Mvcc principle in MySQL

Catalog

What is? mvcc?

What is snapshot reading , And current reading ？

mvcc The role of ？

mvcc Implementation principle of ？

RC,RR Under the inoodb What's the difference ？

What is? mvcc?

MVCC, Full name Multi-Version Concurrency Control, Multi version concurrent control .MVCC It's a method of concurrency control , Generally in the database management system , Realize concurrent access to database , Implement transaction memory in programming language .

What is snapshot reading , And current reading ？

1、 The current reading

select lock in share mode( Shared lock ), select for update ; update, insert,delete( Exclusive lock ) These operations are all a current read
The current read is to read the latest version of the record , When reading, it is also necessary to ensure that other concurrent transactions cannot modify the current record , Will lock the read record
2、 Read the snapshot

Operation without locking (select) That's snapshot reading , Non blocking read without lock ;
The premise of snapshot read is that isolation level is not serial level , Snapshot reads at the serial level degrade to current reads ;
namely MVCC, It can be said that MVCC It's a variant of a line lock , But it's in many cases , Avoid lock operation , Lower the cost
The implementation of snapshot read is based on multi version concurrency control , Since it's based on multiple versions , That is to say, what the snapshot read may not be the latest version of the data , It could be the previous version of history

mvcc The role of ？

mvcc The function of is mainly in the multi-threaded environment , Improve database read and write performance , Solution reading （ Read the snapshot ）- Lock free concurrency control for write problems .

mvcc Implementation principle of ？

mvcc The implementation principle of is through the hidden fields and undolog as well as  readview Realized

Hide fields ：

DB_TRX_ID

6 byte , Of the recently modified transaction id, Record the record created or last modified id.

DB_ROLL_PTR

7 byte , rollback pointer , Point to the previous version of this record for matching undolog, Point to the previous old version .

DB_ROW_ID

6 byte , Hidden primary key , If the data table has no primary key , that inodb It will automatically generate a 6 Bytes of row_id.

uodolog:

uodolog It is called rollback log , It means in progress delete,update,insert Log for easy rollback .

readview：

readview It is a view generated when a transaction performs a snapshot read operation , At the moment when the transaction performs snapshot reading , Will generate a snapshot of the current data system , Record and maintain the current active transactions id, Business id The value of is increasing .

readview Three global attributes in ：

trx_list: A list of values , Used for maintenance readview Generate transactions that are active in the system at the time id

up_limit_id： Record trx_list Transactions in the list id The smallest id

low_limit_id：readview Generate the next transaction that the system has not yet allocated ID

mvcc Implementation process ：

The following is a simulation diagram （ Suppose transaction one is id by 1, And so on ）

The specific comparison rules are as follows ：

1. Compare first DB_TRX_ID（ Of the recently modified transaction id   6）< up_limit_id( Record trx_list Transactions in the list id The smallest id

1) , You can see DB_TRX_ID Records , If it is greater than or equal to, go to the next judgment .（ obviously 6>1 Enter the second judgment ）

2. Then judge DB_TRX_ID(6)>=low_limit_id( Generate the next transaction that the system has not yet allocated ID 7) , If greater than or equal to, it means DB_TRX_ID The location is recorded in readview Only after generation , So it must be invisible to the current transaction , If it is less than , Then proceed to the next judgment .（6<7 Means not readview Those that appear only after generation enter the next judgment ）

3. Judge DB_TRX_ID Whether in an active transaction , If "in" means "in" readview The moment of creation , This transaction is still active , Not yet commit, The modified data cannot be seen in the current transaction , If not, it means that the transaction is readview It started before the generation commit, Then the modification result is visible .

RC,RR Under the inoodb What's the difference ？

Mainly because RC And RR Of readview Generation timing is different , stay RC Under the isolation mechanism of , Each snapshot read generates and gets the latest readview, and RR Is the first snapshot read in the transaction readview, After that readview It's all the same .

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