How to Prevent Deadlock in DBMS: Causes and Solutions

Deadlocks in a DBMS happen when two or more transactions wait forever for each other’s locks. To prevent deadlocks, ensure transactions acquire locks in a consistent order and keep transactions short and simple to reduce waiting time.

Why This Happens

A deadlock occurs when two or more transactions hold locks on resources the others need, creating a cycle of waiting that never ends. For example, Transaction A locks Resource 1 and waits for Resource 2, while Transaction B locks Resource 2 and waits for Resource 1. Neither can proceed, causing a deadlock.

sql

BEGIN TRANSACTION;
-- Transaction A locks Resource 1
SELECT * FROM accounts WHERE id = 1 FOR UPDATE;
-- Transaction A waits for Resource 2
-- (but Resource 2 is locked by Transaction B)

BEGIN TRANSACTION;
-- Transaction B locks Resource 2
SELECT * FROM accounts WHERE id = 2 FOR UPDATE;
-- Transaction B waits for Resource 1
-- (but Resource 1 is locked by Transaction A);

Output

ERROR: deadlock detected
DETAIL: Process 1 waits for ShareLock on transaction 2; blocked by process 2.
Process 2 waits for ShareLock on transaction 1; blocked by process 1.

The Fix

To fix deadlocks, make sure all transactions lock resources in the same order. This breaks the circular wait condition. Also, keep transactions short to reduce lock time and avoid user interaction during transactions.

sql

BEGIN TRANSACTION;
-- Both transactions lock Resource 1 first
SELECT * FROM accounts WHERE id = 1 FOR UPDATE;
-- Then lock Resource 2
SELECT * FROM accounts WHERE id = 2 FOR UPDATE;
-- Perform updates
COMMIT;

Output

COMMIT
-- Transactions complete without deadlock

Prevention

To avoid deadlocks in the future, follow these best practices:

• Consistent Lock Ordering: Always acquire locks in the same sequence across all transactions.
• Keep Transactions Short: Commit or rollback quickly to free locks sooner.
• Use Lower Isolation Levels: When possible, use isolation levels like Read Committed to reduce locking.
• Detect and Retry: Implement deadlock detection in your application to retry transactions if a deadlock occurs.
• Avoid User Interaction: Don’t wait for user input inside transactions.

Related Errors

Other concurrency issues similar to deadlocks include:

• Lock Timeout: Transactions wait too long for a lock and then fail.
• Lost Updates: Two transactions overwrite each other's changes without proper locking.
• Phantom Reads: Reading new rows inserted by other transactions during a transaction.

These can be mitigated by proper transaction management and isolation levels.

Key Takeaways

Always acquire locks in a consistent order to prevent circular waiting.

Keep transactions short to minimize lock holding time.

Use appropriate isolation levels to balance concurrency and consistency.

Implement deadlock detection and retry logic in your application.

Avoid user input or long processing inside transactions.