Isolation Levels control how transactions interact with each other in a database. They define how visible data changes are between concurrent transactions and help balance data consistency with performance.
? Key Concepts
Transaction: A sequence of SQL operations executed as a single unit
Dirty Read: Reading uncommitted data from another transaction
Non-Repeatable Read: Same row gives different values within a transaction
Phantom Read: New rows appear when re-running a query
? Isolation Levels in MySQL
READ UNCOMMITTED – Lowest isolation
READ COMMITTED
REPEATABLE READ – Default in MySQL
SERIALIZABLE – Highest isolation
? Syntax / Theory
Isolation levels can be set globally, per session, or per transaction.
? View Code Example
-- Set isolation level for current session
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
? Code Examples
? View Code Example
-- Start a new transaction
START TRANSACTION;
-- Read data inside the transaction
SELECT * FROM orders WHERE order_id = 101;
-- Commit the transaction
COMMIT;
? Live Output / Explanation
What Happens?
Under REPEATABLE READ, the SELECT query will always return the same result within the transaction, even if another transaction modifies the data and commits it.
? Interactive Simulator
Simulate two concurrent sessions (A and B) accessing the same Bank Account.
Initial Balance: $1000
Session A You
Waiting to start...
Session B Background
Waiting to start...
? Tips & Best Practices
Use REPEATABLE READ for most applications (default)
Use SERIALIZABLE only when strict consistency is required
Lower isolation improves performance but increases anomalies
Always understand concurrency needs before changing isolation levels
? Try It Yourself
Open two database sessions and run transactions in parallel
Test how data visibility changes with each isolation level
Compare READ COMMITTED vs REPEATABLE READ behavior