Interview › Databases & Caching
What are the SQL isolation levels, and what anomalies does each prevent?
Databases & Caching · Intermediate level
Answer
Isolation levels define what concurrent transactions are allowed to see. The standard levels are Read Uncommitted, Read Committed, Repeatable Read, and Serializable; stronger isolation prevents more anomalies but can increase locking or retry needs.
Technical explanation
Stronger isolation can improve correctness but may increase blocking or retry requirements.
Deadlocks are handled by aborting a victim transaction; the application must retry the whole transaction safely.
Short transactions, consistent lock ordering, proper indexes, and idempotent retry logic are essential.
Hands-on example
Safe transfer example:
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SELECT id FROM accounts WHERE id IN (1,2) ORDER BY id FOR UPDATE;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;
On serialization failure or deadlock, retry the whole transaction, not just one statement.
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