ACID

1. 原子性（Atomicity）

事务被视为不可分割的最小单元，事务的所有操作要么全部提交成功，要么全部失败回滚。

回滚可以用回滚日志（Undo Log）来实现，回滚日志记录着事务所执行的修改操作，在回滚时反向执行这些修改操作即可。

2. 一致性（Consistency）

数据库在事务执行前后都保持一致性状态。在一致性状态下，所有事务对同一个数据的读取结果都是相同的。

3. 隔离性（Isolation）

一个事务所做的修改在最终提交以前，对其它事务是不可见的。

4. 持久性（Durability）

一旦事务提交，则其所做的修改将会永远保存到数据库中。即使系统发生崩溃，事务执行的结果也不能丢失。

系统发生崩溃可以用重做日志（Redo Log）进行恢复，从而实现持久性。与回滚日志记录数据的逻辑修改不同，重做日志记录的是数据页的物理修改。

Atomicity

The atomicity aspect of the ACID model mainly involves InnoDBtransactions. Related MySQL features include:

• The autocommit setting.
• The COMMIT statement.
• The ROLLBACK statement.

Consistency

The consistency aspect of the ACID model mainly involves internal InnoDBprocessing to protect data from crashes. Related MySQL features include:

• The InnoDB doublewrite buffer. See Section 15.6.4, “Doublewrite Buffer”.
• InnoDB crash recovery. See InnoDB Crash Recovery.

Isolation

The isolation aspect of the ACID model mainly involves InnoDBtransactions, in particular the isolation level that applies to each transaction. Related MySQL features include:

• The autocommit setting.
• Transaction isolation levels and the SET TRANSACTION statement. See Section 15.7.2.1, “Transaction Isolation Levels”.
• The low-level details of InnoDB locking. Details can be viewed in the INFORMATION_SCHEMA tables (see Section 15.15.2, “InnoDB INFORMATION_SCHEMA Transaction and Locking Information”) and Performance Schema data_locks and data_lock_waits tables.

Durability

The durability aspect of the ACID model involves MySQL software features interacting with your particular hardware configuration. Because of the many possibilities depending on the capabilities of your CPU, network, and storage devices, this aspect is the most complicated to provide concrete guidelines for. (And those guidelines might take the form of “buy new hardware”.) Related MySQL features include:

• The InnoDB doublewrite buffer. See Section 15.6.4, “Doublewrite Buffer”.
• The innodb_flush_log_at_trx_commit variable.
• The sync_binlog variable.
• The innodb_file_per_table variable.
• The write buffer in a storage device, such as a disk drive, SSD, or RAID array.
• A battery-backed cache in a storage device.
• The operating system used to run MySQL, in particular its support for the fsync() system call.
• An uninterruptible power supply (UPS) protecting the electrical power to all computer servers and storage devices that run MySQL servers and store MySQL data.
• Your backup strategy, such as frequency and types of backups, and backup retention periods.
• For distributed or hosted data applications, the particular characteristics of the data centers where the hardware for the MySQL servers is located, and network connections between the data centers.