What is a Transaction in DBMS?

A transaction in the context of a Database Management System (DBMS) is a logical unit of work that performs one or more operations (such as INSERT, UPDATE, DELETE, or SELECT) on a database. These operations are grouped together to ensure that they are executed as a single, indivisible unit, which either completes fully (commit) or fails entirely (rollback). This ensures data consistency, integrity, and reliability.

Key Properties of Transactions (ACID Properties)

1. Atomicity:
   • A transaction is treated as a single unit.
   • Either all operations within the transaction are executed successfully or none are.
   • Example: In a bank transfer, both debit from one account and credit to another must occur together; otherwise, neither occurs.
2. Consistency:
   • Ensures that the database transitions from one valid state to another.
   • All integrity constraints and rules must remain valid after a transaction.
   • Example: After a stock purchase, the total available shares remain constant.
3. Isolation:
   • Transactions are executed independently of one another.
   • Concurrent transactions do not interfere with each other.
   • Example: While one transaction updates a customer's balance, another cannot see the intermediate state.
4. Durability:
   • Once a transaction is committed, the changes are permanent, even in case of a system crash.
   • Example: After transferring funds, the database reflects the transaction even after a power failure.

States of a Transaction

A transaction progresses through the following states:

1. Active: The transaction is in progress.
2. Partially Committed: All operations have been executed, but changes are not yet permanent.
3. Committed: The transaction successfully completes, and changes are saved permanently.
4. Failed: An error occurs, and the transaction cannot proceed further.
5. Aborted: Changes made during the transaction are rolled back, restoring the database to its original state.