Brief introduction to mvcc

MVCC（Multi-Version Concurrency Control） Multi version concurrent control .

We are learning about MVCC Before , First, let's learn about some common locks .

Read the lock ： Also called Shared locks 、S lock , If business T For data objects A add S lock , The transaction T You can read A But it can't be modified A, Other affairs can only be right again A Add S lock , Instead of X lock , until T Release A Upper S lock . This ensures that other things can be read A, But in T Release A Upper S You can't do it before you lock it A Make any changes .

Write lock ： It's also called exclusive lock 、X lock . If business T For data objects A add X lock , Business T You can read A You can also modify A, Other affairs can no longer be right A Add any locks , until T Release A The lock on the . This ensures that the rest of the business is T Release A The lock on can't be read or modified before A.

Table locks ： The object of operation is the data table .Mysql Most lock strategies support , It is a locking strategy with the lowest system overhead but the lowest concurrency . Business t Lock the whole watch , Other transactions are readable and not writable , If write lock is added , Then the addition, deletion and modification of other affairs are not allowed .

Row-level locks ： The operand is a row in the data table . yes MVCC Technology is used more . Row level lock has a high cost to the system , But it's better to deal with high concurrency .

MVCC Most transaction engines that support row locking , No longer simply use row lock to control database concurrency , Instead, combine the row lock of the database with multiple versions of the row , It's just a small expense , You can read without locking , So as to greatly improve the concurrency performance of the database system .

MVCC working process

InnoDB Of MVCC, This is achieved by saving two hidden columns behind each row of records . These two columns , A created version number that holds the line , A deleted version number that saves the row . Every time you start a new business , The system version number will increase automatically . The system version number at the start of the transaction will be the version number of the transaction , It is used to compare with the version number of each line of records found .

Innodb Hidden columns in

Innodb adopt undo log Save a snapshot of the information of the old version of the changed row . InnoDB Three hidden columns are added for each row of data in the internal implementation of MVCC.