Summary of process and principle of solution to concurrency problem (form repeated submission problem)

With the help of databases

insert Use unique index update Use Optimism lock version Edition method

This kind of efficiency depends on the ability of database hardware in large amount of data and high concurrency , For non core business
Optimistic lock mode 1： version control + The spin *（ Where does spin reflect ）*
Considering that the probability of these update conflicts is not high . Here, we use database optimistic lock to realize
The principle is based on CAS, Compare and exchange data , If the discovery has been updated , Direct update failed
Then add spin （ Self circulation return ） Then update , Until success . Optimism lies in our belief that the probability of conflict is low
If it happens , Just use a cheap mechanism to quickly find . Fast failure .
Let's discuss how to implement it , Database table GoodsSale Add a new line data_version To record the updated version number of the data . The new table structure is as follows ：

GoodsSaleDao#uodateCount Corresponding mapper Of SQL Statement , The data is updated at the same time data_version = data_version+!, Execute this sql At this time, the row lock has been applied to the data , So this data_version Add 1 The operation of is atomic operation

1. Row lock and table lock

stay mysql Of InnoDB The engine supports row locking , And Oracle Different ,mysql The row locks are loaded through the index , That is, the row lock is added to the row of the index response , If the corresponding SQL The statement doesn't follow the index , It will scan the whole table , Row locks cannot be implemented , Instead, the table lock .

Mysql In the field of innodb The storage engine supports row level locking ,innodb The row lock of is realized by locking the index entries , This means that only when retrieving data through index conditions ,innodb To use the row lock , Otherwise, use the watch lock .

Update the statement based on the current data (UPDATE user set name=‘11111’ where account=‘1’), This condition field account No Index added , So the data table is locked .