MySQL锁[1]--共享、排他、意向锁

Shared and Exclusive Locks --- 共享锁和排他锁

InnoDB implements standard row-level locking where there are two types of locks, shared (S) locks and exclusive (X) locks.

InnoDB实现了两种标准的 行级锁 ，分别是 共享锁(S锁) 和 排他锁(X锁)：

• A shared (S) lock permits the transaction that holds the lock to read a row. 共享锁(S锁)容许(当前)持有这个锁的事务去读这个行。

• An exclusive (X) lock permits the transaction that holds the lock to update or delete a row. 排他锁(X锁)容许(当前)持有这个锁的事务去更新或删除这个行。

If transaction T1 holds a shared (S) lock on row r, then requests from some distinct transaction T2 for a lock on row r are handled as follows:

• A request by T2 for an S lock can be granted immediately. As a result, both T1 and T2 hold an S lock on r.

• A request by T2 for an X lock cannot be granted immediately.

若是事务T1带有一个在r行记录上的共享锁，那么另外一个事务T2在请求对r行的锁时，会被处理以下:

• T2请求共享锁将会被马上放行。这样，T1和T2事务都有r行的共享锁。
• T2请求排他锁，将不会被当即放行。

If a transaction T1 holds an exclusive (X) lock on row r, a request from some distinct transaction T2 for a lock of either type on rcannot be granted immediately. Instead, transaction T2 has to wait for transaction T1 to release its lock on row r.

若是事务T1在r行上带有排他锁，那么另外一个事务T2无论请求对r行的哪一种锁，都不会被当即放行。相反，事务T2必须等待事务T1释放在r行上的锁。

排他锁和共享锁的兼容性以下

	X	S
X	Conflict	Conflict
S	Conflict	Compatible

Intention Locks 意向锁

InnoDB supports multiple granularity locking which permits coexistence of row locks and table locks. For example, a statement such asLOCK TABLES ... WRITE takes an exclusive lock (an X lock) on the specified table. To make locking at multiple granularity levels practical, InnoDB uses intention locks. Intention locks are table-level locks that indicate which type of lock (shared or exclusive) a transaction requires later for a row in a table. There are two types of intention locks:

• An intention shared lock (IS) indicates that a transaction intends to set a shared lock on individual rows in a table.

• An intention exclusive lock (IX) indicates that a transaction intends to set an exclusive lock on individual rows in a table.

InnoDB 支持 多粒度锁，它容许 行锁 和 表锁的同时存在。例如，‘LOCK TABLES ... WRITE’ 声明语句 给明确的表加上一个排他锁(X锁)。为了锁在多粒度级别的实践，InnoDB 使用 意向锁。意向锁是表级锁，它代表一个事务在(申请表级锁以后)接下来对表中的某一行请求何种类型的锁(共享仍是排他)。意向锁有两种:

• 意向共享锁(IS锁) 代表一个事务打算(在得到表锁以后)对表中的个别行设置一个共享锁(S锁)。
• 意向排他锁(IX锁) 代表一个事务打算(在得到表锁以后)对表中的个别行设置一个排他锁(X锁)。

For example, SELECT ... FOR SHARE sets an IS lock, and SELECT ... FOR UPDATE sets an IX lock.

例如，SELECT ... FOR SHARE 会设置一个 意向共享锁(IS锁)，SELECT ... FOR UPDATE 会设置一个意向排他锁。

The intention locking protocol is as follows:

• Before a transaction can acquire a shared lock on a row in a table, it must first acquire an IS lock or stronger on the table.

• Before a transaction can acquire an exclusive lock on a row in a table, it must first acquire an IX lock on the table.

意向锁的协议以下：

• 在一个事务获取表中某行的共享锁以前，它必须首先获取这个表的一个意向共享锁或者一个更强的锁。
• 在一个事务获取表中某行的排他锁以前，它必须首先获取这个表的一个意向排他锁。

Table-level lock type compatibility is summarized in the following matrix.

表级锁的兼容性总结以下:

	X	IX	S	IS
X	Conflict	Conflict	Conflict	Conflict
IX	Conflict	Compatible	Conflict	Compatible
S	Conflict	Conflict	Compatible	Compatible
IS	Conflict	Compatible	Compatible	Compatible

A lock is granted to a requesting transaction if it is compatible with existing locks, but not if it conflicts with existing locks. A transaction waits until the conflicting existing lock is released. If a lock request conflicts with an existing lock and cannot be granted because it would cause deadlock, an error occurs.

若是某个事务请求锁，而这个锁与已存在的锁是兼容的，那么这个锁将会被放行。可是若是这个锁与已有的锁冲突，则不会被放行。事务将会等待，直到那个已存在的引发冲突的锁被释放。若是一个锁请求和一个已存在的锁发生冲突，且不会被放行，由于若是放行，会形成死锁，会发生错误。

Intention locks do not block anything except full table requests (for example, LOCK TABLES ... WRITE). The main purpose of intention locks is to show that someone is locking a row, or going to lock a row in the table.

意向锁不会阻塞任何东西，除了全表扫描(好比 LOCK TABLES ... WRITE)。意向锁的主要目的是为了展现有人正在锁住表中的某一行，或者将要锁住那一行。

Transaction data for an intention lock appears similar to the following in SHOW ENGINE INNODB STATUS and InnoDB monitor output:

TABLE LOCK table `test`.`t` trx id 10080 lock mode IX

备注：在执行层面，当MySQL-Server向InnoDB提交一个SQL语句时，意向锁是InnoDB本身去实现的，该语句最终会得到对应行的锁都是S锁或者X锁。