Understanding Transaction Isolation and Concurrency Issues in Databases

Why Transaction Isolation Is Needed

Databases serve multiple clients simultaneously, so concurrent requests can lead to data safety issues such as lost updates, dirty reads, non‑repeatable reads, and phantom reads; transaction isolation is designed to solve these problems.

Understanding Multi‑Transaction Concurrency Problems

Lost Update (Dirty Write) When multiple transactions modify the same row based on the original value, updates can be overwritten because each transaction is unaware of the others.

In the illustration, two transactions update id=1; the later commit overwrites the earlier one, causing the first transaction’s change to disappear.

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

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

Non‑Repeatable Reads A transaction reads the same row at different times and gets different values because another transaction has modified or deleted the row in between.

The example shows transaction A reading a changing value for id=1, making consistent processing impossible.

Phantom Reads A transaction re‑executes a query and sees additional rows inserted by another transaction, even though the query criteria are unchanged.

Transaction A initially finds 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.

Understanding Transaction Isolation Levels

MySQL provides four isolation levels:

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

Read Committed : Only committed data is visible, preventing dirty reads but still permitting non‑repeatable and phantom reads.

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

Serializable : Eliminates dirty, non‑repeatable, 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 using SET tx_isolation='REPEATABLE-READ'; .