background
The new project is about to go online. When testing the function, I found that the page was stuck after clicking the button. At first I thought it was a network problem, but the page stuck and reproduced 100%. Check the background log and found that it was locked while executing the update statement.
Query via sql
select * from information_schema.innodb_trx;
We found that it was caused by updating the table sys_sn_rule, so let's sort out the code and see why LOCK_WAIT appears.
Cause investigation
By looking at the interface call, we locate a method, here I will simplify the method. The simplified code is as follows:
@Override
@Transactional(rollbackFor = Exception.class)
public Result funA() {
//更新表table1;
funB();
...
}
public void funB() {
//更新表table1;
....
}
The problem lies in that funA calls funB, and the two methods operate on the same piece of data in table1 at the same time. There will be two transactions here. When the data is updated, both transactions will be waiting for each other to close the transaction, which will deadlock at that time. Let's illustrate with a picture:
Deadlock diagram
As shown in the figure above, when funA is executed, table 1 will be updated, transaction A will be opened before the table is updated, and this row of data will be locked when the table is updated (in order to protect data consistency). Next, call funB, open transaction B, and update table table1. Because the row of table1 has been locked, transaction B needs to wait for the lock to be released before it can continue. However, if transaction A wants to be closed, it needs to be closed after funA is executed. FunB is called in funA, and funB has to wait for table1 to release the lock before it can be executed. This leads to an endless loop.
For the knowledge of database locking, you can read this article:
“The Road to Solving Deadlock-Learning Transaction and Isolation Level-aneasystone's blog
(https://www.aneasystone.com/archives/2017/10/solving-dead-locks-one.html)
”
Solution
We can use multi-threading to solve:
@Override
@Transactional(rollbackFor = Exception.class)
public Result funA() {
//更新表table1;
taskExecutor.execute(() -> {
try {
Thread.sleep(5 * 1000);
funB();
} catch (InterruptedException e) {
e.printStackTrace();
}
});
...
}
public void funB() {
//更新表table1;
....
}
With multithreading, the two transactions are in two different threads, and there will be no loop waiting.
Deadlock Diagram-Multithreading
After the code was modified, the test passed smoothly.
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