Database transactions (often asked)

One 、 Database transaction concept

Database transactions , It refers to some column operations performed as a single logical unit of work , Or both , Either not

A simple example ： Xiao Ming turns to Xiao Hei 100 Yuan , Can be divided into Xiao Ming minus 100 element , Black plus 100 element . But you can't just perform one of these steps , Or both , Either not . This is the business .

  Two 、 The nature of transactions （ACID）【 a key 】

• Atomicity （Atomicity）： A business （transaction） All operations in , Or complete it all , Or not at all , It doesn't end in the middle . An error occurred during the execution of the transaction , Will be rolled back （Rollback） Status to the beginning of the transaction , It's like this transaction has never been executed . namely , The business is indivisible 、 Irreducible .

• Uniformity （Consistency）： Before and after transaction start , The integrity of the database is not compromised . This means that the data to be written must fully comply with all default constraints 、 trigger 、 Cascade rollback, etc . such as ： Before we transfer money, we add up to 200 element , After that, we always want to 100 element .

• Isolation, （Isolation）： The ability of a database to allow multiple concurrent transactions to read, write, and modify its data simultaneously , Isolation can prevent data inconsistency caused by cross execution when multiple transactions are executed concurrently . Transaction isolation is divided into different levels , Include uncommitted reads （Read uncommitted）、 Submit to read （read committed）、 Repeatable （repeatable read） And serialization （Serializable）.

• persistence （Durability）: After transaction ends , Changes to data are permanent , Even if the system fails, it will not be lost .

3、 ... and 、 What are the problems of concurrent transactions ?( For example )

In a typical application , 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 .

  Example ： Business 1 Inquire about a data a , Business 2 Yes a Modify the data a‘ , Business 1 Query again a —>a’, Business 2 Roll back a’--->a, Business 1 Next, all operations are wrong

• 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 .

  Example ： Business 1 Inquire about a data a , Business 2 Yes a Modify the data to a‘ Submit , Business 1 Inquire about a data —>a’, Business 1 The two queries are different

• 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 .

  Example ： Business 1 Inquire about surface a Number of all data pieces n strip , Business 2 Yes a Add table Submit , Business 1 Query again n+1 strip , The number of data pieces before and after is inconsistent

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 .

Four 、 What are the transaction isolation levels ?MySQL The default isolation level for is ?

SQL The standard defines four isolation levels ： Default REPEATABLE-READ( Repeatable )

• READ-UNCOMMITTED( Read uncommitted )： Lowest isolation level , Allow read of uncommitted data changes , Can cause dirty reading 、 Phantom or unrepeatable reading .

• READ-COMMITTED( Read committed )： Allow to read data submitted by concurrent transactions , Can prevent dirty reading , 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 .