Understanding Database Isolation Levels and ACID Properties

Database Isolation Level Classification

1. Read Uncommitted : Data can be read before a transaction commits.

2. Read Committed : Data is visible only after the transaction commits.

3. Repeatable Read : MySQL’s default level; data read is locked until commit.

4. Serializable : Highest isolation, transactions execute one after another, reducing concurrency.

Dirty Read, Non‑Repeatable Read, Phantom Read

Dirty read : Reading uncommitted data.

Non‑repeatable read : Re‑reading yields different data because another transaction modified it.

Phantom read : New rows appear or disappear in a subsequent query due to other transactions.

MySQL Commands for Isolation Level

View current isolation level: select @@tx_isolation; Set isolation level to read uncommitted:

set transaction isolation level read uncommitted;

ACID Properties of Transactions

Atomicity

A transaction’s operations are all‑or‑nothing; either every change is applied or none are.

Consistency

Before and after a transaction, the database must satisfy all integrity constraints.

Isolation

Concurrent transactions are isolated from each other; DBMS uses locking to provide different isolation levels.

Durability

Once a transaction commits, its changes survive system failures, ensured by backup and recovery mechanisms.