Redis transaction mechanism

One 、Redis Orders related to the transaction ：

1、MULTI：

Used to mark the opening of the transaction block .MULTI After performing ,Redis The subsequent commands will be put into a cache queue one by one , When EXEC When the command is called , All the commands in the queue will be executed atomically .

2、EXEC：

Execute all previously queued commands in a transaction , Then restore the normal connection state . When using WATCH On command , Only if the monitored key has not been modified ,EXEC Command will execute the command in the transaction .

3、DISCARD：

Give up the business , Clear the command in the transaction queue , Then restore the normal connection state . If used UNWATCH command , that DISCARD The command will cancel all keys currently connected to the monitor .

4、WATCH：

When a transaction needs to be executed conditionally , You will use this command to key Set as monitored . If this is before the transaction is executed key Altered by other orders , Then the whole business will be interrupted .WATCH The command can be used to provide CAS function .

5、UNWATCH：

Clear all monitored keys in the transaction . If called EXEC or DISCARD command , So you don't have to call it manually UNWATCH command .

Two 、Redis How things work ：

1、 Definition of transaction ：

Redis The essence of a transaction is a set of commands , All commands in a transaction are either executed , Either not . The principle of transaction is to send the command belonging to a transaction to Redis, Put in a queue , let Redis Execute these orders in turn . If it's sending EXEC The client is disconnected before the command , be Redis The transaction queue will be cleared , None of the commands in the transaction will execute . And once the client sends EXEC command , All orders will be executed , Even after that, it doesn't matter if the client is disconnected , because Redis All commands to be executed have been recorded in .

besides ,Redis The transaction can also ensure that the commands in a transaction are executed in turn without being inserted by other commands . in other words , During the execution of the transaction , The server will not interrupt the transaction and execute the command requests of other clients , That is, it will not be inserted by other commands , Don't jam , Wait until all commands in the transaction have been executed before processing command requests from other clients . It's one-off 、 Sequence 、 To execute a series of orders exclusively .

2、Redis The nature of transactions ：

（3） Atomicity ：Redis The atomicity of can only guarantee the one-time execution of batch operations , And traditional mysql The difference is ,Redis Rollback is not supported , In execution EXEC On command , If Redis The execution of a command in the transaction failed , Subsequent orders will still be executed , No rollback .

Redis Why rollback is not supported rollback？

• Redis The reason for the operation failure can only be syntax error or wrong data type operation , These are problems that can be found during development , Will not enter the production environment , Therefore, no rollback is required .
• Redis The interior design advocates simplicity and high performance , Supporting the ability to roll back transactions can lead to design complexity , This is related to Redis Against the original intention of , Therefore, rollback capability is not required .
• Redis The application scenario is obviously not designed for high reliability and strong consistency of data storage , It is designed for high performance of data access , Designers have partially abandoned atomicity for simplicity and high performance .

（2） Isolation, ： A transaction is a separate isolation operation , There is no concept of isolation level , The commands in the transaction queue will not be actually executed before they are committed . In business , All commands are serialized , To execute in order . Transaction is in the process of execution , Command requests sent by other clients will not be inserted into the transaction execution command sequence .

（3） persistence ： If Redis When running in a particular persistence mode , Transactions are also persistent .

3、Redis Wrong handling of transactions ：

If there is an error in the execution of a command in a transaction ,Redis How will it be handled ？ Answer that question , First, you need to know what causes the command execution error .

（1） Grammar mistakes ：

Syntax error means that the command does not exist or the number of command parameters is incorrect . such as ：

redis＞MULTI
OK
redis＞SET key value
QUEUED
redis＞SET key
(error)ERR wrong number of arguments for 'set' command
redis＞ errorCOMMAND key
(error) ERR unknown command 'errorCOMMAND'
redis＞ EXEC
(error) EXECABORT Transaction discarded because of previous errors.

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With the MULTI The command was executed 3 An order ： One is the right command , Successfully joined the transaction queue ; The other two commands have syntax errors . And as long as there is a syntax error in one command , perform EXEC After the command Redis It will directly return an error , Can't even execute commands with correct syntax .

Here's a little bit of attention ： Redis 2.6.5 Previous versions ignored commands with syntax errors , Then execute other syntactically correct commands in the transaction . In this case ,SET key value Will be performed ,EXEC The command will return a result ：1) OK.

（2） Running error ：

Run error refers to an error that occurs when a command is executed , For example, using hash type commands to manipulate keys of collection type , This error occurs before actual execution Redis It's impossible to discover , So in a transaction, such an order will be Redis Accept and execute . If a command in the transaction appears Running error , Other commands in the transaction will continue to be executed （ Including the command after the error command ）, Examples are as follows ：

redis＞MULTI
OK
redis＞SET key 1
QUEUED
redis＞SADD key 2
QUEUED
redis＞SET key 3
QUEUED
redis＞EXEC
1) OK
2) (error) ERR Operation against a key holding the wrong kind of value
3) OK
redis＞GET key
"3"

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It can be seen that although SADD key 2 There was a mistake , however SET key 3 Still implemented .

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