一、死鎖案例

MySQL版本：Percona MySQL Server 5.7.19

隔離級別：可重復讀(RR)

業務邏輯：并發下按某個索引字段先delete記錄，再insert記錄

比如：begin;delete?from?tb?where?order_id?=?xxx;insert?into?tb(order_id)?values(xxx);

commit;

二、MySQL鎖基本概念

S：共享鎖(行級鎖)

X：排他鎖(行級鎖)

IS：意向共享鎖(表級鎖)

IX：意向排他鎖(表級鎖)

以上4種鎖的兼容性見下表：

鎖模式兼容性表gap鎖與gap鎖之間不沖突

rec insert intention(插入意向鎖)與gap鎖沖突。

三、模擬復現死鎖

打開參數，從innodb status獲取更多的鎖信息。

set GLOBAL innodb_status_output_locks=ON;

表結構：CREATE?TABLE?`tb`?(??`order_id`?int(11)?DEFAULT?NULL,

KEY?`idx_order_id`?(`order_id`)

)?ENGINE=InnoDB?DEFAULT?CHARSET=utf8

表中數據：mysql>?select?*?from?tb;

+----------+|?order_id?|+----------+|???????10?||???????20?|+----------+2?rows?in?set?(0.00?sec)

事務執行步驟：session1session2begin

begin

delete from tb where order_id=15;

delete from tb where order_id=15;

insert into tb select 15;(等待鎖)

insert into tb select 15;(死鎖)當session1執行delete from tb where order_id=15;，由于條件order_id=15的記錄不存在，session1 獲得2個鎖結構，分別是意向排他鎖IX(表級鎖)、gap鎖(行級鎖)，如下：---TRANSACTION?1055191443,?ACTIVE?20?sec2?lock?struct(s),?heap?size?1136,?1?row?lock(s)

MySQL?thread?id?315642,?OS?thread?handle?139960342456064,?query?id?150462030?localhost?root

TABLE?LOCK?table?`db`.`tb`?trx?id?1055191443?lock?mode?IX

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191443?lock_mode?X?locks?gap?before?rec當session2執行delete from tb where order_id=15;，同樣由于order_id=15的記錄不存在，session2 也獲得2個鎖結構，分別是意向排他鎖IX(表級鎖)、gap鎖(行級鎖)，如下：---TRANSACTION?1055191444,?ACTIVE?3?sec2?lock?struct(s),?heap?size?1136,?1?row?lock(s)

MySQL?thread?id?315336,?OS?thread?handle?139960562685696,?query?id?150462412?localhost?root

TABLE?LOCK?table?`db`.`tb`?trx?id?1055191444?lock?mode?IX

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191444?lock_mode?X?locks?gap?before?rec當session2執行insert into tb select 15;， session2 已經獲取到IX鎖，gap鎖，等待 rec insert intention(插入意向鎖)---TRANSACTION?1055191444,?ACTIVE?68?sec?inserting

mysql?tables?in?use?1,?locked?1LOCK?WAIT?3?lock?struct(s),?heap?size?1136,?2?row?lock(s),?undo?log?entries?1MySQL?thread?id?315336,?OS?thread?handle?139960562685696,?query?id?150462778?localhost?root?executing

insert?into?tb?select?15-------?TRX?HAS?BEEN?WAITING?2?SEC?FOR?THIS?LOCK?TO?BE?GRANTED:

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191444?lock_mode?X?locks?gap?before?rec?insert?intention?waiting

------------------

TABLE?LOCK?table?`db`.`tb`?trx?id?1055191444?lock?mode?IX

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191444?lock_mode?X?locks?gap?before?rec

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191444?lock_mode?X?locks?gap?before?rec?insert?intention?waiting當session1執行insert into tb select 15;，session1 已獲取到IX鎖，gap鎖， 等待rec insert intention(插入意向鎖)， session1, session2 都在等待插入意向鎖， 插入意向鎖與gap鎖沖突，雙方都沒有釋放gap鎖，又都在等待插入意向鎖，死鎖發生。LATEST?DETECTED?DEADLOCK

------------------------2018-11-03?17:15:11?0x7f4b0e7ea700***?(1)?TRANSACTION:

TRANSACTION?1055191444,?ACTIVE?135?sec?inserting

mysql?tables?in?use?1,?locked?1LOCK?WAIT?3?lock?struct(s),?heap?size?1136,?2?row?lock(s),?undo?log?entries?1MySQL?thread?id?315336,?OS?thread?handle?139960562685696,?query?id?150462778?localhost?root?executing

insert?into?tb?select?15***?(1)?WAITING?FOR?THIS?LOCK?TO?BE?GRANTED:

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191444?lock_mode?X?locks?gap?before?rec?insert?intention?waiting

***?(2)?TRANSACTION:

TRANSACTION?1055191443,?ACTIVE?201?sec?inserting,?thread?declared?inside?InnoDB?5000mysql?tables?in?use?1,?locked?13?lock?struct(s),?heap?size?1136,?2?row?lock(s),?undo?log?entries?1MySQL?thread?id?315642,?OS?thread?handle?139960342456064,?query?id?150463172?localhost?root?executing

insert?into?tb?select?15***?(2)?HOLDS?THE?LOCK(S):

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191443?lock_mode?X?locks?gap?before?rec

***?(2)?WAITING?FOR?THIS?LOCK?TO?BE?GRANTED:

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191443?lock_mode?X?locks?gap?before?rec?insert?intention?waiting

***?WE?ROLL?BACK?TRANSACTION?(2)

四、案例擴展

以上死鎖案例，業務代碼邏輯是多線程并發下，有可能多個線程會執行相同order_id的job，比如兩個線程執行的order_id 都是15。

另外一種情況，多個線程間，不會執行到相同order_id的情況，也可能發生死鎖。比如一個線程order_id=15，另外一個線程order_id=16，如下所示：

事務執行步驟：session1session2begin

begin

delete from tb where order_id=15;

delete from tb where order_id=16;

insert into tb select 16;(等待鎖)

insert into tb select 15;(死鎖)

鎖情況與上述相同，不再贅述，死鎖信息如下：LATEST?DETECTED?DEADLOCK

------------------------2018-11-03?17:28:30?0x7f4b0e667700***?(1)?TRANSACTION:

TRANSACTION?1055191450,?ACTIVE?18?sec?inserting

mysql?tables?in?use?1,?locked?1LOCK?WAIT?3?lock?struct(s),?heap?size?1136,?2?row?lock(s),?undo?log?entries?1MySQL?thread?id?316221,?OS?thread?handle?139960338228992,?query?id?150467652?localhost?root?executing

insert?into?tb?select?16***?(1)?WAITING?FOR?THIS?LOCK?TO?BE?GRANTED:

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191450?lock_mode?X?locks?gap?before?rec?insert?intention?waiting

***?(2)?TRANSACTION:

TRANSACTION?1055191449,?ACTIVE?28?sec?inserting,?thread?declared?inside?InnoDB?5000mysql?tables?in?use?1,?locked?13?lock?struct(s),?heap?size?1136,?2?row?lock(s),?undo?log?entries?1MySQL?thread?id?316222,?OS?thread?handle?139960340870912,?query?id?150467681?localhost?root?executing

insert?into?tb?select?15***?(2)?HOLDS?THE?LOCK(S):

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191449?lock_mode?X?locks?gap?before?rec

***?(2)?WAITING?FOR?THIS?LOCK?TO?BE?GRANTED:

RECORD?LOCKS?space?id?1337?page?no?4?n?bits?72?index?idx_order_id?of?table?`db`.`tb`?trx?id?1055191449?lock_mode?X?locks?gap?before?rec?insert?intention?waiting

***?WE?ROLL?BACK?TRANSACTION?(2)

作者：偏執的工匠

鏈接：https://www.jianshu.com/p/f8495015dace

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