Understanding MySQL Transaction Isolation Levels and Best Practices

Why Transactions Matter

Transferring money is a classic example of a transaction: the operations of checking balance, deducting, and updating must be atomic, otherwise concurrent reads could cause double‑spending.

Isolation and Isolation Levels

Isolation (the "I" in ACID) prevents anomalies when multiple transactions run concurrently. The main problems are:

Dirty read

Non‑repeatable read

Phantom read

SQL defines four isolation levels, each balancing consistency and performance:

Read Uncommitted : Uncommitted changes are visible to other transactions.

Read Committed : Changes become visible only after the transaction commits.

Repeatable Read : All reads within a transaction see a snapshot taken at the start; uncommitted changes remain invisible.

Serializable : Full locking on rows; later transactions wait for earlier ones to finish.

Illustrative Example

Consider a table T(c int) ENGINE=InnoDB with a single row value 1. Two transactions run sequentially, producing different results under each isolation level:

create table T(c int) engine=InnoDB;
insert into T(c) values(1);

Read Uncommitted: V1, V2, V3 = 2

Read Committed: V1 = 1, V2 & V3 = 2

Repeatable Read: V1 = V2 = 1, V3 = 2

Serializable: Transaction B is locked until A commits; from A’s view V1 = V2 = 1, V3 = 2

Implementation Details

MySQL creates a view (read‑view) for each isolation level:

Repeatable Read: view is created at transaction start and reused.

Read Committed: view is recreated for each statement.

Read Uncommitted: reads the latest row value directly, no view.

Serializable: uses explicit row locks.

Oracle defaults to Read Committed, so when migrating to MySQL you may need to set transaction_isolation='READ-COMMITTED' to keep behavior consistent.

Configuring Isolation

Set the global or session variable transaction_isolation to the desired level and verify with:

show variables like 'transaction_isolation';