Detailed explanation of transaction isolation level

Four isolation levels

SQL The standard defines four isolation levels ：

• READ-UNCOMMITTED( Read uncommitted )： Lowest isolation level , Allow read of uncommitted data changes , It may cause changes to be lost 、 Dirty reading 、 Phantom or unrepeatable reading . Can pass " Exclusive write lock " Solve the problem of missing modifications .
• READ-COMMITTED( Read committed )： Allow to read data submitted by concurrent transactions , Can prevent dirty reads and lost modifications , But phantom or unrepeatable reads can still occur .
• REPEATABLE-READ( Repeatable )： Multiple reads of the same field are consistent , Unless the data is modified by the transaction itself , Can prevent dirty and unrepeatable read , But phantom reading can still happen .
• SERIALIZABLE( Serializable )： Highest isolation level , Completely obey ACID Isolation level . All transactions are executed one by one , In this way, there is no interference between transactions , in other words , This level prevents dirty reads 、 Unrepeatable reading and phantom reading .

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Isolation level	Dirty reading	It can't be read repeatedly	Fantasy reading
READ-UNCOMMITTED	√	√	√
READ-COMMITTED	×	√	√
REPEATABLE-READ	×	×	√
SERIALIZABLE	×	×	×

Problems caused by different isolation levels

Multiple transactions run concurrently , Often operate the same data to complete their own tasks （ Multiple users operate on the same data ）. Concurrency is necessary , But it may cause the following problems .

• Dirty reading （Dirty read）: When a transaction is accessing data and modifying the data , This modification has not yet been committed to the database , At this time, another transaction also accesses the data , And then I used this data . Because this data is not submitted yet , So the data read by another transaction is “ Dirty data ”, basis “ Dirty data ” The operation may not be correct .
• Missing changes （Lost to modify）: When a transaction reads a data , Another transaction also accesses the data , After modifying the data in the first transaction , The second transaction also modifies the data . In this way, the modification result in the first transaction will be lost , So it's called lost modification . for example ： Business 1 Read data from a table A=20, Business 2 Also read A=20, Business 1 modify A=A-1, Business 2 Also modify A=A-1, final result A=19, Business 1 The modification of is lost .
• It can't be read repeatedly （Unrepeatableread）: Reading the same data multiple times in a transaction . Before the end of the business , Another transaction also accesses the data . that , Between two reads in the first transaction , Due to the modification of the second transaction, the data read by the first transaction twice may be different . This happens when the data read twice in a transaction is different , So it's called unrepeatable reading .
• Fantasy reading （Phantom read）: Unreal reading is similar to nonrepeatable reading . It happens in a transaction （T1） Read a few lines of data , Then another concurrent transaction （T2） When some data is inserted . In the subsequent query , The first thing （T1） There will be more records that don't exist , It's like an illusion , So it's called Unreal reading .

The difference between nonrepeatable reading and unreal reading ：
The key point of non repeatable reading is to modify. For example, read a record many times and find that the values of some columns have been modified , The key point of magic reading is to add or delete a record. For example, read a record many times and find that the number of records increases or decreases .

MySQL What is the default isolation level for ?

MySQL InnoDB The default isolation level supported by the storage engine is REPEATABLE-READ（ Can be reread ）. We can go through SELECT @@tx_isolation; Order to see ,MySQL 8.0 The order was changed to SELECT @@transaction_isolation;

mysql> SELECT @@tx_isolation;+-----------------+| @@tx_isolation  |+-----------------+| REPEATABLE-READ |+-----------------+

MySQL InnoDB Of REPEATABLE-READ（ Can be reread ） It's not guaranteed to avoid unreal reading , Lock reading is needed to ensure . And the mechanism used for this locking degree is Next-Key Locks.
Because the lower the isolation level , The fewer locks the transaction requests , So the isolation level of most database systems is READ-COMMITTED( Read submissions ) , But what you need to know is InnoDB The storage engine uses... By default REPEATABLE-READ（ Can be reread ） There will be no loss of performance .
InnoDB Storage engine in Distributed transactions In general, we use SERIALIZABLE( Serializable ) Isolation level .
Expand the ( The following is excerpted from 《MySQL Technology insider ：InnoDB Storage engine ( The first 2 edition )》7.7 Chapter )：
InnoDB The storage engine provides the ability to XA Transaction support , And pass XA Transaction to support the implementation of distributed transaction . Distributed transaction refers to allowing multiple independent transaction resources （transactional resources） Participate in a global transaction . Transaction resources are usually relational database systems , But it can also be other types of resources . A global transaction requires that all participating transactions in it either commit , Either roll back , This is for the original ACID The demands have been raised again . in addition , When using distributed transactions ,InnoDB The transaction isolation level of the storage engine must be set to SERIALIZABLE.

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