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

Introduction

In the world of databases, transactions play a vital role in ensuring the consistency and integrity of data. A transaction is a logical unit of work that consists of one or more database operations. To better understand how transactions work, it's crucial to grasp the concept of transaction states. In this article, we'll dive into the different states a transaction can be in and explore their significance. We'll also provide simple code examples and explanations to illustrate each transaction state.

What is a Transaction?

A transaction is a sequence of one or more database operations (such as INSERT, UPDATE, or DELETE) that are treated as a single logical unit of work. The main purpose of a transaction is to ensure the consistency and reliability of data within a database. Transactions follow the ACID properties (Atomicity, Consistency, Isolation, Durability) to maintain data integrity.

Transaction States

Transactions progress through various states during their execution. Let's explore each state in detail:

• Active State: The active state is the initial state of a transaction. In this state, the transaction is actively executing its operations. It may read or modify data in the database. Once a transaction completes its operations, it transitions to the next state.
• Partially Committed State: In the partially committed state, a transaction has executed all its operations successfully, but it has not been officially committed yet. The database holds the changes made by the transaction temporarily until it is committed. Other transactions can still access the modified data, but they cannot see the changes made by the partially committed transaction.
• Committed State: When a transaction reaches the committed state, it means all its operations have been successfully executed, and the changes made by the transaction are permanently saved in the database. In this state, the changes become visible to other transactions, ensuring data consistency.
• Failed State: If any operation within a transaction fails due to an error or constraint violation, the transaction enters the failed state. In this state, the transaction is unable to proceed further, and all its changes are rolled back. The database restores the data to its original state before the transaction started.
• Aborted State: An aborted state occurs when a transaction is explicitly aborted or rolled back due to some external factors, such as a system failure or a user's request. In this state, all the changes made by the transaction are undone, and the database restores the data to its original state.

Code Examples

Let's illustrate each transaction state with simple code examples using SQL.

(a) Active State Example:

BEGIN TRANSACTION;

UPDATE employees SET salary = salary + 500 WHERE department = 'Sales';

COMMIT;

Explanation: In this example, a transaction is initiated using the 'BEGIN TRANSACTION' statement. The transaction then performs an update operation to increase the salary of employees in the Sales department. Finally, the 'COMMIT' statement confirms the successful execution of the transaction, transitioning it to the committed state.

(b) Partially Committed State Example:

BEGIN TRANSACTION;

INSERT INTO orders (order_id, customer_id, total_amount) VALUES (123, 'ABC123', 1000);

INSERT INTO order_items (order_id, product_id, quantity) VALUES (123, 'P1', 2);

COMMIT;

Explanation: This example demonstrates a transaction that inserts a new order and its corresponding items into two tables. The 'BEGIN TRANSACTION' statement starts the transaction, followed by the insertion operations. The 'COMMIT' statement temporarily holds the changes made by the transaction in the partially committed state.

(c) Failed State Example:

BEGIN TRANSACTION;

UPDATE employees SET salary = salary * 1.1 WHERE department = 'Sales';

UPDATE departments SET budget = budget - 10000 WHERE department_name = 'Sales';

COMMIT;

Explanation: In this example, a transaction attempts to update the salaries of employees in the Sales department and decrease the budget of the Sales department. However, an error occurs during the second operation, causing the transaction to enter the failed state. The changes made by the transaction are rolled back, and the database restores the original data.

Sample Problems

Problem 1: Identify the transaction states
Consider the following sequence of operations:

1. Begin transaction
2. Insert a new record
3. Update an existing record
4. Commit transaction

Identify the transaction states for each operation.

1. Begin transaction - Active state
2. Insert a new record - Active state
3. Update an existing record - Active state
4. Commit transaction - Committed state

Problem 2: Determine the final state
Given a transaction that encounters an error during execution, what will be the final state?

The final state will be the failed state. All changes made by the transaction will be rolled back, and the data will be restored to its original state.

Conclusion

Transaction states are essential to understand in DBMS as they provide a clear understanding of how transactions progress and affect the database. By grasping the different states (active, partially committed, committed, failed, and aborted), you can better manage the consistency and integrity of your database. Remember, transactions are crucial for maintaining data reliability and following the ACID properties.