Preface
MySQL deadlockIt refers to the phenomenon that two or more transactions are waiting for each other's locks held by the other party, which makes all transactions unable to continue execution. In the InnoDB storage engine, deadlocks are generated through lock mechanisms, especially when concurrency is high and business logic is complex, deadlocks are more likely to occur.
1. Causes of MySQL deadlock
MySQL deadlock usually occurs inRow-level locksuperior. Common causes of deadlock include:
- Transaction A and transaction B hold locks that each other needs: Transaction A locks record 1, Transaction B locks record 2, Transaction A tries to acquire the lock of record 2, and Transaction B tries to acquire the lock of record 1, causing a deadlock.
- Locking in different orders: Two transactions request locking on the same set of resources, but the locking order is different, resulting in waiting for each other. For example, transaction A locks record 1 and record 2 in order, while transaction B locks record 2 and record 1 in reverse order.
- Gap lock is used: Under InnoDB's Next-Key Locking mechanism, gap locking can also cause deadlocks, especially when multiple transactions attempt to lock the same gap during range query.
- Long transactions and locks waiting time too long: The transaction execution time is long, and the lock is not released in time, causing other transactions to wait for the lock to time out or deadlock.
2. Deadlock detection and processing
MySQL usageDeadlock detectionTo deal with deadlock problem. MySQL automatically detects whether the transaction is deadlocked and aborts one of the transactions, releasing the lock to allow the other transaction to continue execution. The InnoDB storage engine solves this problem by introducing a deadlock detection mechanism. When a deadlock is detected, a transaction is selected to roll back to break the deadlock. The rolled-back transaction will be thrownDeadlock found when trying to get lockmistake.
3. How to avoid and deal with MySQL deadlocks?
1. Reasonable design of index
Using appropriate indexes can reduce the range of locks and reduce the probability of deadlocks. When there is no index, MySQL locks all records in the table, increasing the chance of lock conflicts. Therefore, rationally designing and using indexes ensures that queries can quickly find data, avoid unnecessary lock contention, and significantly reduce the risk of deadlocks.
2. Keep the locking order consistent
When multiple records in the transaction operation table, maintaining a consistent locking order can effectively reduce deadlock problems. For example, if both transactions need to lock the same resource, make sure they request the lock in the same order to avoid deadlocks.
3. Reduce transaction lock time
Try to shorten the execution time of transactions and reduce the holding time of locks. Divide transactions into smaller logical units to avoid long-term resource occupancy. At the same time, move non-essential complex operations to be performed outside the transaction as much as possible.
4. Reduce concurrency
In the case of high concurrency, the chance of increasing lock collision and deadlock is higher. Lock contention can be reduced by controlling the concurrency, such as using an optimistic lock mechanism to avoid frequent locking.
5. Use table locks instead of row locks
For some scenarios in write operations, consider using table locks instead of row locks to avoid deadlocks caused by row-level locks. However, table locks will cause concurrency performance to decline, so you need to choose the appropriate lock according to the business scenario.
6. Locking a smaller range
Try to reduce the range of transaction locked rows by using primary key indexes and appropriate conditions. Especially inUPDATEorDELETEDuring operation, accurate query conditions are used to limit the scope of the lock.
7. Submit transactions in batches
For batch operations, consider disassembling large transactions into multiple small transactions, reducing the number of rows and operating ranges of locks at one time, and reducing the lock holding time.
8. Select the appropriate transaction isolation level
Appropriately reducing the transaction isolation level can reduce the chance of lock conflict. For example, the transaction isolation level can be taken fromSerializableAdjusted toRead CommittedorRepeatable Readto reduce row locking.
9. Use SELECT for locking operation... FOR UPDATE
When you need to update immediately after querying data, you can useSELECT ... FOR UPDATETo explicitly lock the rows to avoid deadlocks caused by locking them during updates.
4. Common deadlock examples
Here is a common deadlock example where two transactions try to lock the same record in different order:
-- Transactions A START TRANSACTION; UPDATE orders SET status = 'shipped' WHERE id = 1; -- Lock the record 1 -- at this time，Transactions B Waiting for locking the record 1 -- Transactions B START TRANSACTION; UPDATE orders SET status = 'shipped' WHERE id = 2; -- Lock the record 2 -- at this time，Transactions A Waiting for locking the record 2 -- Transactions A Try updating records 2，但Transactions B Hold the lock，Transactions A wait UPDATE orders SET status = 'shipped' WHERE id = 2; -- Transactions B Try updating records 1，但Transactions A Hold the lock，Transactions B wait -- Deadlock occurs，MySQL 自动检测并回滚其中一个Transactions
5. How to detect and analyze deadlocks?
Deadlocks in MySQL can be detected and analyzed by:
1. Enable the innodb_print_all_deadlocks parameter
By settinginnodb_print_all_deadlocks=ON, can output all deadlock information in MySQL logs, for easy analysis and debugging.
2. Use the SHOW ENGINE INNODB STATUS command
After MySQL deadlock occurs, you can useSHOW ENGINE INNODB STATUSCommand to view deadlock information. This command will output the recent deadlock situation and help the developer find the source of the deadlock.
SHOW ENGINE INNODB STATUS\G
The information contained in the output includes:
- Which transaction is rolled back
- When a deadlock occurs, what locks are held by the transaction and which locks are waiting for?
- SQL statements for transaction operations
3. MySQL slow query log
Turning on MySQL slow query logs can also indirectly help discover performance problems caused by lock waiting. Although deadlocks cannot be displayed directly, they can be used as an auxiliary tool for troubleshooting lock conflict problems.
6. Coping strategies after deadlock
When a deadlock occurs, MySQL automatically rolls back one of the transactions, and the developer needs to catch and handle this exception.
In the code, you can handle deadlocks using the following method:
try {
// Execute transactions ...
} catch (SQLException e) {
if (() == 1213) { // 1213 represents deadlock error code // Deadlock detection, try again retryTransaction();
} else {
// Other exception handling throw e;
}
}
By catching deadlock exceptions and performing appropriate retry, the system can continue to execute after a deadlock occurs, thereby improving the robustness of the system.
7. Summary
MySQL deadlock is a common problem for databases in concurrency scenarios, especially for large-scale and complex business systems, deadlock problems are more frequent. Through reasonable index design, keeping the locking order consistent, shortening transaction time, and optimizing locking strategies, the occurrence of deadlocks can be effectively reduced. At the same time, when a deadlock occurs, MySQL has a deadlock detection and automatic rollback mechanism, and developers can improve the stability and reliability of the system through reasonable exception handling and retry mechanisms.
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