A database transaction represents a fundamental unit of work executed within a database management system (DBMS) against a database.

In essence, a database transaction consists of a sequence of one or more operations that are executed as a single logical unit of work. These operations typically include reading data from the database (through a ‘select’ operation), writing data (via ‘insert’ or ‘update’ operations), or deleting data. Transactions are a core concept in all database systems, as they play a crucial role in ensuring data integrity and consistency.

The concept of a transaction is underpinned by four key properties, commonly referred to as the ACID properties. ACID stands for Atomicity, Consistency, Isolation, and Durability:

• Atomicity ensures that a transaction is treated as a single, indivisible unit of work. This means that either all changes made by the transaction are committed to the database, or none are.

• Consistency guarantees that a transaction moves the database from one valid state to another, preserving the integrity of the data.

• Isolation ensures that the concurrent execution of multiple transactions does not compromise the database’s consistency. It guarantees that the outcome is the same as if the transactions had been executed sequentially, one after the other.

• Durability ensures that once a transaction has been committed, it remains so, even in the event of a system failure.

Transactions are vital in database systems, especially in environments where multiple users may access and modify the same data simultaneously. By grouping related operations into a single transaction, we can maintain a consistent state in the database, even if a system failure occurs during the execution of these operations.

For instance, consider a banking system where a user is transferring money from one account to another. This operation involves two critical steps: first, deducting the specified amount from the first account, and second, adding that amount to the second account. If a system failure occurs after the first step but before the second, the database could be left in an inconsistent state, resulting in missing funds. By treating these two steps as a single transaction, we can ensure that both steps are completed together, or neither is executed, thereby preserving the integrity of the data.