SET salary = 5000 WHERE id = 1;

INSERT INTO employees (id, name, salary) VALUES (4, 'John Doe', 4000);

COMMIT;

In the code above, we start the transaction using the 'BEGIN TRANSACTION' statement. Then, we perform the required operations within the transaction, including updating an employee's salary and inserting a new employee. Finally, we commit the transaction using the 'COMMIT' statement, making the changes permanent.

Sample Problems and Solutions

Problem 1: Consider a banking system where a user transfers money from one account to another. Ensure that the transaction deducts the correct amount from the sender's account and adds it to the recipient's account.

BEGIN TRANSACTION;

-- Deduct amount from sender's account

UPDATE accounts SET balance = balance - 500 WHERE account_number = '123456789';

-- Add amount to recipient's account

UPDATE accounts SET balance = balance + 500 WHERE account_number = '987654321';

COMMIT;

Problem 2: How can you prevent a transaction from being committed automatically in some scenarios?

To prevent a transaction from being committed automatically, we can use the 'SET IMPLICIT_TRANSACTIONS ON' statement. This statement enables implicit transactions, meaning each SQL statement is treated as a separate transaction unless explicitly committed. Here's an example:

SET IMPLICIT_TRANSACTIONS ON;

UPDATE employees SET salary = 5000 WHERE id = 1;

-- The transaction is not committed automatically

-- You can explicitly commit it using COMMIT statement

Conclusion

Transaction management is a vital aspect of database systems to maintain data consistency and reliability. By following the ACID properties, understanding transaction states, and choosing appropriate isolation levels, you can ensure that your database operations are executed reliably. Remember to use transaction management techniques like rollback and commit to handle errors and maintain data integrity. With the knowledge gained from this article, you are well-equipped to start working with transactions in DBMS.