MYSQL transactions

Transactions

Transaction:
One or more SQL statements form an execution unit, which is either all executed or not executed at all

1, ACID attributes

Atomicity

Atomicity means that a transaction is an indivisible unit of work, and the operations in the transaction either all occur or none of them occur.

Consistency

The transaction must be in a consistent state before and after execution.For example, accounts a and b have a total of 1,000 yuan. After the transfer between the two people succeeds or fails, the sum of their accounts is still 1,000.

ISOLATION

Transaction isolation means that the execution of a transaction cannot be interfered by other transactions, that is, the operations and data used within a transaction are isolated from other concurrent transactions, and concurrently executed transactions cannot be mutuallyinterference.

Durability

Durability means that once a transaction is committed, its changes to the data in the database are permanent, and subsequent operations and database failures should not have any impact on it

2. Common concurrency issues

For multiple transactions running at the same time, when these transactions access the same data in the database, if the necessary isolation mechanism is not adopted, it will lead to various concurrency problems:

Dirty read: A transaction reads data that was not committed by another transaction

For two transactions T1, T2, T1 reads fields that have been updated by T2 but not yet committed. After that, if T2 rolls back, the content read by T1 is temporary and invalid.
Not repeatableRead: In the process of a transaction reading a row of data multiple times, another transaction modifies the data and commits it, resulting in different data read by the current transaction

For two transactions T1, T2, T1 reads a field, then T2 updates the field. After that, T1 reads the same field again, and the value is different.

Phantom read: When one transaction reads data in a certain range, another transaction inserts new records, causing the current transaction to read more data

For two transactions T1, T2, T1 reads a field from a table, then T2 inserts some new rows into that table. After that, if T1 reads the same table again, there will be morea few lines.

3. Isolation

Isolation of database transactions: The database system must have the ability to isolate and run various transactions concurrently, so that they will not affect each other and avoid various concurrency problems.
Isolation level: The degree to which a transaction is isolated from other transactions is calledIsolation level. The database specifies a variety of transaction isolation levels. Different isolation levels correspond to different interference levels. The higher the isolation level, the better the data consistency, but the weaker the concurrency

Set the isolation level to solve concurrency problems

The 4 transaction isolation levels provided by the database:
Isolation level

READ UNCOMMITTED

Allows transactions to read changes that have not been committed by other transactions, dirty reads,
Non-repeatable reads and phantom reads problems will occur
(If transaction 1 modifies the data and does not commit, transaction 2 reads data changesinto the modified data,
If transaction 1 is rolled back and transaction 2 reads the data again and becomes the original value [the problem of dirty read, non-repeatable read and phantom read]

READ COMMITTED

Only allow transactions to read changes that have been committed by other transactions [avoid dirty reads]
Dirty reads can be avoided, but non-repeatable reads and phantom reads still exist
(If transaction 1 modifies dataIf it is not committed, the data read by transaction 2 is the data that has been submitted, which can avoid dirty reading, but if the transaction is over, it will be committed,
The value read by transaction 2 again will be different, non-repeatable reading, and the problem of phantom reading still exists.will appear)

REPEATABLE READ

Ensure that a transaction can read the same value from a field multiple times. During the duration of the transaction, prohibiting other transactions from updating this field can avoid dirty reads and non-duplication, but the problem of phantom reads still exists
(If transaction 1 modifies the data, transaction 2 reads the original data [avoiding dirty reads], when transaction 1 commits, transaction 2 reads it again and avoids [repeatable read], only transaction 2After submitting, the data will be updated after other transactions are performed)
(The transaction modifies a certain column of the table [not yet carriage return], and the transaction inserts a column, when the transaction returns, it is found that the affectedIt is four lines, obviously there are three lines in the table,
hallucination problem)

SERIALIZABLE

Ensures that a transaction can read the same rows from a table, during the duration of the transaction, prohibit other transactions from performing insert, update and delete operations on the table,
All concurrency problems can be avoided, but performance is very slow
(Transaction 1 executes the modification operation, at this time the transaction 2 insert operation press Enter and find that it has been waiting, only after transaction 1 commits it will be executed)
Oracle supports 2 transaction isolation levels: READ COMMITED,
SERIALIZABLE.Oracle's default transaction isolation level is: READ

COMMITED

Mysql supports 4 transaction isolation levels. Mysql default transaction isolation level is: REPEATABLE READ

 View the current isolation level: SELECT @@tx_isolation;
 Set the isolation level of the current mySQL connection:
 SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
 Set the global isolation level of the database system:
 SET GLOBAL TRANSACTION ISOLATION LEVEL READ COMMITTED