Linux server development, MySQL transaction principle analysis

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1. What is the business

A transaction is going from one consistency state to another consistency state .

sentence

• START TRANSACTION | BEGIN
• COMMIT
• ROLLBACK
• SAVEPOINT identifier
• ROLLBACK TO SAVEPOIN

2.ACID characteristic

Atomicity （A）

Commit transactions or do all , Either not .

Uniformity （C）

A transaction unit can only be visible by other transactions after committing transactions .

Isolation, （I）

Multiple connections process data at the same time , Don't affect each other . To improve performance , Through granularity lock , Set different isolation levels .

persistence （D）

DML Operations are persisted , Even if it goes down , Data can be retrieved according to the log .

3. Isolation level

READ UNCOMMITTED

Read uncommitted , Reads at this level are unlocked , Write and lock it , Write locks release locks after a transaction is committed or rolled back .

READ COMMITTED

Read submitted RC, Support... From this level MVCC, That is, to provide consistent, unlocked reads ; At this time, read the historical snapshot data ; Read the latest data of the historical version under this isolation level , So read the submitted data .

REPEATABLE READ

Repeatable RR, Support MVCC, At this time, the read operation reads the version data at the beginning of the transaction .

SERIALIZABLE

Serialization , Support MVCC, The highest level of isolation .

command ：

– Set level isolation
SET GLOBAL |SESSION TRANSACTION ISOLATION LEVE REPEATABLE READ;
SET @@tx_isolation=‘REPEATABLE READ’;
SET @@global.tx_isolaction =‘REPEATABLE READ’;
– View global level isolation
SELECT @@global.tx_isolation;
– View the current level of isolation
SELECT @@session.tx_isolation;
SELECT @@tx_isolation;
– Manually add... To the reading S Read the lock
SELECT … LOCK IN SHARE MODE;
– Manually add... To the reading X Write lock
SELECT …FOR UPDATE;
– View the current lock information
SELECT &FROM information_schema.innodb_locks;

S Shared lock ：

Lock added by transaction read operation , Lock every line , Only serialization will add S lock .

x Exclusive lock ：

The transaction deletes or updates the lock added , Lock on a line . This lock is added to all four isolation levels .

IS Intention sharing lock ：

Intention lock is to tell other things , This form is in use . Function exclusion table lock reading . Insert with IS, add X. Use exclusive locks indirectly .

IX Intention exclusive lock ：

Intention locks and read-write locks at the full table level will conflict , Intention will only conflict with row lock , Row lock conflicts with row lock .

GAP LOCK Clearance lock and NEXT-KEY Lock these two locks only in READ REPEATE Exists under level . The clearance lock locks a range , But not the record itself , Fully open section NEXT-KEY It's a record lock + Clearance lock , Lock a range and record itself , Left open right closed interval . These two locks often cause deadlocks .
INSERT INTENTION LOCK, For example 1,7 Row insertion , If there is an intention to insert a lock, it can be inserted simultaneously 3,4. Improve the concurrency performance of the insertion gap .
AUTO INCR-LOCK Low probability of self incrementing lock B+ The split of the tree , Improved self growth value insertion performance .

Dirty reading

Business A Read another uncommitted transaction .

It can't be read repeatedly

Business A You can read about affairs B Data submitted , It usually happens in a transaction that reads data different from the transaction .

Fantasy reading

Read the same range of data twice , Result set （ More or different data ） Dissimilarity . Non repeatable reading refers to row data , Unreal reading refers to the scope .
Locking before a transaction starts is a persistent lock , The lock is released only when the transaction is committed or rolled back .

Lost update

Both transactions are written , It is divided into commit overrides and rollback overrides . Rollback overwrite has been rejected by the database .

Look at deadlocks

– Turn on standard monitoring

• CREATE TABLE innodb_monitor(a INT) ENGINE=INNODB;
  – Turn off standard monitoring
• DROP TABLE innodb_monitorl
  – Turn on the lock monitor
• CREATE TABLE innodb_lock_monitor ( INT ) ENGINE=INNOB;
  – Turn off lock monitoring
• DROP TABLE innodb_lock_monitor

– Turn on standard monitoring

• set GLOBAL innodb_status_output_ON;
  – Turn off standard monitoring
• set GLOBAL innodb_status_status_output=OFF;
  – Turn on the lock monitor
• set GLOBAL innodb_status_outpus_locks=ON;
  – Turn off lock monitoring
• set GLOBAL innodb_status_output_locks=OFF;
  – Record deadlock information in the error log
• set GLOBAL innodb_print_all_deadlocks=ON;