• Shared and Exclusive Locks

• Intention Locks

• Record Locks

• Gap Locks

• Next-Key Locks

• Insert Intention Locks

• AUTO-INC Locks

• Predicate Locks for Spatial Indexes

Shared and Exclusive Locks

• latch: A lightweight structure used by InnoDB to implement a lock for its own internal memory structures
• latches Mutexes and rw-locks are known collectively as latches
• lock: The high-level notion(概念) of an object that controls access to a resource, such as a table, row, or internal data structure, as part of a locking strategy.
• InnoDB implements standard row-level locking
• rw-lock types include s-locks (shared locks), x-locks (exclusive locks), and sx-locks (shared-exclusive locks).
• A shared (S) lock （共享）, provides read access to a common resource
• An exclusive (X) lock （排它）, provides write access to a common resource while not permitting inconsistent reads by other threads.
• An sx-lock provides write access to a common resource while permitting inconsistent reads by other threads. sx-locks were introduced in MySQL 5.7 to optimize concurrency and improve scalability for read-write workloads.

Intention Locks

• Intention locks are table-level locks
• Intention shared (IS):
• Intention exclusive (IX):
• main purpose of IX and IS locks is to show that someone is locking a row, or going to lock a row in the table.
• intention locks do not block anything except full table requests (for example, LOCK TABLES ... WRITE).
• The intention locking protocol is as follows:
  • Before a transaction can acquire an S lock on a row in table t, it must first acquire an IS or stronger lock on t. （在获得row的 S lock前，必须在表t上面先获得 IS 或者 stronger lock)
  • Before a transaction can acquire an X lock on a row, it must first acquire an IX lock on t. (在获得row的 X lock前，必须在表t上面先获得 IX)
  • intention lock互相兼容

Record Locks

• A record lock is a lock on an index record
• Record locks always lock index records, even if a table is defined with no indexes.For such cases, InnoDB creates a hidden index and uses this index for record locking. (如果表没有定义索引，那么Innodb会创建一个隐式的clustered index聚簇[cù]索引，然后用这个索引加锁）

Gap Locks

• A gap lock is a lock on a gap between index records, or a lock on the gap before the first or after the last index record.
• SELECT c1 FOR UPDATE FROM t WHERE c1 BETWEEN 10 and 20;
• A gap might span a single index value, multiple index values, or even be empty.（gap可以是单个index，多个index，empty这个怎么理解？)
• conflicting locks can be held on a gap by different transactions

Next-Key Locks

• InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it .

AUTO-INC Locks

• An AUTO-INC lock is a special table-level lock taken by transactions inserting into tables with AUTO_INCREMENT columns

Predicate Locks for Spatial Indexes