Isolation in ACID and Transaction Isolation Levels
Fast recall
Quick revision
Core answer
Whether the four SQL isolation levels and their minimum guarantees are distinguished from product-specific snapshot and locking behavior
Not knowing the four standard isolation levels or confusing isolation with atomicity
The SQL levels define minimum behavior; PostgreSQL and InnoDB differ, and SERIALIZABLE may abort work that the whole application transaction must retry
Interview-ready guide
Interview answer
Isolation controls what concurrent transactions can observe and how their effects combine.
SQL names four levels. READ UNCOMMITTED permits dirty reads in the standard, although PostgreSQL maps it to READ COMMITTED. READ COMMITTED prevents dirty reads, but a later statement can see a newer committed value.
REPEATABLE READ prevents non-repeatable reads; phantom and snapshot behavior vary by database. PostgreSQL's implementation is stronger than the standard minimum, and InnoDB uses MVCC plus locking rules rather than simply locking every row read. SERIALIZABLE must produce a result equivalent to some serial order and may abort a transaction, so retry logic is part of using it.
Choose the level together with constraints, locks, version checks, or conditional updates that express the actual business invariant.
Isolation in ACID and Transaction Isolation Levels
Direct Answer
Isolation controls how concurrent transactions observe and affect one another. SQL names READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE, but databases implement them differently. SERIALIZABLE must produce an outcome equivalent to some serial order and may require retries; weaker levels allow product-specific anomalies in exchange for concurrency.
Code Example
-- PostgreSQL: choose the level as the transaction begins.
BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;
SELECT balance FROM accounts WHERE id = 1;
-- A later statement can see rows committed after this statement's snapshot.
UPDATE accounts
SET balance = balance - 50
WHERE id = 1 AND balance >= 50;
COMMIT;
Isolation level and the conditional update solve different parts of the concurrency rule.
Common Mistakes & Pitfalls
- Assuming one universal default: PostgreSQL defaults to READ COMMITTED, while MySQL InnoDB defaults to REPEATABLE READ; verify the selected database and connection settings.
- Confusing isolation with atomicity: Isolation concerns concurrent visibility and outcomes, while atomicity concerns whether one transaction partly commits.
- Treating SERIALIZABLE as a lock switch: Implementations may block, detect conflicts, or abort transactions. Choose from the invariant and measured workload, then retry the complete transaction when required.
Check your understanding
Answer these without looking back at the article.
- What is a dirty read in SQL?
- What is the default isolation level in MySQL InnoDB?
- How do you set isolation level in Spring @Transactional?
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