Why Transaction Isolation Is Needed and an Overview of MySQL Isolation Levels

Why Transaction Isolation Is Needed

Databases serve multiple clients concurrently, leading to situations where several transactions operate on the same data at the same time. This concurrency can cause data safety issues such as lost updates (dirty writes), dirty reads, non‑repeatable reads, and phantom reads. Transaction isolation is designed to address these problems.

Understanding Multi‑Transaction Concurrency Issues

Lost Update (Dirty Write) : When two transactions modify the same row based on the original value without awareness of each other, the later update overwrites the earlier one.

Both transactions update id=1, but only one value persists, causing confusion for the transaction that committed earlier.

Dirty Reads : A transaction reads data modified by another transaction that has not yet committed. If the uncommitted transaction rolls back, the reading transaction has acted on invalid data.

Transaction A updates a=50, Transaction B reads a=50 and proceeds, but later A rolls back, leaving B with dirty data.

Non‑Repeatable Reads : A transaction reads the same row twice and gets different results because another transaction modified or deleted the row in between.

During transaction A, each query for id=1 returns a different value for column a, making consistent processing impossible.

Phantom Reads : A transaction re‑executes a query and finds new rows inserted by another transaction that satisfy the query condition.

Transaction A initially sees two rows with id<10; after transaction B inserts a new row, A sees three rows, illustrating a phantom read.

Understanding Transactions

A transaction is a logical unit of work consisting of a group of SQL statements and follows the ACID properties:

Atomicity : All operations succeed or none do.

Consistency : Data remains in a valid state before and after the transaction.

Isolation : Transactions operate independently without affecting each other.

Durability : Once committed, changes survive system failures.

MySQL Transaction Isolation Levels

MySQL defines four isolation levels:

Read Uncommitted : Allows reading uncommitted changes, leading to dirty reads, non‑repeatable reads, and phantom reads.

Read Committed : Prevents dirty reads but still permits non‑repeatable reads and phantom reads.

Repeatable Read : Prevents dirty reads and non‑repeatable reads; phantom reads can still occur.

Serializable : Eliminates dirty reads, non‑repeatable reads, and phantom reads by using strict locking, at the cost of performance.

MySQL's default isolation level is Repeatable Read . You can view the current level with SHOW VARIABLES LIKE 'tx_isolation'; and set it with SET tx_isolation='REPEATABLE-READ'; .