MySQL has multiple timeouts variables to control its operations. This presentation focus on the purpose of each timeout variables and how it can be used.

1. MySQL Timeout Variables Explained
Presented by,
Sri Sakthivel MD
Senior MySQL DBA
www.mydbops.com info@mydbops.com

2. About Mydbops
● Founded in 2015, HQ in Bangalore India with 150+ customer base across the globe.
● Mydbops is on Database Consulting with core specialization on MySQL and MongoDB Administration and
Support.
● We have expert team with 20+ certified DBA’s providing full time support and currently managing 300+
servers on cloud.
● Mydbops was created with a motto of developing a Devops model for Database administration offering
24*7 expert remote DBA support.
● We help organisations to architect and scale systems in MySQL/Mongo by implementing the advanced
technologies in industry which are completely open source.
● We are leading solution provider for MySQL database in the market for all sort of cloud based deployments
and management.

3. About Me
Sri Sakthivel MD
● MySQL/MariaDB DBA Expert.
● Active MySQL Blogger.
● Interested on DB Fine Tuning and SQL Load Balancers.
● Twitter : @hercules7sakthi
● Hobbies : Bike riding and like good Food.

4. Agenda
● Introduction
● Purpose Of Timeout
● Timeout Variables
● connect_timeout
● delayed_insert_timeout
● have_statement_timeout
● innodb_flush_log_at_timeout
● innodb_lock_wait_timeout

5. Agenda
● innodb_rollback_on_timeout
● interactive_timeout
● lock_wait_timeout
● net_read_timeout
● net_write_timeout
● rpl_stop_slave_timeout
● slave_net_timeout
● wait_timeout
● Conclusion

6. Introduction
- We have n number of timeout variables, which defines the connection or session or
transaction or SQL threads expire length based on its own parameters.
- In MySQL, timeout variables are playing important roles for maintain the
connections and security as well.
- By defining the accurate thresholds for timeout variables we can effectively
maintain the database traffic .

7. Why timeout variables ?
Below are some of the important things we can achieve by setting the proper timeout
thresholds .
● Predetermine the transaction length, which is waiting for the response .
● Can reclaim the resource by cleaning up the completed transactions ( sleeping
transactions ).
● Can keep the server with minimum transactions ( active transactions )
● Can define the session timeouts for security purpose .

8. Timeout Variables
[sakthi@mydbopslabs14]# mysql --login-path=mydbops -e "show global variables like '%timeout%';" -s -N | awk '{
print $1 }'
connect_timeout
delayed_insert_timeout
have_statement_timeout
innodb_flush_log_at_timeout
innodb_lock_wait_timeout
innodb_rollback_on_timeout
interactive_timeout
lock_wait_timeout
net_read_timeout
net_write_timeout
rpl_stop_slave_timeout
slave_net_timeout
wait_timeout

9. connect_timeout
● connect_timeout
default 10 seconds
connect_timeout is an client variable. It defines the number of seconds that
the mysqld waits for a connect packet before display the ERROR .
Example -
[sakthi@mydbopslabs14]# less /etc/my.cnf | grep -i connect_timeout
connect_timeout=10
[sakthi@mydbopslabs14]# date | awk '{ print $4 }' ; mysql --login-path=mydbops ; date | awk '{ print $4 }'
15:52:20
ERROR 2003 (HY000): Can't connect to MySQL server on 'localhost' (110)
15:52:30

10. delayed_insert_timeout
● delayed_insert_timeout
default 300 seconds
delayed_insert_timeout is mainly for MyISAM engine . It defines the timeout
threshold for the INSERT statements with DELAYED option .
INSERT DELAYED means, the insert queue will be populated , when the table is
not use by other threads .
This variable is deprecated , because the delayed insert will not supported in
future release. It is removed on latest GA release ( 8.0 )

11. have_statement_timeout
● have_statement_timeout
default value is YES
have_statement_timeout is the variable for define the timeout is enabled or
not ( y/n ) with max_execution_time. It was introduced in MySQL 5.7
default value is 0
max_execution_time was introduced in MySQL 5.7. This is the variable only
applicable for SELECT statements . It defines the timeout threshold for SELECT
statements .

12. have_statement_timeout
Example -
mysql> select @@have_statement_timeoutG
*************************** 1. row ***************************
@@have_statement_timeout: YES # enabled
1 row in set (0.00 sec)
mysql> set max_execution_time=1; #
setting for 1 second in session level
Query OK, 0 rows affected (0.00 sec)
mysql> pager grep md5sum #
short result
PAGER set to 'grep md5sum'
mysql> select * from information_schema.tables; # trying SELECT
statement
ERROR 3024 (HY000): Query execution was interrupted, maximum statement execution time exceeded # facing
ERROR as it is exceeded

13. innodb_flush_log_at_timeout
● innodb_flush_log_at_timeout
default 1 second
innodb_flush_log_at_timeout defines the how often the Innodb do the fsync
from OS buffer to Disk . It will be slightly based on the innodb_flush_log_at_trx_commit
(2)
innodb_flush_log_at_trx_commit = 2 ; ( every commit the log written happens from log
buffer to OS buffer . fsync happens based on the innodb_flush_log_at_timeout )

14. innodb_lock_wait_timeout
● innodb_lock_wait_timeout
default 50 seconds
innodb_lock_wait_timeout defines the transaction how much time need to be
wait for ROW LOCK . Once the wait time is reached the innodb_lock_wait_timeout
threshold, it will exceed the connection .
Example -
I have taken two session terminals for explain this . mysql1 is session 1 and the mysql2 is
session 2.

15. innodb_lock_wait_timeout
Session 1 -
mysql1> start transaction; # starting
transaction
Query OK, 0 rows affected (0.00 sec)
mysql1> select * from lock where id= 1 for updateG # making row lock for record id = 1
*************************** 1. row ***************************
id: 1
name: aaa
1 row in set (0.00 sec)

16. innodb_lock_wait_timeout
Session 2 -
mysql2> set innodb_lock_wait_timeout=10; # setting innodb_lock_wait_timeout = 10 sec
Query OK, 0 rows affected (0.00 sec)
mysql2> select now()G update lockk set name='zzz' where id=1G select now()G # trying to update
*************************** 1. row ***************************
now(): 2018-04-25 05:11:52
1 row in set (0.00 sec)
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction # ERROR after 10 sec
*************************** 1. row ***************************
now(): 2018-04-25 05:12:03
1 row in set (0.00 sec)

17. innodb_rollback_on_timeout
● innodb_rollback_on_timeout
default is OFF
By default the Innodb rollback the last statement on transaction if timeout
happens. If we enabled innodb_rollback_on_timeout, we can rollback the entire
transactions .
We can avoid the partial success of the transactions .
Transaction Structure -
start transaction / begin ;
SQL statement ;
commit / rollback;

18. innodb_rollback_on_timeout
Without rollback_on_timeout -
start transaction / begin ; ( success ) # rollback will not happen
SQL statement ; ( killed ) # rollback will happen
commit / rollback; # rollback will happen
With rollback_on_timeout -
start transaction / begin ; ( success ) # rollback will happen
SQL statement ; ( killed ) # rollback will happen
commit / rollback; # rollback will happen

19. interactive_timeout
● interactive_timeout
default 28800 seconds
Interactive_timeout is the variable, which defines the timeout for interactive
connections . Which means directly logged client connections.
The timeout will be taken effect only over the idle connections (sleep). When
the timeout happens, it will exit the old thread and make the fresh connection with
new thread.
Example -
mysql> set global interactive_timeout=5;
Query OK, 0 rows affected (0.00 sec)

20. interactive_timeout
mysql> select now()G
*************************** 1. row *************************** # current time
now(): 2018-04-25 05:43:41
ERROR 2006 (HY000): MySQL server has gone away # ERROR after 5
seconds
No connection. Trying to reconnect…
Connection id: 5744052
Current database: *** NONE ***
mysql> select now()G
*************************** 1. row *************************** # after 5 seconds time
now(): 2018-04-25 05:43:46
1 row in set (0.00 sec)

21. wait_timeout
● wait_timeout
default 28800 second
wait_timeout is the variable, which defines the timeout for non-interactive
connections . Which means direct application connections. When the timeout happens,
it will exit the old thread and make the fresh connection with new thread. The timeout
will taken effect only over the idle connections (sleep) .
We can set this variable based on the application timeout threshold for avoid
the MySQL server gone away error .
We will be have the same ERROR what we had in interactive_timeout session .
error - ERROR 2006 (HY000): MySQL server has gone away

22. lock_wait_timeout
● lock_wait_timeout
default 31536000 seconds
lock_wait_timeout is the variable only take effect over the transactions which
acquired METADATA LOCKS.
It will affect all the DDL & DML statements, VIEWS, STORED PROCEDURES,
FUNCTIONS .
Example -
mysql1> lock table lock write;
Query OK, 0 rows affected (0.00 sec)

23. lock_wait_timeout
mysql2> set lock_wait_timeout=10; # setting
threshold 10 seconds
Query OK, 0 rows affected (0.00 sec)
mysql1> show processlist;
| 5744430 | mydbops | localhost | mydbops | Query | 5 |Waiting for table metadata lock
| alter table lock add column lwtoo blob not null |
# executing ALTER
mysql2> select now()G alter table lockk add column lwtoo blob not null G select now()G
*************************** 1. row ***************************
now(): 2018-04-25 06:01:46
1 row in set (0.00 sec)
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction # getting ERROR after
10 seconds
*************************** 1. row ***************************
now(): 2018-04-25 06:01:56
1 row in set (0.00 sec)

24. rpl_stop_slave_timeout
● rpl_stop_slave_timeout
default 31536000 seconds
rpl_stop_slave_timeout is the variable defines the timeout for STOP SLAVE
statements .
When the big transaction is being apply, that time if we trigger the STOP SLAVE
statement , then it will be wait .

25. slave_net_timeout
● slave_net_timeout
default value is 60 seconds
slave_net_timeout is the variable that slave will wait for more data to make
the connection to master . If the expected data is not received , then the slave will broke
the connection and retry again . The retry will be happen based on the
master_connect_retry .

26. net_read_timeout
● net_read_timeout
default 30 seconds
The number of seconds to wait for more data from the connections to perform
the read operation. If the wait is reached the net_read_timeout threshold, then it
will about the connection .

27. net_write_timeout
● net_write_timeout
default value is 30 seconds
net_write_timeout defines the number of seconds to be wait for a block to be
written to a connection before aborting the write .

28. Conclusion
However, MySQL provides more timeout variables for the effective operations . By
tuning the timeout variables based on the production environment settings, we can
achieve the neat and fair performance .

29. Ensure Your Database is Safe hands with
Mydbops DB Support
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