Queries

A transaction is a fundamental concept in DBMS that represents a unit of work involving one or more database operations (e.g., INSERT, UPDATE, DELETE, SELECT). Here are some key concepts associated with transactions:

Properties of Transaction (ACID Properties)

To carry out a transaction within a Database Management System (DBMS), it must exhibit a set of characteristics commonly referred to as the ACID properties.

• A – Atomicity

• C – Consistency

• I – Isolation

• D – Durability 

Transaction States

Transactions in database management systems (DBMS) have several key properties that define their behavior and ensure data integrity. These properties are often referred to as the ACID properties:

1. **Atomicity (A):** Atomicity ensures that a transaction is treated as a single, indivisible unit of work. 

It follows the "all or nothing" principle, meaning that all the operations within a transaction are either fully completed or fully undone in case of an error or failure. 

If any part of the transaction fails, the entire transaction is rolled back, and the database remains unchanged. 

Atomicity ensures that the database remains in a consistent state.

2. **Consistency (C):** Consistency guarantees that a transaction brings the database from one valid state to another valid state. 

This means that a transaction must adhere to all integrity constraints, rules, and validations defined in the database schema. 

If a transaction violates consistency rules, it is rolled back, and the database remains unchanged. Consistency ensures that data remains in a coherent state during and after the transaction.

3. **Isolation (I):** Isolation defines the degree to which one transaction is isolated from the effects of other concurrent transactions. 

It ensures that concurrent transactions do not interfere with each other and maintains data integrity. 

Different isolation levels (e.g., Read Uncommitted, Read Committed, Repeatable Read, Serializable) specify the level of isolation between concurrent transactions.

Higher isolation levels provide stronger guarantees but may impact performance due to locking and resource contention.

4. **Durability (D):** Durability guarantees that once a transaction is committed, its effects are permanent and will survive any subsequent system failures, such as power outages, crashes, or hardware failures. 

This is typically achieved by writing transaction changes to non-volatile storage (e.g., disk) and maintaining a transaction log for recovery purposes. Durability ensures data persistence and reliability.

These ACID properties collectively provide a framework for ensuring the reliability, consistency, and integrity of database transactions, even in complex and concurrent environments. They are essential in applications where data accuracy is critical, such as financial systems, e-commerce platforms, and any scenario where data integrity is paramount. Transactions that adhere to the ACID properties can be trusted to maintain data integrity and consistency.

States of Transaction 

Transactions in a database management system (DBMS) go through various states during their lifecycle. These states represent the progress and outcome of a transaction. The common states of a transaction include:

1. **Active (or Running):** The transaction is in the active state when it is executing its operations. During this phase, the transaction is interacting with the database, reading, and writing data. It may issue one or more SQL statements.

2. **Partially Committed (or Preparing):** After the transaction has executed its operations successfully, it enters the partially committed state. In this state, the DBMS prepares to make the changes permanent but has not yet committed the transaction. The system ensures that all necessary conditions are met before moving to the committed state.

3. **Committed:** In the committed state, the transaction has been successfully completed, and all its changes have been made permanent in the database. Once a transaction is committed, its changes are considered permanent and cannot be undone.

4. **Aborted (or Rolled Back):** If an error occurs during the execution of the transaction or if the transaction encounters a condition that causes it to fail, it may enter the aborted state. In this state, the transaction is rolled back, which means that any changes it made to the database are undone, and the database is restored to its previous state.

5. **Failed:** The failed state is different from the aborted state. A transaction enters the failed state when it encounters a critical error that prevents it from continuing. In this case, the DBMS automatically rolls back the transaction to maintain data integrity and consistency.

6. **Terminated (or Completed):** After a transaction has reached a terminal state (committed, aborted, or failed), it is considered terminated. The transaction can no longer be modified or interact with the database. It may still be examined for auditing or debugging purposes.

7. **Pending:** In some systems, a transaction may enter a pending state temporarily. This typically occurs when a transaction is waiting for a resource or a lock that is held by another transaction. While in the pending state, the transaction is not actively executing but is waiting for its turn to proceed.