How Can I Prevent Deadlocks in MySQL?

How to Avoid Deadlocks in MySQL

Deadlocks are a common issue in database systems, which can cause significant performance issues. In MySQL, deadlocks occur when two or more threads attempt to lock the same resources in opposite orders.

Understanding Deadlocks

Let's analyze a typical deadlock scenario:

• Connection 1: Locks key(1) and then tries to lock key(2).
• Connection 2: Locks key(2) and then tries to lock key(1).

If both connections execute simultaneously, deadlock occurs since both are waiting for each other to release the locked keys.

Avoiding Deadlocks

To prevent deadlocks, it's crucial to ensure that connections lock keys in the same order.

Reorder Queries

• Non-Deadlock-Prone Queries: For queries that do not lock multiple keys, the order is irrelevant.
• Deadlock-Prone Queries: For queries that lock multiple keys, such as your UPDATE and DELETE statements, reorder the WHERE clauses to use an ascending order for the keys. This ensures a consistent locking order across all connections.

Example

In your specific case, the DELETE statement should be modified as follows:

DELETE FROM onlineusers 
WHERE id IN (
    SELECT id FROM onlineusers
    WHERE datetime <= now() - INTERVAL 900 SECOND 
    ORDER BY id
) u;

By using this subquery approach, the DELETE operation will lock rows in ascending order of the id column, avoiding potential deadlocks.

Client-Side Retry Logic

If deadlocks still occur in your system, it's recommended to implement client-side retry logic. This involves having the client automatically retry the operation multiple times (e.g., three attempts) in case of a deadlock error.

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