The default isolation level of MySQL is RR. Why does Alibaba and other large manufacturers change to RC?

Mainly because MySQL The process of master-slave replication is through bin log For data synchronization , and MySQL Early only statement such bin log Format , In this format ,bin log The record is SQL The original text of the sentence .

When the transaction is out of order , This will cause the standby database to be in SQL After playback , The result is inconsistent with the content of the main library .

To solve this problem ,MySQL By default Repetable Read This level of isolation , because stay RR in , When updating the data, the record lock will be added and the gap lock will be added at the same time . This can be avoided .

Before I know MySQL The default isolation level is RR after , For a long time, I thought that no one should modify the default configuration .

But until one day , There was a deadlock on our line , I was in the process of checking , It was found that the isolation level of our database did not use the default RR, It's changed to Read Committed .（ About the deadlock investigation process , You can refer to ：[ A database deadlock troubleshooting process ](https://blog.csdn.net/agonie201218/article/details/122217626）

You can use this command to view the current isolation level of the database ：

select @@tx_isolation;

that , There are questions here , Why did Ali change the database isolation level to RC Well , Is there any thinking behind it ？

RR and RC The difference between

Trying to figure this out , We need to find out first RR and RC The difference between , Analyze their advantages and disadvantages .

Read consistency

Read consistency , Also known as snapshot read . Snapshot is the historical version before the current row of data . Snapshot read is to use snapshot information to display query results based on a certain point in time , Regardless of changes made by other transactions running at the same time .

stay MySQL in , Only READ COMMITTED and REPEATABLE READ These two transaction isolation levels use consistent read .

stay RC in , Each read regenerates a snapshot , Always read the latest version of the line .

stay RR in , The snapshot is taken for the first time in the transaction SELECT Generated when the statement is executed , Only changes made to the data in this transaction will update the snapshot .

In the database RC In this isolation level , And support " Semi consistent reading " , One update sentence , If where Records matched by conditions have been locked , that InnoDB The most recently submitted version of the record will be returned , from MySQL The upper layer determines whether this needs to be locked .

Locking mechanism

Database lock , Under different transaction isolation levels , Different mechanisms are used . stay MySQL in , There are three types of locks , Namely Record Lock、Gap Lock and Next-Key Lock.

Record Lock Represents a record lock , The lock is the index record .

Gap Lock It's the gap lock , The lock is the gap between index records .

Next-Key Lock yes Record Lock and Gap Lock The combination of , Lock the index record and clearance at the same time . His range is left open and right closed .

stay RC in , It will only increase the index Record Lock, Not add Gap Lock and Next-Key Lock.

stay RR in , In order to solve the problem of unreal reading , In support of Record Lock At the same time , And support Gap Lock and Next-Key Lock;

Master slave synchronization

During data master-slave synchronization , In different formats binlog There are also requirements for transaction isolation level .

MySQL Of binlog It mainly supports three formats , Namely statement、row as well as mixed, however ,RC Isolation level only supports row Format binlog. If you specify mixed As binlog Format , So if you use RC, The server automatically uses row Format of logging .

and RR The isolation level also supports statement、row as well as mixed Three .

Why do Internet companies choose to use RC

Enhance concurrency

What is the biggest difference between Internet companies and traditional enterprises ？

High concurrency ！

you 're right , The concurrency of Internet business is much higher than that of traditional enterprises .2020 On the eleventh day of the new year , Order creation peaked at 58.3 Ten thousand brush / second .

Many people ask , What can we do to withstand such a large amount of concurrency . Actually , The optimization behind this can't be finished for hours , Because what we have to do 、 There are so many things you can do .

There is an optimization related to our article today , That is to improve concurrency by modifying the isolation level of the database .

Why? RC Than RR The degree of concurrency is better ？

First ,RC In the process of locking , No need to add Gap Lock and Next-Key Lock Of , Just add row level locks to the records to be modified .

This makes the degree of concurrency higher than RR Much higher .

in addition , because RC And support " Semi consistent reading ", It can greatly reduce the conflict of row lock when updating statements ; For records that do not meet the update conditions , The lock can be released in advance , Increase degree of concurrency .

Reduce deadlocks

because RR This level of transaction isolation increases Gap Lock and Next-Key Lock, This makes the granularity of the lock larger , Then it will increase the probability of deadlock .

Deadlock ： A transaction locks the table A, And then you access the table B; Another transaction locks the table B, And then try to access the table A; At this time, they will wait for each other to release the lock , It leads to deadlock .

summary

This paper introduces some MySQL Database RR and RC Two transaction isolation levels . They are mainly in the locking mechanism 、 There are some differences in master-slave synchronization and consistent reading .

And many big factories , In order to improve concurrency and reduce the probability of deadlock , The isolation level of the database will be changed from the default RR Adjust to RC.

Of course , This is not entirely without problems , use first RC after , You need to solve the problem of fantasy reading by yourself , This is actually ok , Many times, the problem of unreal reading can be ignored , Or it can be solved by other means .

There is also the use of RC When , Out of commission statement Format binlog, This effect is actually negligible , because MySQL Is in 5.1.5 Version starting support row Of 、 stay 5.1.8 Support for mixed, The latter two can replace statement Format .