MySQL's transaction rollback mechanism and ACID characteristics ensure data reliability and consistency. 1. Transaction rollback refers to the undo log when an error occurs or is triggered actively, and the database is restored to the state before the transaction starts; 2. ACID includes atomicity, consistency, isolation and persistence, which ensures the overall execution of the transaction, consistent state transition, concurrent isolation and permanent storage after commit; 3. The working principle of rollback is to record the reverse operation of the modification operation to the undo log, and execute it in reverse order when needed; 4. In actual use, transactions need to be explicitly enabled, isolation levels reasonably set, long transactions, and pay attention to deadlock processing. These mechanisms jointly ensure high requirements for data accuracy in finance, e-commerce and other systems.

MySQL's transaction rollback and ACID characteristics are very critical parts of database operations, especially when dealing with systems such as finance and e-commerce that require high data consistency. If you want to know why MySQL can "undo" operations when an error occurs, or why multiple operations either succeed or fail together, it is actually the transaction rollback mechanism and the support of ACID attributes.

What is transaction rollback?

Transaction rollback refers to the state that restores the database to the state before the transaction starts when an error occurs during transaction execution or when a rollback command is actively triggered. This ensures that even if an exception occurs, partially completed data changes are not left.

To give a simple example: you transfer money from Account A to Account B, deduct the money first and then add the money. If the increase of money fails, you need to "repay back" the deducted money, which is the role of rollback.

To implement rollback, MySQL uses undo log (revoke log) . Whenever you perform a modification operation (such as UPDATE or DELETE), the system will record the old value so that it can restore the original state when rollback is required.

What is ACID? Why does it matter?

ACID refers to the four core characteristics of a transaction: atomicity, consistency, isolation, and persistence . Together, they guarantee the reliability of transaction processing.

• Atomicity : All operations in a transaction are either completed or not completed. If one step fails, the entire transaction will be rolled back.
• Consistency : The transaction must ensure that the database changes from one consistent state to another. For example, before and after the transfer, the total amount remains unchanged.
• Isolation : When multiple transactions are executed concurrently, they cannot interfere with each other. Different isolation levels determine which intermediate states can be seen.
• Persistence : Once a transaction is committed, changes to the database are permanent and cannot be lost even if the system crashes.

These properties do not exist out of thin air, but are achieved through logging mechanisms (such as redo log and undo log), lock mechanisms, and storage engines working together.

How does transaction rollback work?

When the transaction begins, MySQL records the reverse information of each modification operation into the undo log. For example:

• If you insert a row of data, undo log will record an operation to "delete the row";
• If you update a row, undo log will record the original value;
• If you delete a line, undo log will record an "insert the original line" operation.

When a transaction is rolled back, MySQL will perform these "inverse operations" in reverse order according to undo log, restoring the data to what it was before the transaction started.

One thing to note is that transaction rollback only affects uncommitted operations . Once the transaction is committed, you cannot roll back again - unless you write compensation logic manually or use flashback technology.

Things to note in actual use

• Explicitly enable transactions : By default, MySQL is in autocommit mode (autocommit=1). If you want to control transactions, remember to explicitly turn on with BEGIN or START TRANSACTION .
• Set isolation levels reasonably : Different isolation levels will affect concurrent behavior and performance. For example, read submitted COMMITTED and repeatable read (REPEATABLE READ) perform differently under MVCC.
• Don't abuse long transactions : transactions that have not been committed for a long time can take up a lot of undo log space, which may lead to performance problems or even insufficient space.
• Pay attention to deadlocks and lock waiting : When a transaction involves multiple tables or rows, deadlocks are prone to occur. innodb_deadlock_detect=ON can be set to automatically detect and roll back one of the transactions.

Basically that's it. Transaction rollback and ACID seem a bit abstract, but as long as you understand the role of undo log and the respective meaning of ACID, many database behaviors can be explained. In actual development, especially when designing high-concurrency systems, this knowledge can often help you avoid many pitfalls.

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