Database transaction Transanction

What is business transanction ？

Business , A logical set of operations , The units that make up this set of operations are either all successful , All or nothing .

The role of affairs ： Ensure that a set of operations all succeed or fail .

Business （transanction） Four characteristics of （ ACID ）： Atomicity (Atomicity)、 Uniformity (Correspondence)、 Isolation, (Isolation)、 persistence (Durability).

Atomicity (Atomicity)	All operations in the whole transaction , Or it's all done , Or not at all , It's impossible to stop in the middle	Or they all succeed , Or they all fail
Uniformity (Correspondence)	Before and after transaction start , The database integrity constraint is not broken	The integrity of the data before and after the transaction must be consistent
Isolation, (Isolation)	Isolated state execution transactions , Make them seem like the only operations that the system performs at a given time	Transactions do not interfere with each other
persistence (Durability)	After the transaction is completed , Changes made by the firm to the database are persisted in the database , It will not be rolled back	Persistence means that once a transaction is committed , It changes the data in the database permanently

How things work

Four isolation levels of transactions

Level	name	Isolation level	Dirty reading	It can't be read repeatedly	Fantasy reading	Database default isolation level
1	Read uncommitted	read uncommitted	yes	yes	yes
2	Read submitted	read committed	no	yes	yes	Oracle
3	Repeatable	repeatable read	no	no	yes	MySQL
4	Serialization	serializable	no	no	no

Set isolation level

• Query the current transaction isolation level

        select @@tx_isolation;

• Set the transaction isolation level

        set session transaction isolation level   Isolation level ;
eg:  Set the transaction isolation level to :read uncommitted,read committed,repeatable read,serializable
eg: set session transaction isolation level read uncommitted;

Manual transaction management

Mode one : The way to manually start a transaction 【 master 】

start transaction; Open transaction

commit; Submit

rollback; Roll back

--   scene : zs towards ls Transfer accounts 100 element 
--  No abnormal 
--  Open transaction 
start transaction;
​
-- zs money -100
update account set money = money - 100 where name = 'zs';
-- ls money +100
update account set money = money + 100 where name = 'ls';
​
--  Commit transaction 
commit;
​
​
--  There are abnormal 
--  Open transaction 
start transaction;
​
-- zs money -100
update account set money = money - 100 where name = 'zs';
--  An exception may occur here 
-- ls money +100
update account set money = money + 100 where name = 'ls';
​
--  Roll back the transaction 
rollback;
--  Committing a transaction or rolling back a transaction will end the transaction 

Mode two : Set up MYSQL Auto submit parameters in 【 understand 】

see MYSQL Whether the transaction is automatically committed in

 show variables like '%commit%';

Set the parameter of automatic submission as OFF

  set autocommit = 0;        //0:OFF    1:ON