Interview › Databases & Caching
What is a transaction, and what do the ACID properties mean?
Databases & Caching · Intermediate level
Answer
A transaction is an all-or-nothing unit of work. ACID means Atomicity, Consistency, Isolation, and Durability: the database commits complete valid changes, controls concurrency, and preserves committed data across crashes.
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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