Middleware_ Redis_ 06_ Redis transactions

Redis Business

1、 Business profile

problem ：Redis During the execution of instructions , Multiple consecutive instructions are disturbed , interrupt , Jump the queue , How to solve ？

redis Concept of transactions ：redis A transaction is a queue of command execution , Wrap a series of predefined commands into （ all in one ） A line .

When executed , One time sequential execution , There will be no interruption in the middle , That is, in a queue , Disposable 、 Sequence 、 Execute a series of commands exclusively .

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2、 Transaction basic operations

2.1、 The basic operation of a transaction

#  Open transaction 
multi

#  effect 
 Set the start position of the transaction , After this instruction is executed , All subsequent instructions are added to the transaction 

#  Perform transactions 
exec

#  effect 
 Set the end position of the transaction , Execute transactions at the same time .

#  Be careful ：
1. multi and exec Pairs appear , Use in pairs 
2.  The command to add a transaction is stored in the task queue first , Not immediately , Only execute exec It's the order that begins 

problem ： A problem was found during the definition of the transaction , What do I do ？

Solution ：

#  Cancel the business 
discard

#  effect 
 Terminate the definition of the current transaction , It happened in multi after ,exec Before 

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2.2、 The workflow of a transaction

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2.3、 Transaction considerations

problem 1： In the process of defining transactions , What if the command format is wrong ？

• Grammar mistakes

  It means that the command writing format is incorrect

• Processing results

  There is a syntax error in the command contained in the defined transaction , All commands in the overall transaction will not be executed . Including commands with correct syntax

problem 2： In the process of defining transactions , What if there is an error in command execution ？

• Running error

  Indicates that the command format is correct , But it can't be executed correctly . for example , Yes list Conduct incr operation

• Processing results

  Commands that run correctly execute , Running the wrong command will not be executed

Be careful ： The data corresponding to the executed command will not be rolled back automatically , You need to implement rollback by yourself .

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2.4、 Manual transaction rollback

1. During recording operation , The status of the affected data
   • Single data ：string
   • More data ：hash、list、set、zset
2. Set command , Restore all modified items
   • Single data ： direct set（ Pay attention to the surrounding properties , For example, prescription ）
   • More data ： Modify the corresponding value or overall clone assignment

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3、 lock

3.1、 Transaction execution based on specific conditions —— lock

Business scenario ：

• Tmall double 11 During the hot sale , Replenish the sold out goods ,4 Every salesman has the right to replenish . Replenishment may be a series of operations , Multiple continuous operations are involved , How to ensure that the operation will not be repeated ？

Business analysis ：

• Multiple clients may modify the same set of data at the same time , And once the data is modified , Will not apply to continue operation
• Lock the data to be operated before operation , As soon as it changes , Terminate the current operation

Solution ：

#  Yes key Add monitoring lock , In execution exec If key change , Terminate transaction execution 
watch key1 [key2] ……

#  Cancel for all key Surveillance 
unwatch

Tips：Redis It is applied to batch task execution based on state control

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3.2、 Transaction execution based on specific conditions —— Distributed lock

Business scenario ：

• Tmall double 11 During the hot sale , Replenish the sold out goods , And the replenishment is completed . Customers are enthusiastic about buying ,3 Buy all goods in seconds . The inventory has been completely emptied in this replenishment , How to prevent the last item from being purchased by more than one person at the same time ？【 Oversold problem 】

Business analysis ：

• Use watch Monitor one key Is there any change that can't solve the problem , The specific data to be monitored here
• although redis It's single threaded , But when multiple clients operate on the same data at the same time , How to avoid not being modified at the same time ？

Solution ：

#  Use setnx Set a public lock 
setnx lock-key value

utilize setnx The return value characteristic of the command , If there is a value, the setting fails , If there is no value, the setting is successful

• For the return setting successful , Have control , Carry out specific business operations in the next step
• For return settings that failed , No control , Line up or wait

After the operation, pass del Operate the release lock

Be careful ： The above solution is a design concept , Rely on norms to ensure , Risky

Tips：Redis It is applied to scene control based on distributed lock correspondence

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3.2、 Transaction execution based on specific conditions —— Distributed lock improvement

Business scenario ：

• Mechanisms that rely on distributed locks , When a user operates, the corresponding client goes down , And the lock has been obtained at this time . How to solve ？

Business analysis ：

• Since the lock operation is controlled by the user, lock and unlock , There must be a risk of not unlocking after locking
• The unlocking operation that needs to be performed cannot rely solely on user control , At the system level, the corresponding minimum treatment scheme shall be given

Solution ：

#  Use  expire  For lock key Add time limit , Don't release , Abandon lock 
expire lock-key second
pexpire lock-key milliseconds

Because operations are usually microseconds or milliseconds , Therefore, the locking time should not be set too large . The specific time needs to be confirmed after business test .

• for example ： The longest execution time for holding a lock 127ms, The shortest execution time 7ms.
• Testing the maximum execution time of a million times corresponds to the maximum time consumption of a command , Testing millions of network delays takes an average of
• Lock time setting is recommended ： Most time consuming x 120% + Average network latency x 110%
• If the business takes the most time << Average network latency , Usually it is 2 An order of magnitude , It takes a long time to take one of them

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