Deadlock is a circular dependency state caused by multiple transactions waiting for each other to release lock resources in MySQL concurrent operations. The reasons for this include cross-updating different records in transactions, and inversely requesting the lock held by the other party after holding the lock. To view the deadlock log, you can use the SHOW ENGINE INNODB STATUS\G command to analyze the SQL statements, tables, lock types and request lock situations in the "LATEST DETECTED DEADLOCK" section. Common avoidance strategies include: 1. Unified access order; 2. Shorten transaction time; 3. Use appropriate indexes; 4. Sort by primary key in batches; 5. Set a retry mechanism. In addition, range locks and different locking paths may also cause deadlocks, and query conditions and index design need to be comprehensively considered. The deadlock problem can be effectively reduced through rational design and log analysis.

MySQL's transaction deadlock is a common problem in database concurrent operations. Especially in high concurrency environments, multiple transactions may fall into a deadlock state when they wait for each other to release resources. If this type of problem is not handled in time, it will affect system performance and even cause the service to be unavailable. The key to the solution is to identify the root cause of deadlock and reasonably adjust the transaction logic or index design .

How does deadlock happen?

Deadlocks in MySQL usually occur when two or more transactions hold locks on each other and try to acquire lock resources that the other party already holds. for example:

• Transaction A updates a record in the table orders ;
• Transaction B also updated another record in orders ;
• Then transaction A tries to update the record that transaction B has modified, and transaction B wants to update the record that transaction A is locking;
• At this time, MySQL detects a circular dependency and will throw a deadlock error (usually Error 1213).

This phenomenon does not necessarily mean that the code is incorrect, but a competition situation under concurrent access.

How to view and analyze deadlock logs?

MySQL provides a command to view information about the last deadlock:

 SHOW ENGINE INNODB STATUS\G

In the "LATEST DETECTED DEADLOCK" section of the output result, you will see detailed transaction waiting diagrams, SQL statements, held locks and requested locks.

Key points include:

• SQL statements executed by each transaction;
• The tables, rows and indexes used;
• The lock types (such as record lock, gap lock) they hold;
• Why is the requested lock not available?

This information can be used to locate which order of operations caused the deadlock.

Common deadlock avoidance strategies

To reduce the occurrence of deadlocks, you can start from the following aspects:

• Unified access order : Ensure that all transactions access the involved data tables and records in the same order. For example, first operate orders and then customers , and do not have transactions that are not allowed to happen.
• Try to shorten the transaction time : the longer the transaction, the longer the lock will be held, and the higher the probability of conflict. Transactions can be submitted as early as possible if the business allows.
• Use the right index : No suitable index can lead to full table scans, thereby increasing the granularity of the lock and the possibility of conflict. Confirm that your query has gone through the correct index.
• Sort by primary key during batch update : If multiple records need to be updated in the transaction, it is recommended to sort them by primary or unique key before executing them, which can reduce the chance of cross-locking of different transactions.
• Appropriate retry mechanism : The application level can catch deadlock exceptions (such as 1213) and automatically retry the transaction within a certain number of times.

Small details in actual cases are easy to ignore

Sometimes deadlocks occur even if it seems that the transaction order is consistent. A common reason is the impact of range locks . For example, using SELECT ... FOR UPDATE with a range lock, and another transaction just inserted or updated within this range, may also cause a conflict.

Another situation is that different query conditions lead to different locking paths . For example, if one transaction is updated with the primary key, and the other transaction is updated with the secondary index, the order of their application for the lock may also cause deadlocks.

So we should not only focus on SQL itself, but also on the locking behavior behind it.

Basically that's it. Although MySQL deadlock cannot be completely avoided, most problems can be discovered and fixed through good design and log analysis.

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