Understanding Transaction Isolation and Concurrency Issues in MySQL

In a multi‑client environment, concurrent requests to a database can cause data safety problems, including lost updates, dirty reads, non‑repeatable reads, and phantom reads. These issues arise from multiple transactions operating on the same data without proper isolation.

Lost update (or dirty write) occurs when two transactions modify the same row based on the original value, causing one update to overwrite the other. Dirty reads happen when a transaction reads uncommitted changes made by another transaction, leading to decisions based on data that may later be rolled back. Non‑repeatable reads occur when a transaction reads the same row twice and sees different values because another transaction has modified it in between. Phantom reads happen when a transaction re‑executes a query and discovers new rows inserted by another transaction that satisfy the query condition.

A transaction is a logical unit of work consisting of one or more SQL statements and is characterized by the ACID properties: Atomicity, Consistency, Isolation, and Durability.

MySQL provides four isolation levels:

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

2. Read Committed : Prevents dirty reads by only exposing committed data, but non‑repeatable reads and phantom reads can still occur.

3. Repeatable Read : Eliminates dirty reads and non‑repeatable reads; phantom reads may still happen.

4. Serializable : Provides the highest isolation by serializing transactions, preventing dirty reads, non‑repeatable reads, and phantom reads at the cost of performance.

MySQL’s default isolation level is Repeatable Read , which can be checked with SHOW VARIABLES LIKE 'tx_isolation'; and changed with SET tx_isolation='REPEATABLE-READ'; .

The article concludes by encouraging discussion and offering additional resources such as interview question collections and open‑source project links.