MySQL query timeout control

mysql Query timeout control

mysql The communication between client and server can adopt A long connection And Short connection Two ways .

• Short connection The number of suitable clients is uncertain Small amount of data interaction . Release immediately after the interaction tcp link . Do not occupy the link
• A long connection Suitable for a fixed number of clients Scenarios with frequent data interaction save tcp Frequent creation of disconnection overhead

Generally, our servers are connected with mysql The second method is adopted . Server side And mysql The server Maintain a connection pool . hold n Multiple long connections .
If a long connection is used without any operation on the database for a long time , So in timeout After value ,mysql server Will close the connection

There is a scene dba Worry about slow queries sent by clients . Therefore, it is necessary to establish a database overload protection mechanism . That is, for the submitted query Apply to set a reasonable execution time in advance .
Control the performance overhead of slow queries , So as to eliminate the hidden dangers that affect the service quality during the peak period .

show global variables like 'wait_timeout';

wait_timeout 
connect_timeout
interactive_timeout
long_query_time
max_execution_time
net_read_timeout  
net_write_timeout
wait_timeout

Choose some important explanations

• wait_timeout connect_timeout

stay MYSQL In the default settings of , If a database connection exceeds 8 Hours are not used ( idle 8 Hours ), The server will disconnect this connection , Subsequent query operations on this connection will fail .

• net_read_timeout The default is 30S
• net_write_timeout The default is 60S

The number of seconds to wait for more data from a connection before aborting the read.
When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort.
When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort.
See also replica_net_timeout and slave_net_timeout.

These two parameters control the abnormal timeout caused by the network . such as server In from client The client reads a large amount of data , Reading and reading, I suddenly find that I can't read , No end identifier was encountered ,
In this case ,server Waiting for the net_read_timeout The following data has not been read in seconds , Then the server will terminate this read operation . Note that this operation is terminated Instead of disconnecting a long connection ;
Or when server select A large amount of data is sent to the client , Hair, hair, hair , I suddenly found that I couldn't move , The client does not receive , The data has not been sent yet ,
At this time server Waiting for the net_write_timeout Seconds later, the write operation is terminated ..

!!! But our goal is ： Prevent slow query from killing the server . Instead of solving the problem of network sending and receiving .

• max_execution_time

Query statement execution timeout control

There are two ways to set the client

• _, err := data.MySQLConn.Exec("SET max_execution_time=1")
• SELECT /*+ MAX_EXECUTION_TIME(10) */ * FROM prop where sleep(10)

To verify this result, we need to use sleep function

• When sleep returns normally (without interruption), it returns 0:
• This statement is interrupted using KILL QUERY from another session: return 1
• This statement is interrupted by timing out: return 1
• When SLEEP() is only part of a query that is interrupted, the query returns an error:

eg.1

SET max_execution_time=1
`select *,sleep(1) from prop where id=2` //  Can be in select The last observation of the result sleep Return value .

Note that this usage mode does not time out err Suggestive . because sleep(1) stay 1ms The interrupt result is returned in 1. This is equivalent to using sleep(1) The return value of . Error details are masked .

eg.2

exec:SET max_execution_time=1    
exec:SELECT * from prop where SLEEP(1);

And this way of use , You can return the error to the client Query execution was interrupted, maximum statement execution time exceeded

also select Statement actual sleep Time and where The number of conditionally filtered items is related . actual sql execution time = sleep(10) * len(result) + Actual query time

Reference documents

• sleep Function introduction official documentation https://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html
• https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html
• https://dev.mysql.com/doc/refman/8.0/en/error-lost-connection.html
• qa https://stackoverflow.com/questions/71261028/how-to-set-mysql-max-execution-time-property-use-go