Repeatable read

Definition

Repeatable read is a level of transaction isolation that ensures that if a transaction reads a row of data, it will always see the same data if it reads that row again during the same transaction. This means that no other transactions can modify or delete that data until the current transaction completes, thus preventing non-repeatable reads and providing a level of consistency in data retrieval.

5 Must Know Facts For Your Next Test

1. Repeatable read prevents non-repeatable reads by locking rows of data that are being accessed, ensuring data consistency within the transaction.
2. This isolation level is often considered a middle ground between lower isolation levels like read committed and higher levels like serializable.
3. While repeatable read guarantees consistent reads, it does not prevent phantom reads, which can occur when new rows are added by other transactions.
4. Most database management systems implement repeatable read using locks on the rows that are read, which can lead to increased contention in high-concurrency environments.
5. Understanding repeatable read is crucial for applications requiring strong consistency guarantees, especially in financial and critical systems.

Review Questions

• How does repeatable read help maintain data consistency during a transaction?
  • Repeatable read maintains data consistency by ensuring that once a transaction reads a row of data, it cannot see any changes made to that row by other transactions until it completes. This isolation prevents non-repeatable reads, where subsequent reads within the same transaction might return different values due to concurrent modifications. By locking the accessed rows, repeatable read allows users to rely on the stability of their data during the course of their transaction.
• Compare repeatable read with other isolation levels such as read committed and serializable.
  • Repeatable read offers a stronger guarantee than read committed by preventing non-repeatable reads, ensuring that any data retrieved remains consistent throughout the transaction. However, it is less strict than serializable, which eliminates all concurrency issues by ensuring that transactions are executed one after the other. This can lead to greater performance issues due to increased locking. Choosing the right isolation level depends on the specific needs for consistency and concurrency in different applications.
• Evaluate how repeatable read impacts performance in high-concurrency database environments.
  • In high-concurrency environments, repeatable read can significantly impact performance due to its use of locks on rows being accessed. While it ensures consistent data views for transactions, the locking mechanism can lead to contention and increased waiting times for transactions as they compete for access to the same data. As a result, while repeatable read provides strong consistency guarantees, it may require careful management of transactions to minimize bottlenecks and maintain system efficiency.

Related terms

ACID Properties: ACID stands for Atomicity, Consistency, Isolation, and Durability; these properties ensure reliable processing of database transactions.

Transaction Isolation Levels: These levels define how transactions interact with each other in terms of visibility and the degree to which they are isolated from one another.

Non-Repeatable Read: A phenomenon where a transaction reads the same row multiple times and gets different values due to changes made by other transactions.