How To Avoid DB Deadlock

To avoid database (DB) deadlocks, you need to understand how they occur: a deadlock happens when two or more transactions hold locks and wait indefinitely for each other’s resources. Proper transaction design, locking strategy, and indexing can significantly reduce the risk.

🧠 1. Access Resources in a Consistent Order

Always acquire locks in the same sequence across all transactions.
Inconsistent ordering is a top cause of deadlocks.

🔄 2. Keep Transactions Short and Simple

Minimize the time locks are held by:
- Avoiding long-running queries in a transaction
- Deferring non-critical work until after commit

🚫 3. Avoid User Input During Transactions

Don’t pause to ask for user input after a transaction starts.
It delays the release of locks and increases the chance of deadlocks.

🔒 4. Use Lower Isolation Levels (If Safe)

Consider READ COMMITTED or READ UNCOMMITTED if your data access patterns allow it.
Higher isolation levels (like SERIALIZABLE) increase locking and deadlock risk.

📊 5. Properly Index Your Tables

Well-indexed queries reduce full table scans, lowering the number of locks needed.
This improves performance and reduces conflict chances.

💻 6. Use Retry Logic in Application Code

Deadlocks can’t always be avoided. Implement logic to:
- Catch deadlock exceptions (e.g., SQL error 1205 in SQL Server)
- Retry the transaction after a short delay

🧼 7. Avoid Unnecessary Locks

Don’t lock more rows than needed (e.g., use SELECT ... FOR UPDATE only when necessary).
Avoid escalating locks from row to table level by controlling lock behavior if your DBMS allows.

🧮 8. Batch Updates Carefully

Update in small batches, especially when modifying large datasets.
This reduces the scope and duration of locks.

🕵️ 9. Monitor and Analyze Deadlocks

Use your DBMS’s tools (e.g., SQL Server Profiler, MySQL’s SHOW ENGINE INNODB STATUS) to:
- Log and analyze deadlock patterns
- Find root causes and tune queries accordingly

Example:

sqlCopyEdit-- Bad (different lock order)
Transaction A: SELECT ... FROM orders THEN FROM customers;
Transaction B: SELECT ... FROM customers THEN FROM orders;

-- Better (same lock order)
Both: SELECT ... FROM customers THEN FROM orders;