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  • gap lock/next-key lock浅析 Basic-Paxos协议日志同步应用

    http://www.cnblogs.com/renolei/p/4673842.html 

    当InnoDB在判断行锁是否冲突的时候, 除了最基本的IS/IX/S/X锁的冲突判断意外, InnoDB还将锁细分为如下几种子类型:

    • record lock (RK)

      记录锁, 仅仅锁住索引记录的一行

    • gap lock (GK)

      区间锁, 仅仅锁住一个区间(开区间)

    • insert intention lock (IK)

      意向插入锁

    • next key lock (NK)

      record lock + gap lock, 半开半闭区间, 且下界开, 上界闭

    以下锁兼容矩阵:

    request与granted之间的兼容矩阵:

         | Type of active  |
 Request |  lock (granted) |
  lock   | RK   GK  IK  NK |
---------+-----------------+
  RK     |  0   1   1  0   |
  GK     |  1   1   1  1   |
  IK     |  1   0   1  0   |
  NK     |  0   1   1  0   |

    下面构造集中场景简单描述下record lock/gap lock/next-key lock

    • Table schema

      CREATE TABLE `reno` (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `name` varchar(10) DEFAULT NULL,
    PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8

    • 构造数据

      insert into reno select 5, 'aa';
insert into reno select 7, 'bb';
insert into reno select 9, 'cc';
insert into reno select 18, 'dd';
insert into reno select 23, 'ee';
insert into reno select 30, 'ff';
insert into reno select 40, 'gg';
insert into reno select 45, 'hh';
insert into reno select 99, 'ii';

    • 查看结果

      select * from reno;
+----+------+
| id | name |
+----+------+
|  5 | aa   |
|  7 | bb   |
|  9 | cc   |
| 18 | dd   |
| 23 | ee   |
| 30 | ff   |
| 40 | gg   |
| 45 | hh   |
| 99 | ii   |
+----+------+
9 rows in set (0.00 sec)

    • 查看tx_isolation

      SELECT @@GLOBAL.tx_isolation, @@tx_isolation;
+-----------------------+----------------+
| @@GLOBAL.tx_isolation | @@tx_isolation |
+-----------------------+----------------+
| READ-COMMITTED        | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.00 sec)

      next-key lock只有在repeatable-read级别下才有意义, 防止出现幻读

      设置tx_isolation级别为REPEATABLE-READ级别:

      SET @@GLOBAL.tx_isolation = 'REPEATABLE-READ';
Query OK, 0 rows affected (0.00 sec)

SET @@SESSION.tx_isolation = 'REPEATABLE-READ';
Query OK, 0 rows affected (0.00 sec)

SELECT @@GLOBAL.tx_isolation, @@tx_isolation;
+-----------------------+-----------------+
| @@GLOBAL.tx_isolation | @@tx_isolation  |
+-----------------------+-----------------+
| REPEATABLE-READ       | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

    • case 1:

      sesion 1sesion 2sesion 2 insert status
      start transaction;    
      select * from reno where id = 9 for update;    
        start transaction;  
        insert into reno select 8,'jj'; ok
        insert into reno select 10,'kk'; ok
        insert into reno select 3,'ll'; ok
        insert into reno select 111,'mm'; ok
        rollback  
      rollback    
      • 加record lock, id = 9

    • case 2:

      sesion 1sesion 2sesion 2 insert status
      start transaction;    
      select * from reno where id = 15 for update;    
        start transaction;  
        insert into reno select 8, 'jj'; ok
        insert into reno select 10, 'kk'; block
        insert into reno select 16, 'll'; block
        insert into reno select 19, 'mm'; ok
        rollback  
      rollback    

      • 加next-key lock, (9, 18]

      • innodb lock info:

        ------------
TRANSACTIONS
------------
Trx id counter 2990040255
Purge done for trx's n:o < 2990040253 undo n:o < 0 state: running but idle
History list length 323
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 38753, OS thread handle 0x7f377c68f700, query id 140937 localhost root init
show engine innodb status
---TRANSACTION 0, not started
MySQL thread id 9, OS thread handle 0x7f370817d700, query id 140906 127.0.0.1 root cleaning up
---TRANSACTION 2990040254, ACTIVE 8 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 38773, OS thread handle 0x7f377c60e700, query id 140924 localhost root executing
insert into reno select 10, 'kk'
------- TRX HAS BEEN WAITING 8 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 64257 page no 3 n bits 80 index `PRIMARY` of table `test`.`reno` trx id 2990040254 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 4; hex 80000012; asc     ;;
 1: len 6; hex 0000b238648b; asc    8d ;;
 2: len 7; hex d70001c00c0110; asc        ;;
 3: len 2; hex 6464; asc dd;;

------------------
---TRANSACTION 2990040253, ACTIVE 17 sec
2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 38758, OS thread handle 0x7f370807b700, query id 140919 localhost root cleaning up
        从上面的trx lock信息里看到此时等待的锁是: lock_mode X locks gap before rec

    • case 3:

      sesion 1sesion 2sesion 2 insert status
      start transaction;    
      select * from reno where id = 200 for update;    
        start transaction;  
        insert into reno select 1, 'jj'; ok
        insert into reno select 88, 'kk'; ok
        insert into reno select 100, 'll'; block
        insert into reno select 500, 'mm'; block
        rollback  
      rollback    

      • 加next-key lock, (99, ~)

      • innodb lock info:

        ------------
TRANSACTIONS
------------
Trx id counter 2990040257
Purge done for trx's n:o < 2990040253 undo n:o < 0 state: running but idle
History list length 323
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 38753, OS thread handle 0x7f377c68f700, query id 141561 localhost root init
show engine innodb status
---TRANSACTION 0, not started
MySQL thread id 9, OS thread handle 0x7f370817d700, query id 141535 127.0.0.1 root cleaning up
---TRANSACTION 2990040256, ACTIVE 137 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 360, 2 row lock(s)
MySQL thread id 38773, OS thread handle 0x7f377c60e700, query id 141548 localhost root executing
insert into reno select 500, 'kk'
------- TRX HAS BEEN WAITING 10 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 64257 page no 3 n bits 80 index `PRIMARY` of table `test`.`reno` trx id 2990040256 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;

------------------
---TRANSACTION 2990040255, ACTIVE 153 sec
2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 38758, OS thread handle 0x7f370807b700, query id 141432 localhost root cleaning up

        从上面的trx lock信息里看到此时等待的锁是: lock_mode X insert intention waiting

    简单总结下:

    • 在case 1中, 实际上加的next-key lock是(9,9], 也就是id=9这一条记录被lock住, 其他所有的插入都没有关系.
    • 在case 2中, 因为id=15记录不存在, 且记录中上下两个边界是id=9, id=18, 因此加的next-key lock是(9, 18], 在这个区间内插入的数据都会被block, 此区间外的数据写入则不受影响.
    • 在case 3中, id=200的记录不存在, 并且比表中所有的记录都大, 因此innodb则认为next-key lock是(99, ~), 任何大于99的id记录插入都会被block, 小于99的id记录写入则不受影响.

    next-key lock是为防止幻读的发生,而只有REPEATABLE-READ以及以上隔离级别才能防止幻读, 所以在READ-COMMITTED隔离级别下面没有next-key lock这一说法.

    参考:

    Love truth but pardon errors
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  • 原文地址：https://www.cnblogs.com/zengkefu/p/5720885.html
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