MySQL transactions and lock mechanisms are key to concurrent control and performance tuning. 1. When using transactions, be sure to explicitly turn on and keep the transactions short to avoid resource occupation and undo log bloating caused by long transactions; 2. Locking operations include shared locks and exclusive locks, SELECT ... FOR UPDATE plus X locks, SELECT ... LOCK IN SHARE MODE plus S locks, write operations automatically locks, and indexes should be used to reduce the lock granularity; 3. The isolation level is repetitively read by default, suitable for most scenarios, and modifications should be cautious; 4. Deadlock inspection can analyze the details of the latest deadlock through the SHOW ENGINE INNODB STATUS command, and the optimization methods include unified execution order, increase indexes, and introduce queue systems. Mastering these core points can help solve concurrency problems.

MySQL's transaction and lock mechanism is the core part of database performance tuning and concurrent control. Many people will encounter deadlocks, blocking, data inconsistency during use. In fact, in most cases, it is because they do not have a deep understanding of transactions and lock behaviors. Let’s start from several practical scenarios and talk about how to manage MySQL transactions and locks well.

Basic behavior of transactions

MySQL's InnoDB engine supports transaction processing, and each transaction follows ACID principles (atomicity, consistency, isolation, and persistence). By default, MySQL is in autocommit mode (autocommit=1), which means that each statement is executed as an independent transaction.

If you want multiple operations as a whole, you need to manually start the transaction:

 START TRANSACTION;
-- Execute multiple SQL statements COMMIT;

Or roll back when an error occurs:

 ROLLBACK;

suggestion :

• For multi-step operations that require consistency, be sure to use transactions explicitly.
• In applications with frequent writing, closing autocommit appropriately can improve performance, but you should also be careful to avoid long transactions occupying resources.
• Transactions that are not committed for a long time may cause undo log to bloat, affect performance and even cause space problems.

Lock type and lock timing

MySQL locks are divided into shared locks (S locks) and exclusive locks (X locks), also called read locks and write locks. In addition, there are gap locks, next-Key locks, etc. to prevent phantom reading.

Common locking statements include:

• SELECT ... FOR UPDATE : Add exclusive lock
• SELECT ... LOCK IN SHARE MODE : Add a shared lock
• Insert, update, and delete operations will also be automatically locked

InnoDB usually adds row-level locks when the query condition hits the index, otherwise it may be upgraded to a table lock.

Common phenomena :

• When multiple transactions modify the same row of data at the same time, the subsequent transaction will be blocked until the previous transaction is committed or rolled back.
• If two transactions are waiting for each other's locks held by the other party, a deadlock will occur. After MySQL detects it, it will automatically roll back one of the transactions.

suggestion :

• Try to make the business short and concise, and reduce the time to hold the lock.
• Unified access order, such as always operating the user table first and then the order table, can effectively reduce the probability of deadlock.
• Use appropriate indexes to avoid unnecessary lock range expansion in full table scanning.

Impact of isolation level

MySQL supports four transaction isolation levels:

• Read not submitted (READ UNCOMMITTED)
• Read ComMITTED
• Repeatable read (REPEATABLE READ) - InnoDB default
• Serialization (SERIALIZABLE)

Different isolation levels determine the visibility between transactions and the behavior of locks. For example:

• Under "Repeatable", InnoDB prevents phantom reading through Next-Key lock;
• Under "Reads submitted", a new snapshot will be generated for each read;
• "Serialization" will turn all normal SELECTs into SELECT ... FOR SHARE , forcing serial execution.

suggestion :

• The default "repeatable" is suitable for most business scenarios, and it is not recommended to change it at will unless you have special needs.
• Modifying the isolation level will affect concurrency behavior and locking strategies, and you need to test it carefully before going online.

Deadlock checking and optimization techniques

When MySQL detects a deadlock, an error log is output and one of the transactions is rolled back. You can view the latest deadlock details through the following command:

 SHOW ENGINE INNODB STATUS\G

The detailed transaction waiting diagram can be seen in the "LATEST DETECTED DEADLOCK" section in the output.

Stroke thoughts :

• See which transactions are involved in the deadlock
• Analyze which locks they hold and which locks are requesting
• Check the SQL statements involved to confirm whether they can be optimized by adjusting the order, narrowing the transaction scope, increasing indexes, etc.

Optimization tips :

• Encapsulate similar operations into a unified service interface to ensure consistent execution order
• Establish appropriate indexes for WHERE conditions to reduce the granularity of locks
• In high concurrent write scenarios, consider introducing a queue system for asynchronous processing

Basically that's it. Transactions and locks seem complex, but in fact, as long as you master a few key points, many problems can be solved easily.

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