Description: Repeatable read is a transaction isolation level in database management systems that ensures that if a transaction reads a row, it will read the same row with the same values again if no other transaction modifies it. This concept is fundamental in managing concurrency in databases, as it allows transactions to operate in a more predictable and consistent manner. At this isolation level, transactions can read data without worrying about other processes changing it during their execution. This is achieved through the use of locks or data versions, ensuring that the results of reads are stable and unaffected by external changes. Repeatable read is particularly useful in applications where data accuracy and consistency are critical, such as in financial systems, inventory systems, or any other applications requiring high data integrity. However, this isolation level can lead to higher resource usage and potentially lower performance, as transactions may become blocked waiting for resources to be released. In summary, repeatable read provides a balance between data consistency and system performance, making it an essential tool in the design of robust databases.
History: The concept of isolation levels in databases was formalized in the SQL-92 standard, which defined several isolation levels, including repeatable read. Over the years, the understanding and implementation of these levels have evolved, especially with the growth of distributed databases and high-concurrency systems. Repeatable read has become a standard in many relational databases, allowing developers to manage data integrity in complex environments.
Uses: Repeatable read is used in applications where data consistency is crucial, such as inventory management systems, banking applications, and reservation systems. It allows transactions to perform reads without fear of data changing during their execution, which is essential for maintaining the integrity of operations.
Examples: An example of repeatable read is in a banking system where a customer checks their balance. If the balance is read at a certain moment, the customer should see the same balance in all subsequent reads during the transaction, even if other customers are making deposits or withdrawals at the same time.
