Redis' transaction mechanism

When it comes to business , I believe we are all familiar , The transaction ACID Four characteristics , It is also often asked during the interview , But in general , We may think of transactions in traditional relational databases , Actually ,Redis It also provides a transaction mechanism , This blog will explain Redis Transaction mechanism of .

One 、 Transaction presentation

Redis Transaction provides a way to package multiple command requests , And then one time 、 A mechanism for executing multiple commands sequentially .

During the execution of the transaction , The server does not interrupt transactions to execute command requests from other clients , It will complete the execution of all commands in the transaction , Then you can process the command requests of other clients .

The figure below shows a Redis The execution of a transaction ：

It can be seen that , Business to MULTI Command start , Then put multiple commands into the transaction , Finally by EXEC The command submits this transaction to the server for execution .

Two 、 Transaction implementation principle

A transaction will experience the following from the beginning to the end 3 Stages ：

• The transaction start
• Order to join the team
• Transaction execution

2.1 The transaction start

MULTI The execution of the command marks the beginning of the transaction .

After executing the order , Client state flags Property will open REDIS_MULTI identification , Indicates that the client switches from non transactional state to transactional state .

2.2 Order to join the team

When a client is in a non transactional state , The command sent by this client will be executed by the server immediately ：

When a client is in a transactional state , The command sent by this client , Whether the server will execute immediately , Divided into the following 2 In this case ：

• If the command sent by the client is MULTI、EXEC、WATCH、DISCARD One of the four commands 1 individual , The server will execute this command immediately .
• If the command sent by the client is the above 4 Commands other than commands , The server will not execute this command immediately , Instead, put it in the transaction queue , And back to the client QUEUED reply .
  

The above process can be represented by the following flow chart ：

First of all DISCARD command , This command is used to cancel the transaction , Give up executing all commands within the transaction block , As shown below ：

Then lift the transaction queue , Every Redis Each client has its own transaction state , The transaction state is stored in the client state mstates In the attribute ：

The transaction state contains 1 Transaction queues and 1 Number of queued orders , As shown below ：

The transaction queue is a multiCmd An array of types , Every... In the array multiCmd Structure stores information about a queued command , such as ：

• Pointer to the command implementation function , Such as GET command 、SET command
• Arguments to the command
• Number of parameters

Transaction queue with fifo （FIFO） The way to keep the order to join the team .

2.3 Transaction execution

When a client in a transactional state executes EXEC On command , The server will traverse the transaction queue of this client , Execute all commands saved in the queue （ In first in first out order ）, Then the result of executing the command is returned to the client at one time .

3、 ... and 、WATCH Command implementation principle

WATCH The command is used to monitor any number of database keys , And in EXEC When the order is executed , Detect whether the monitored key is modified , If it's modified , The server will refuse to execute the transaction , And return an empty reply to the client .

For better understanding , Let's make a demonstration , First , We open the client 1, perform WATCH Command monitor key “name”, And then start a transaction ：

here , Don't execute EXEC command , Open client 2, Execute the following command to modify “name” Value of key ：

then , On the client side 1 perform EXEC On command , An empty reply will be returned , because “name” The value of the key is on the client 2 Has been modified ：

that ,WATCH What is the principle of command implementation ？ We start from 3 Analyze... In every way ：

• Use WATCH Command monitor database key
• Trigger of monitoring mechanism
• Determine whether the transaction is safe

3.1 Use WATCH Command monitor database key

Every Redis Databases are kept 1 individual watched_keys Dictionaries , The key of this dictionary is a certain quilt WATCH Database key for command monitoring , The value of a dictionary is a linked list , All clients monitoring the corresponding database key are recorded in the linked list .

for instance , If the client 1 Monitoring key “name”, client 2 Monitoring key “age”, that watched_keys The data stored in the dictionary is roughly as follows ：

If the client at this time 3 Executed the following WATCH command ：

that watched_keys The data stored in the dictionary becomes ：

3.2 Trigger of monitoring mechanism

So here comes the question , since watched_keys The dictionary is stored by WATCH Command monitor key , So how is the monitoring mechanism triggered ？

The answer is all commands to modify the database , such as SET、LPUSH、SADD etc. , After the implementation, they will be right watched_keys Dictionary check , If a client is monitoring the key just modified by the command , Then all clients that monitor this key REDIS_DIRTY_CAS The logo will be opened , Indicates that the transaction security of the client has been compromised .

Pictured above is an example , If key “name” The value of is modified , So the client 1、 client 3 Of REDIS_DIRTY_CAS The logo will be opened .

3.3 Determine whether the transaction is safe

The last critical step is , When the server receives a client sent EXEC On command , The server will depend on whether the client is open REDIS_DIRTY_CAS Flag to decide whether to execute the transaction , The flow chart of judgment is as follows ：

Four 、 Example of transaction execution failure

First of all, let's see No 1 An example , This transaction was rejected by the server due to an error in the command queue , All commands in the transaction will not be executed ：

Look at the first 2 An example , A nonexistent command appeared when the transaction was queued , The server will refuse to execute this transaction ：

Look at the first 3 An example ,RPUSH An error is reported during the execution of the order , But the subsequent orders continue to be executed , And the previously executed commands have not been affected ：

This example also shows Redis Transaction does not support rollback mechanism .

5、 ... and 、 summary

Redis Transaction provides a way to package multiple commands , And then one time 、 A mechanism for orderly execution ,

Its principle is that multiple commands will be queued into the transaction queue , Then press first in, first out （FIFO） Sequential execution ,

And the transaction will not be interrupted during execution , When all the commands in the transaction queue have been executed , It's the end of the business .

from ：https://www.cnblogs.com/zwwhnly/p/13395106.html