Database Deadlock in Concurrent Updates
Application encounters deadlock errors intermittently when multiple users perform operations simultaneously. Specific sequences of operations cause deadlock: Thread A waits for lock held by Thread B, which waits for lock held by Thread A. Application must retry deadlocked operations.
Deadlocks are hard to reproduce but fail under load testing or in production.
Error Messages You Might See
Common Causes
- Transactions acquire locks in different order (A locks table1 then table2, B locks table2 then table1)
- Long-running transactions holding locks too long
- Missing indexes causing full table scans and excessive row locks
- Row-level locks held while waiting for other operations
- No deadlock retry logic in application
How to Fix It
Always acquire locks in same order across application. Use consistent transaction isolation level. Keep transactions short. Add indexes to reduce lock contention. Implement deadlock retry: catch deadlock exception, wait briefly, retry. Check database: SHOW PROCESSLIST shows locks held. Configure deadlock victim: database kills one transaction to resolve.
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Get HelpFrequently Asked Questions
How to prevent deadlocks?
Always acquire locks in same order. If A acquires table1 then table2, B must too (never table2 then table1).
How to detect deadlock cause?
Enable deadlock logging: innodb_print_all_deadlocks = ON (MySQL). Logs show which queries caused it and lock order.
Should deadlocks be retried?
Yes. Deadlocks are normal under load. Catch DeadlockException, wait briefly (exponential backoff), retry. Usually succeeds on second try.