One means of limiting use of MySQL server resources is to set the max_user_connections system variable to a non-zero value. However, this method is strictly global, and does not allow for management of individual accounts. In addition, it limits only the number of simultaneous connections made using a single account, and not what a client can do once connected. Both types of control are interest to many MySQL administrators, particularly those working for Internet Service Providers.

In MySQL 5.1, you can limit the following server resources for individual accounts:

Any statement that a client can issue counts against the query limit. Only statements that modify databases or tables count against the update limit.

It is also possible to limit the number of simultaneous connections to the server on a per-account basis.

An account in this context is a single row in the user table. Each account is uniquely identified by its User and Host column values.

As a prerequisite for using this feature, the user table in the mysql database must contain the resource-related columns. Resource limits are stored in the max_questions, max_updates, max_connections, and max_user_connections columns. If your user table doesn't have these columns, it must be upgraded; see Section 5.5.2, “mysql_upgrade — Check Tables for MySQL Upgrade”.

To set resource limits with a GRANT statement, use a WITH clause that names each resource to be limited and a per-hour count indicating the limit value. For example, to create a new account that can access the customer database, but only in a limited fashion, issue this statement:

mysql> GRANT ALL ON customer.* TO 'francis'@'localhost'
    ->     IDENTIFIED BY 'frank'
    ->     WITH MAX_QUERIES_PER_HOUR 20
    ->          MAX_UPDATES_PER_HOUR 10
    ->          MAX_CONNECTIONS_PER_HOUR 5
    ->          MAX_USER_CONNECTIONS 2;

The limit types need not all be named in the WITH clause, but those named can be present in any order. The value for each per-hour limit should be an integer representing a count per hour. If the GRANT statement has no WITH clause, the limits are each set to the default value of zero (that is, no limit). For MAX_USER_CONNECTIONS, the limit is an integer indicating the maximum number of simultaneous connections the account can make at any one time. If the limit is set to the default value of zero, the max_user_connections system variable determines the number of simultaneous connections for the account.

To set or change limits for an existing account, use a GRANT USAGE statement at the global level (ON *.*). The following statement changes the query limit for francis to 100:

mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
    ->     WITH MAX_QUERIES_PER_HOUR 100;

This statement leaves the account's existing privileges unchanged and modifies only the limit values specified.

To remove an existing limit, set its value to zero. For example, to remove the limit on how many times per hour francis can connect, use this statement:

mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
    ->     WITH MAX_CONNECTIONS_PER_HOUR 0;

Resource-use counting takes place when any account has a non-zero limit placed on its use of any of the resources.

As the server runs, it counts the number of times each account uses resources. If an account reaches its limit on number of connections within the last hour, further connections for the account are rejected until that hour is up. Similarly, if the account reaches its limit on the number of queries or updates, further queries or updates are rejected until the hour is up. In all such cases, an appropriate error message is issued.

Resource counting is done per account, not per client. For example, if your account has a query limit of 50, you cannot increase your limit to 100 by making two simultaneous client connections to the server. Queries issued on both connections are counted together.

The current per-hour resource-use counts can be reset globally for all accounts, or individually for a given account:

Counter resets do not affect the MAX_USER_CONNECTIONS limit.

All counts begin at zero when the server starts; counts are not carried over through a restart.