发布时间: 2019-08-09 01:04:00
参考：https://ruby-china.org/topics/28963 （乐观锁和悲观锁)
这个例子说的非常形象。
Process #1 Process #2
------------------ ------------------
Read from Database
Some processing... Read from database
Some processing... Some processing...
Update database Some processing...
Update database
rails 中，使用悲观锁．
ask_account = MyModel.find 1
Account.transaction do
ask_account.lock!
ask_account.balance += self.volume.to_d
ask_account.save!
end
下面是个单元测试：
it '应该实现悲观锁，两个进程同时修改一个model' do
$logger.info "------- 测试悲观锁, start"
a1 = Thread.new {
account = Account.last
Account.transaction do
account.lock!
account.balance += 333
# 这里不sleep. 直接运行
account.save!
end
}
# 第二个进程立刻
a2 = Thread.new {
account = Account.last
Account.transaction do
account.lock!
account.balance += 444
account.save!
end
}
a1.join
Account.last.balance.should == 333 + 100
$logger.info "------- 测试悲观锁, ends"
end
结果如下：
21:01:52 INFO: ------- 测试悲观锁, start 21:01:52 DEBUG: (1.2ms) SET NAMES utf8 COLLATE utf8_general_ci, @@SESSION.sql_mode = CONCAT(CONCAT(@@sql_mode, ',STRICT_ALL_TABLES'), ',NO_AUTO_VALUE_ON_ZERO'), @@SESSION.sql_auto_is_null = 0, @@SESSION.wait_timeout = 2147483 21:01:52 DEBUG: (1.7ms) SET NAMES utf8 COLLATE utf8_general_ci, @@SESSION.sql_mode = CONCAT(CONCAT(@@sql_mode, ',STRICT_ALL_TABLES'), ',NO_AUTO_VALUE_ON_ZERO'), @@SESSION.sql_auto_is_null = 0, @@SESSION.wait_timeout = 2147483 21:01:52 DEBUG: Account Load (0.2ms) SELECT `accounts`.* FROM `accounts` ORDER BY `accounts`.`id` DESC LIMIT 1 21:01:52 DEBUG: Account Load (0.2ms) SELECT `accounts`.* FROM `accounts` ORDER BY `accounts`.`id` DESC LIMIT 1 21:01:52 DEBUG: (0.5ms) BEGIN 21:01:52 DEBUG: (0.5ms) BEGIN 21:01:52 DEBUG: Account Load (1.6ms) SELECT `accounts`.* FROM `accounts` WHERE `accounts`.`id` = 1354 LIMIT 1 FOR UPDATE 21:01:52 DEBUG: Account Update (0.3ms) UPDATE `accounts` SET `balance` = 433.0, `updated_at` = '2019-08-26 21:01:52' WHERE `accounts`.`id` = 1354 21:01:52 DEBUG: Account Load (2.5ms) SELECT `accounts`.* FROM `accounts` WHERE `accounts`.`id` = 1354 LIMIT 1 FOR UPDATE 21:01:52 DEBUG: Account Update (0.3ms) UPDATE `accounts` SET `balance` = 877.0 WHERE `accounts`.`id` = 1354 21:01:52 DEBUG: (14.7ms) COMMIT 21:01:52 DEBUG: Account Load (0.4ms) SELECT `accounts`.* FROM `accounts` ORDER BY `accounts`.`id` DESC LIMIT 1 21:01:52 INFO: ------- 测试悲观锁, ends 21:01:52 DEBUG: (20.1ms) COMMIT
从上面来看，两个事务是先后结束的．不存在rollback. 第二个进程会自动等待第一个进程结束．
哪怕当前程序只是 t1.join, （只等待t1 ) , t2也会生效．
a1 = Thread.new {
account = Account.last
Account.transaction do
account.lock!
account.balance += 333
# 这里休息 0.1s <---------- 增加了这个代码
sleep 0.1
account.save!
end
}
21:03:33 INFO: ------- 测试悲观锁, start 21:03:33 DEBUG: (2.0ms) SET NAMES utf8 COLLATE utf8_general_ci, @@SESSION.sql_mode = CONCAT(CONCAT(@@sql_mode, ',STRICT_ALL_TABLES'), ',NO_AUTO_VALUE_ON_ZERO'), @@SESSION.sql_auto_is_null = 0, @@SESSION.wait_timeout = 2147483 21:03:33 DEBUG: (2.1ms) SET NAMES utf8 COLLATE utf8_general_ci, @@SESSION.sql_mode = CONCAT(CONCAT(@@sql_mode, ',STRICT_ALL_TABLES'), ',NO_AUTO_VALUE_ON_ZERO'), @@SESSION.sql_auto_is_null = 0, @@SESSION.wait_timeout = 2147483 21:03:33 DEBUG: Account Load (1.2ms) SELECT `accounts`.* FROM `accounts` ORDER BY `accounts`.`id` DESC LIMIT 1 21:03:33 DEBUG: Account Load (2.0ms) SELECT `accounts`.* FROM `accounts` ORDER BY `accounts`.`id` DESC LIMIT 1 21:03:33 DEBUG: (0.5ms) BEGIN 21:03:33 DEBUG: (0.8ms) BEGIN 21:03:33 DEBUG: Account Load (0.8ms) SELECT `accounts`.* FROM `accounts` WHERE `accounts`.`id` = 1355 LIMIT 1 FOR UPDATE 21:03:33 DEBUG: Account Update (0.3ms) UPDATE `accounts` SET `balance` = 433.0 WHERE `accounts`.`id` = 1355 # 注意这里，这个 102.4ms . 显示等待了 t1 结束，第二个才生效． 21:03:33 DEBUG: Account Load (102.4ms) SELECT `accounts`.* FROM `accounts` WHERE `accounts`.`id` = 1355 LIMIT 1 FOR UPDATE 21:03:33 DEBUG: Account Update (0.3ms) UPDATE `accounts` SET `balance` = 877.0 WHERE `accounts`.`id` = 1355 21:03:33 DEBUG: (14.5ms) COMMIT 21:03:33 DEBUG: Account Load (0.2ms) SELECT `accounts`.* FROM `accounts` ORDER BY `accounts`.`id` DESC LIMIT 1 21:03:33 INFO: ------- 测试悲观锁, ends 21:03:33 DEBUG: (16.8ms) COMMITBack