Interview › Databases & Caching
What is a deadlock, and how do databases detect and resolve them?
Databases & Caching · Intermediate level
Answer
A deadlock occurs when transactions wait on each other in a cycle. Databases detect the cycle and abort one transaction, so applications must retry the full transaction and engineers should fix lock ordering, transaction length, and indexes.
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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