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How to deal with mysql's deadlock problem once and for all

Popularity：208 ℃/2024-10-17 15:46:48

MySQL Deadlocks It is a phenomenon in which two or more transactions wait for the locks held by each other, thus causing all transactions to be unable to continue execution. In the InnoDB storage engine, deadlocks are generated through the locking mechanism, especially in the case of high concurrency and complex business logic, deadlocks are more likely to occur.

The Causes of MySQL Deadlocks

Deadlocks in MySQL generally occur inline lock On. Common causes of deadlocks include:

  1. Transaction A and transaction B hold the locks they need for each other.: Transaction A locks record 1, transaction B locks record 2, and transaction A tries to acquire a lock on record 2 while transaction B tries to acquire a lock on record 1, resulting in a deadlock.
  2. Locking in different order: Two transactions request locks on the same set of resources, but in different order, causing them to wait for each other. For example, Transaction A locks Record 1 and Record 2 in the same order, while Transaction B locks Record 2 and Record 1 in the opposite order.
  3. Gap lock is used.: With InnoDB's Next-Key Locking mechanism, gap locking can also lead to deadlocks, especially during range queries where multiple transactions attempt to lock the same gap.
  4. Long transactions and long lock wait times: Transactions take a long time to execute and do not release locks in a timely manner, causing other transactions to wait for lock timeouts or deadlocks.

II. Deadlock Detection and Handling

MySQL UsageDeadlock Detection To deal with deadlocks, MySQL automatically detects if a transaction is in a deadlock state and aborts one of the transactions, releasing the lock to allow the other to continue execution.The InnoDB storage engine solves this problem by introducing a deadlock detection mechanism, which, when a deadlock is detected, selects a transaction to be rolled back in order to break the deadlock. The transaction that is rolled back throws theDeadlock found when trying to get lock Error.

How to Avoid and Handle Deadlocks in MySQL?

1. Rational design of the index

Using a proper index reduces the extent of locking and decreases the probability of deadlocks. Without an index, MySQL locks all records in the table, increasing the chance of lock conflicts. Therefore, proper design and use of indexes to ensure that queries can find data quickly and avoid unnecessary lock contention can significantly reduce the risk of deadlocks.

2. Keep the locking sequence consistent

When transactions operate on multiple rows in a table, maintaining a consistent locking order can effectively minimize deadlock problems. For example, if two transactions both need to lock the same resource, make sure they request locks in the same order to avoid deadlocks.

3. Reducing transaction lockup time

Minimize transaction execution time and reduce lock holding time. Divide transactions into smaller logical units to avoid tying up resources for long periods of time. At the same time, move non-essential complex operations outside the transaction as much as possible.

4. Reduced concurrency

In the case of higher concurrency, the chances of increasing lock conflicts and deadlocks are higher. Lock contention can be reduced by controlling the degree of concurrency, such as using an optimistic locking mechanism to avoid frequent locking.

5. Using Table Locks Instead of Row Locks

For some scenarios where write operations are concentrated, you can consider using table locks instead of row locks to avoid deadlocks caused by row-level locks. However, table locks can lead to concurrency performance degradation, so you need to choose the right lock according to the business scenario.

6. Locking in a smaller area

Try to minimize the range of rows locked by transactions by using primary key indexes and appropriate conditions. In particular, theUPDATE maybeDELETE operation, use precise query conditions to limit the scope of the lock.

7. Submission of transactions in batches

For batch operations, consider breaking a large transaction into multiple small transactions to reduce the number of rows and the range of operations that are locked at once and to reduce the lock holding time.

8. Choosing the Right Transaction Isolation Level

Appropriately lowering the transaction isolation level can reduce the chance of lock conflicts. For example, you can reduce the transaction isolation level fromSerializable turnRead Committed maybeRepeatable Readto minimize row locking.

9. Locking operation useSELECT ... FOR UPDATE

When you need to update the data immediately after querying it, you can use theSELECT ... FOR UPDATE to explicitly lock the rows, avoiding deadlocks caused by going back and adding locks during updates.

Common Deadlock Examples

Here is an example of a common deadlock where two transactions attempt to lock the same records but in different orders:

-- Transaction A
START TRANSACTION.
UPDATE orders SET status = 'shipped' WHERE id = 1; -- Lock record 1
-- At this point, transaction B is waiting to lock record 1.

-- Transaction B
START TRANSACTION.
UPDATE orders SET status = 'shipped' WHERE id = 2; -- lock record 2
-- At this point, transaction A is waiting to lock record 2.

-- Transaction A is attempting to update record 2, but transaction B holds the lock while transaction A waits.
UPDATE orders SET status = 'shipped' WHERE id = 2; -- Locked record 2 -- Transaction A is waiting to lock record 2 -- Transaction A tries to update record 2, but transaction B holds the lock and transaction A waits.

-- Transaction B attempts to update record 1, but transaction A holds the lock and transaction B waits.

transaction B attempts to update row 1, but transaction A holds the lock and transaction B waits; -- A deadlock occurs and MySQL automatically detects and rolls back one of the transactions.

V. How to detect and analyze deadlocks?

Deadlocks in MySQL can be detected and analyzed in the following ways:

1. (computing) enable (a feature)innodb_print_all_deadlocks parameters

By setting theinnodb_print_all_deadlocks=ONIf you have a deadlock, you can output all the deadlock information in the MySQL logs for easy analysis and debugging.

2. Using the SHOW ENGINE INNODB STATUS Command

After a deadlock has occurred in MySQL, you can use theSHOW ENGINE INNODB STATUS command to view deadlock information. This command outputs the most recent deadlocks and helps developers find the root cause of deadlocks.

SHOW ENGINE INNODB STATUS\G

The information contained in the output includes:

  • Which transaction is rolled back
  • When a deadlock occurs, which locks are held by the transaction, and which locks are pending
  • Transactional SQL Statements

3. MySQL Slow Query Log

Turning on MySQL slow query logging can also indirectly help find performance problems due to lock waiting. Although it does not directly show deadlocks, it can be used as an aid in troubleshooting lock conflict problems.

VI. Coping strategies after a deadlock

When a deadlock occurs, MySQL automatically rolls back one of the transactions, and developers need to catch and handle this exception.

In code, you can use the following to handle deadlocks:

try {
    // Execute the transaction
    ...
} catch (SQLException e) {
    if (() == 1213) { // 1213 is a deadlock error code.
        // Deadlock detection, retry
        retryTransaction(); } else {
    } else {
        // Other exceptions
        throw e; }
    }
}

By catching deadlock exceptions and retrying appropriately, the system can continue execution after a deadlock occurs, thus improving the robustness of the system.

VII. Summary

MySQL deadlocks are a common problem for databases in concurrency scenarios, especially for large-scale and complex business systems, where deadlocks are more frequent. The occurrence of deadlocks can be effectively reduced by means of reasonable index design, keeping the locking order consistent, shortening transaction time, and optimizing locking strategies. At the same time, when a deadlock occurs, MySQL has a deadlock detection and automatic rollback mechanism, so developers can improve the stability and reliability of the system through reasonable exception handling and retry mechanism.

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