One of the common causes of downtime with MySQL is running out of connections. Have you ever seen this error? “ERROR 1040 (00000): Too many connections.” If you’re working with MySQL long enough you surely have. This is quite a nasty error as it might cause complete downtime… transient errors with successful transactions mixed with failing ones as well as only some processes stopping to run properly causing various kinds of effects if not monitored properly.
There are number of causes for running out of connections, the most common ones involving when the Web/App server is creating unexpectedly large numbers of connections due to a miss-configuration or some script/application leaking connections or creating too many connections in error.
The solution I see some people employ is just to increase max_connections to some very high number so MySQL “never” runs out of connections. This however can cause resource utilization problems – if a large number of connections become truly active it may use a lot of memory and cause the MySQL server to swap or be killed by OOM killer process, or cause very poor performance due to high contention.
There is a better solution: use different user accounts for different scripts and applications and implement resource limiting for them. Specifically set max_user_connections:
mysql> GRANT USAGE ON *.* TO 'batchjob1'@'localhost' -> WITH MAX_USER_CONNECTIONS 10;
This approach (available since MySQL 5.0) has multiple benefits:
Security – different user accounts with only required permissions make your system safer from development errors and more secure from intruders
Preventing Running out of Connections – if there is a bug or miss-configuration the application/script will run out of connections of course but it will be the only part of the system affected and all other applications will be able to use the database normally.
Overload Protection – Additional numbers of connections limits how much queries you can run concurrently. Too much concurrency is often the cause of downtime and limiting it can reduce the impact of unexpected heavy queries running concurrently by the application.
In addition to configuring max_user_connections for given accounts you can set it globally in my.cnf as “max_user_connections=20.” This is too coarse though in my opinion – you’re most likely going to need a different number for different applications/scripts. Where max_user_connections is most helpful is in multi-tenant environments with many equivalent users sharing the system.