Transaction overview of tidb

TiDB Support complete distributed transactions , Provide Optimistic business And Pessimism （TiDB 3.0 Introduction in ） Two transaction models . This article mainly introduces the statements involving transactions 、 Optimistic affairs and pessimistic affairs 、 The isolation level of the transaction , And optimistic transaction application side retry and error handling .

General statement

This chapter is introduced in TiDB How to use . The following example will be used to demonstrate the control flow of a simple transaction ：

Bob To give Alice Transfer accounts 20 Yuan , There are at least two operations ：

• Bob The accounts are down 20 element .
• Alice Account increase 20 element .

Transactions can ensure that either of the above two operations are successful , Or they all fail , No money will disappear or appear in vain .

Use  bookshop  In the database  users  surface , Insert some sample data into the table ：

INSERT INTO users (id, nickname, balance) VALUES (2, 'Bob', 200); INSERT INTO users (id, nickname, balance) VALUES (1, 'Alice', 100);

Now? , Run the following transactions and explain the meaning of each statement ：

BEGIN; UPDATE users SET balance = balance - 20 WHERE nickname = 'Bob'; UPDATE users SET balance = balance + 20 WHERE nickname= 'Alice'; COMMIT;

After the above transaction succeeds , The table shall be as follows ：

+----+--------------+---------+ | id | account_name | balance | +----+--------------+---------+ | 1 | Alice | 120.00 | | 2 | Bob | 180.00 | +----+--------------+---------+

Open transaction

To explicitly open a new transaction , Both available  BEGIN  sentence , You can also use  START TRANSACTION  sentence , They have the same effect . grammar ：

BEGIN;

START TRANSACTION;

TiDB The default transaction mode for is pessimistic transaction , You can also specify the open Optimistic business ：

BEGIN OPTIMISTIC;

Turn on Pessimism ：

BEGIN PESSIMISTIC;

If the above statement is executed , At present Session Is in the middle of a transaction , The system will automatically commit the current transaction first , Start a new transaction .

Commit transaction

COMMIT  Statement for commit TiDB All changes made in the current transaction . grammar ：

COMMIT;

Before enabling optimistic transactions , Please make sure that the application can handle  COMMIT  Statement may return an error . If you are not sure how the application will handle , It is recommended to use pessimistic transactions instead .

Roll back the transaction

ROLLBACK  Statement is used to rollback and undo all modifications of the current transaction . grammar ：

ROLLBACK;

Back to the previous transfer example , Use  ROLLBACK  After rolling back the entire transaction ,Alice and Bob The balance of has not changed , All modifications of the current transaction are canceled together .

TRUNCATE TABLE `users`; INSERT INTO `users` (`id`, `nickname`, `balance`) VALUES (1, 'Alice', 100), (2, 'Bob', 200); SELECT * FROM `users`; +----+--------------+---------+ | id | nickname | balance | +----+--------------+---------+ | 1 | Alice | 100.00 | | 2 | Bob | 200.00 | +----+--------------+---------+ BEGIN; UPDATE `users` SET `balance` = `balance` - 20 WHERE `nickname`='Bob'; UPDATE `users` SET `balance` = `balance` + 20 WHERE `nickname`='Alice'; ROLLBACK; SELECT * FROM `users`; +----+--------------+---------+ | id | nickname | balance | +----+--------------+---------+ | 1 | Alice | 100.00 | | 2 | Bob | 200.00 | +----+--------------+---------+

If the client connection is aborted or closed , The transaction will also be rolled back automatically .

Transaction isolation level

Transaction isolation level is the basis of database transaction processing ,ACID  Medium  “I”, namely Isolation, It refers to the isolation of transactions .

SQL-92 The standard defines 4 Kind of isolation level ： Read uncommitted (READ UNCOMMITTED)、 Read submitted (READ COMMITTED)、 Repeatable (REPEATABLE READ)、 Serialization (SERIALIZABLE). For details, see table below. ：

Isolation Level	Dirty Write	Dirty Read	Fuzzy Read	Phantom
READ UNCOMMITTED	Not Possible	Possible	Possible	Possible
READ COMMITTED	Not Possible	Not possible	Possible	Possible
REPEATABLE READ	Not Possible	Not possible	Not possible	Possible
SERIALIZABLE	Not Possible	Not possible	Not possible	Not possible

TiDB Syntax supports setting  READ COMMITTED  and  REPEATABLE READ  Two isolation levels ：

mysql> SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; ERROR 8048 (HY000): The isolation level 'READ-UNCOMMITTED' is not supported. Set tidb_skip_isolation_level_check=1 to skip this error mysql> SET TRANSACTION ISOLATION LEVEL READ COMMITTED; Query OK, 0 rows affected (0.00 sec) mysql> SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; Query OK, 0 rows affected (0.00 sec) mysql> SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; ERROR 8048 (HY000): The isolation level 'SERIALIZABLE' is not supported. Set tidb_skip_isolation_level_check=1 to skip this error

TiDB Implemented snapshot isolation (Snapshot Isolation, SI) Consistency of levels . As with the MySQL bring into correspondence with , It's also called “ Repeatable ”. This isolation level is different from  ANSI Repeatable read isolation level   and  MySQL Repeatable read isolation level . For more details, please read  TiDB Transaction isolation level .