Use scenarios and deadlock checks for row locks, table locks, and gap locks

The article mainly introduces the three lock mechanisms of row lock, table lock and gap lock in the database and their deadlock problems. 1. The row lock locks a specific data row, with high concurrency, and the InnoDB engine is used by default; 2. The table lock locks the entire table, with low concurrency, and is used for batch operations or database maintenance; 3. The gap lock locks the data row gap to prevent phantom reading. Deadlock occurs when transactions hold each other's resources required by each other. The troubleshooting method includes viewing the database log, analyzing the causes of deadlock (such as circular dependency, lock granularity), and solving it by optimizing code, reducing lock holding time, or adjusting lock sequence. The ultimate goal is to choose the right lock type and properly handle concurrency, avoid deadlocks, and improve database stability and efficiency.

Database lock: The grudges and deadlock secrets of row locks, table locks, and gap locks

Many developers will encounter lock problems in database concurrent control, especially row locks, table locks and gap locks. They are like martial arts masters, each with their own tricks. If used well, they can maintain the integrity of the database. If used poorly, they will easily generate deadlocks, causing your program to fall into trouble. In this article, let’s talk about the grievances of these three locks and how to solve the headache-inducing problem of deadlock.

First of all, it must be clear that these three types of locks are all designed to solve the problem of data inconsistency caused by concurrent access to the database. The difference is the granularity of the lock: the table lock is rough, one lock locks the entire table; the row lock is fine, and one lock only locks one row of data; the gap lock is relatively special, which locks the "slit" between data rows.

Xingsuo is like a martial arts master, focusing only on his own goals and attacking accurately. It only locks specific data rows and has the highest concurrency. MySQL's InnoDB engine uses row locks by default, which is very important in high concurrency scenarios. However, the implementation of line locks is also relatively complicated, and various situations need to be considered, such as next-key lock, which combines the functions of line locks and gap locks to prevent phantom reading.

 <code class="language-sql">-- 一个简单的行锁示例(假设主键是id)<br> SELECT * FROM users WHERE id = 1 FOR UPDATE; -- 加行锁<br>UPDATE users SET name = 'New Name' WHERE id = 1; -- 更新数据</code>

This code has a line lock, so other sessions cannot modify the data with id=1.

The watch lock is like a martial arts leader, who can lock the entire watch directly. It is simple and crude, and all operations have to be queued up, and the concurrency is very low. It is generally used in batch operations that require data consistency or database maintenance operations. For example, when executing the TRUNCATE TABLE command, the table lock will be automatically added.

 <code class="language-sql">-- 表锁示例<br>LOCK TABLES users WRITE; -- 加写锁<br>-- ... 进行一些操作...<br> UNLOCK TABLES; -- 释放锁</code>

The gap lock is more mysterious, it locks the "slit" between data lines. For example, suppose your table already has data (1, 2, 4), if you insert data in this interval (2, 4), the gap lock will prevent other transactions from inserting data in this interval, thus avoiding phantom reading. This is a lock mechanism that prevents data insertion. It is useful in some scenarios, but it is also difficult to understand.

So, how do these three locks produce deadlocks?

Imagine that two masters attack at the same time and stuck each other, which is a deadlock. For example, one transaction locks line A, another transaction locks line B, and the first transaction wants to lock line B, and the second transaction wants to lock line A, and the result is a stalemate.

How to troubleshoot deadlocks?

First of all, the database itself records deadlock information. You can view deadlock information by viewing the database log or using some database tools. Key information includes: transaction IDs involved in deadlocks, locked resources, etc.

You then need to analyze the cause of the deadlock. This usually requires analysis in combination with your business logic and code. Check if there are loop dependencies in your code, or if the granularity of the lock is too large, resulting in fierce competition for locks.

Finally, there are many ways to solve deadlocks, such as optimizing your code, reducing the lock holding time, adjusting the lock order, or using more fine-grained locks. Sometimes, you may need to refactor your database design to make it more suitable for concurrent access.

Remember, select the appropriate lock type and handle concurrency carefully to avoid deadlocks and make your database program run more stably and efficiently. This is like practicing martial arts. You need to constantly learn and practice to become a real database master.

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