What Is Read Committed?

Definition: Read Committed

Read Committed is an isolation level in database management systems that ensures any data read during a transaction is committed at the moment it is read. It prevents the phenomenon known as dirty reads, where a transaction could read uncommitted data from another transaction, potentially leading to inconsistencies.

Understanding Read Committed

Read Committed is one of the most commonly used isolation levels in database management systems like Oracle, SQL Server, and PostgreSQL. It strikes a balance between performance and consistency by allowing read operations to only see committed data, thus avoiding dirty reads, but it may still experience non-repeatable reads and phantom reads.

How Read Committed Works

In a Read Committed isolation level, every read operation within a transaction sees only the data that has been committed by other transactions at the moment the read operation is executed. Here’s a step-by-step breakdown of how it works:

1. Transaction Start: When a transaction begins, it doesn’t set a specific point in time for the reads.
2. Data Reading: Each read operation fetches the latest committed data from the database.
3. Commit/Abort: The transaction continues to read and modify data until it is committed or aborted.

This ensures that the transaction will not read any intermediate, uncommitted changes made by other transactions.

Key Characteristics

• Prevents Dirty Reads: Ensures that a transaction reads only committed data from other transactions.
• Allows Non-Repeatable Reads: Since each read operation might fetch a different version of the data if other transactions commit changes between reads.
• Allows Phantom Reads: New rows that match the search condition can appear in subsequent reads within the same transaction if other transactions commit inserts.

Benefits of Read Committed

1. Data Consistency: Prevents reading uncommitted data, which reduces the chances of encountering inconsistencies.
2. Performance: Offers better performance compared to higher isolation levels like Serializable, as it doesn’t lock data excessively.
3. Simplicity: Easier to implement and manage due to its straightforward mechanism of reading only committed data.

Use Cases

• Web Applications: Where high performance is crucial, and some level of inconsistency can be tolerated.
• Reporting Systems: Where it’s essential to avoid dirty reads but acceptable to have non-repeatable or phantom reads.
• General Purpose Databases: Commonly used in OLTP (Online Transaction Processing) systems where quick access to data is needed.

Drawbacks

While Read Committed provides a good balance between consistency and performance, it has its limitations:

1. Non-Repeatable Reads: The same query can return different results if other transactions commit changes between the reads.
2. Phantom Reads: New records can appear in a query result set if other transactions insert data between reads.

How to Implement Read Committed

Different database systems implement Read Committed in various ways, but the general approach involves setting the transaction isolation level to Read Committed. Here’s how you can do it in some popular DBMSs:

• Oracle: SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
• SQL Server: SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
• PostgreSQL: SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

Example Scenario

Consider an e-commerce application where multiple users can place orders simultaneously. Here’s how Read Committed helps:

1. User A starts a transaction to read product inventory.
2. User B concurrently starts a transaction and updates the inventory by placing an order.
3. User B commits the transaction.
4. User A reads the inventory again and sees the updated inventory, reflecting the committed changes made by User B.

In this scenario, User A never sees the intermediate, uncommitted inventory state during User B’s transaction, ensuring that User A’s transaction operates on consistent data.

Best Practices

• Monitor Performance: Continuously monitor the database performance to ensure that the Read Committed isolation level meets the application’s needs.
• Transaction Management: Keep transactions short and optimize query performance to minimize the time data remains locked.
• Evaluate Consistency Requirements: Understand the consistency requirements of your application to decide if Read Committed is the appropriate isolation level.

Frequently Asked Questions Related to Read Committed