A MySQL server timeout can occur for many reasons, but happens most often when a command is sent to MySQL over a closed connection. The connection could have been closed by the MySQL server because of an idle-timeout; however, in most cases it is caused by either an application bug, a network timeout issue (on a firewall, router, etc.), or due to the MySQL server restarting. Rarely does the
wait_timeout value cause the problem, and changing the value does not fix the problem. For cases where an application fails to close a connection it is no longer using, a low
wait_timeout value can help to avoid hitting
max_connections simply due to "sleeping" idle connections that are not in a transaction and will not be reused.
Follow these steps to resolve the issue:
Login to your server using SSH.
my.cnf (the MySQL configuration file).
sudo vi /etc/my.cnf
Locate the timeout configuration and adjust it to fit your server.
wait_timeout = 28800 interactive_timeout = 28800
The interactive timeout does not affect any web application connections. A high
interactive_timeout but a low
wait_timeout is normal and is the best practice.
Choose a reasonable
wait_timeout value. Stateless PHP environments do well with a 60 second timeout or less. Stateful applications that use a connection pool (Java, .NET, etc.) will need to adjust
wait_timeout to match their connection pool settings. The default 8 hours (
wait_timeout = 28800) works well with properly configured connection pools.
wait_timeout to be slightly longer than the application connection pool's expected connection lifetime. This is a good safety check.
Consider changing the
wait_timeout value online. This does not require a MySQL restart, and the
wait_timeout can be adjusted in the running server without incurring downtime. You would issue
set global wait_timeout=60 and any new sessions created would inherit this value. Be sure to preserve the setting in
my.cnf. Any existing connections will need to hit the old value of
wait_timeout if the application abandoned the connection. If you do have reporting jobs that will do longer local processing while in a transaction, you might consider having such jobs issue
set session wait_timeout=3600 upon connecting.
Save the changes and exit the editor.
Restart MySQL to apply the changes as follows:
sudo /etc/init.d/mysql restart
Once the restart completes, the new changes are applied.
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