In MySQL, transaction isolation levels define how and when the changes made by one transaction become visible to other concurrent transactions. The four standard isolation levels are: Read Uncommitted: Transactions can see uncommitted changes made by others, leading to potential "dirty reads." Read Committed: A transaction only sees changes committed before it began, preventing dirty reads but allowing non-repeatable reads. Repeatable Read: Ensures that if a transaction reads a row, it will see the same data throughout its execution, preventing non-repeatable reads but not phantom reads. Serializable: The strictest level, transactions are executed in a way that ensures complete isolation, preventing dirty reads, non-repeatable reads, and phantom reads, but potentially impacting performance due to increased locking.

Transaction isolation is a critical concept in MySQL that affects how transactions interact with each other and when changes become visible. If you've ever encountered a scenario where you update a record but don't immediately see the changes in another transaction, the reason lies in MySQL's transaction isolation levels.

In this article, we'll explore the four transaction isolation levels, how they impact data visibility, and why MySQL's default setting can sometimes cause updates to appear "invisible" until a transaction commits.

1. What Are Transaction Isolation Levels?

Transaction isolation levels define how changes made in one transaction are isolated from others. The stricter the isolation level, the more protection it offers against concurrency issues like dirty reads, non-repeatable reads, and phantom reads, but it can also impact database performance.

MySQL supports four standard isolation levels:

(1) Read Uncommitted

• Transactions can read uncommitted changes made by other transactions.
• Can lead to dirty reads, where a transaction reads data that may be rolled back later.
• Offers the best performance but the lowest consistency.
• Example: A transaction updates a row but hasn’t committed yet. Another transaction reads this uncommitted data, which may be later rolled back, leading to inconsistent results.

(2) Read Committed

• A transaction only sees committed changes made by other transactions.
• Prevents dirty reads but allows non-repeatable reads (where the same query returns different results if executed twice).
• Used by default in PostgreSQL and Oracle, but not MySQL.
• Example: If you run the same SELECT query twice within a transaction, another transaction's commit could change the data between queries.

(3) Repeatable Read (MySQL Default)

• Ensures that repeated reads within a transaction return the same result.
• Prevents dirty reads and non-repeatable reads but allows phantom reads (where new rows inserted by other transactions appear in repeated queries).
• Default isolation level in MySQL InnoDB.
• Example: If a transaction reads a row and another transaction updates it, the first transaction will still see the old value until it commits or rolls back.

(4) Serializable (Strongest Isolation)

• Transactions are completely isolated from each other.
• Prevents dirty reads, non-repeatable reads, and phantom reads.
• Ensures complete consistency but requires locking and reduces concurrency.
• Example: All queries execute sequentially as if they were happening one at a time, significantly reducing performance in high-traffic environments.

2. Why You Can't See Changes Immediately in MySQL

If you've updated a record in one transaction but don’t see the change in another, it's likely due to MySQL’s default Repeatable Read isolation level.

• Within a transaction, MySQL provides a snapshot of data at the start of the transaction. Even if another transaction modifies and commits changes, your transaction won’t see them until it commits.
• This behavior prevents non-repeatable reads, ensuring that repeated queries within the same transaction return the same result.

Example Scenario

1. Transaction A starts and reads a record: 

2. START TRANSACTION;
   SELECT balance FROM accounts WHERE id = 1;
   
   

3. Transaction B updates and commits: 

4. UPDATE accounts SET balance = balance + 100 WHERE id = 1;
   COMMIT;
   
   

5. Transaction A runs the same SELECT again and still sees the old balance! The change from Transaction B is not visible until Transaction A commits.

3. How MySQL Manages Transaction Isolation

MySQL's InnoDB storage engine uses MVCC (Multi-Version Concurrency Control) to handle transaction isolation efficiently.

• Instead of locking rows aggressively, InnoDB creates consistent snapshots of data for each transaction.
• This is why transactions only see data that was committed before they started, unless the isolation level is explicitly changed.

Using SELECT ... FOR UPDATE

If you need to see the latest data and prevent other transactions from modifying it, use:

SELECT balance FROM accounts WHERE id = 1 FOR UPDATE;

This ensures that other transactions must wait until yours commits before they can update the same row.

4. How to Change the Transaction Isolation Level

You can modify MySQL's isolation level using:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
START TRANSACTION;

To change the default for all sessions:

SET GLOBAL TRANSACTION ISOLATION LEVEL READ COMMITTED;

💡 Note: Changing isolation levels may affect performance and consistency, so choose wisely based on your use case.

5. Choosing the Right Isolation Level for Performance & Consistency

Best Practices:

• Use Read Committed for real-time visibility in applications like banking & finance.
• Stick with Repeatable Read (default) for most general use cases.
• Avoid Serializable unless absolute consistency is required.
• Monitor performance when using stricter isolation levels.

Conclusion

MySQL's transaction isolation levels define how data changes are handled in concurrent transactions. If you don’t see updates immediately, it’s due to MySQL’s Repeatable Read isolation, which ensures consistency but can sometimes be confusing for developers.

Understanding these isolation levels helps in designing scalable, high-performance database systems that maintain both data integrity and efficiency.

🔹 Key Takeaway: If you need real-time visibility into changes, consider switching to Read Committed, but be aware of potential trade-offs in consistency.