Detailed explanation of mvcc and its principle

What do you mean mvcc

Multi version concurrency control , If only one transaction or single thread operates on data .

In the current mainstream production environment, it is impossible for only one thread to operate on the database , At this time, it is necessary to ensure data consistency .

So what are the application scenarios in the case of concurrency ？

1. Read
There is no problem , There is no need to add any locks , There is no need for concurrency control

2. Write about
There may be the possibility of loss and renewal

3. Reading and writing
How to guarantee concurrent reading and writing ？ Lock , But it's inefficient ————mvcc, It is to solve the problem of reading and writing data in the case of concurrency

Pre skills ：

1. The current reading ：
What is read is always the latest data recorded in the database

Which operations will cause the current read ：
1.updata
2.insert
3.delete
4.select…for updata
5.select…lock in share mode

2. Read the snapshot ：
What is read is not the latest data , But the data of a historical version

Those will cause snapshot reading ： Save the essence select

Snapshot problem ： If the database 100g Then I'll take pictures of them all ?

Each row of data can have multiple versions , Who modifies which transaction in each version modifies this transaction_id One by one .

So this 100g The snapshot of is not right 100g, Not copy and paste to form a snapshot , Instead, it reads row trxI_ID, The latest version number of the current transaction is read .
because transaction_id Is increasing in order , We put the submitted transactions ID The maximum value of is recorded as up_limit_id

After we start the transaction , We read the data , If I record more than up_limit_id We don't recognize !

What do you mean , Let me give you an example ：

1 To 5 Execute in order , here Q1,Q2 What's the value of ？

---

Why? b yes 20 No 19？？

It says updata Trigger current read , So I read the latest data

Let's continue to analyze step by step ：

Suppose the last version number submitted in the database is 999

First step ：

First, a Started the business , Then the b Open transaction , Then the c Open transaction , And all records up_limit_id=999（ Maximum ）

The second step

Business c Changed the age

The third step

Business b modify ,updata Trigger snapshot read , Directly modify the values at the database level , Get the latest data in the database

Get is 1002 Number , That's business c Version number of , Then modify the version number . The result is

Business b When reading data again , Because of the latest revision 1001 No. modified , So what I got was 1001, Just match yourself , To recognize, so the age is 20 year

Step four ：

a The last version of the transaction record is 999.1001 If not, continue to look up ,1002 Still not in line with , Read 999, In line with , return . Show 18.