Before MySQL 4.0.2, the only means of limiting use of MySQL server resources is to set the global max_user_connections system variable to a nonzero value. This limits the number of simultaneous connections that can be made by any given account, but places no limits on what a client can do once connected. In addition, this method is strictly global, and does not allow for management of individual accounts. Both types of control are of interest to many MySQL administrators, particularly those working for Internet Service Providers.

Starting from MySQL 4.0.2, you can limit access to the following server resources for individual accounts:

Any statement that a client can issue counts against the query limit (unless its results are served from the query cache). Only statements that modify databases or tables count against the update limit.

An “account” in this context is assessed against the actual host from which a user connects. Suppose that there is a row in the user table that has User and Host values of usera and %.example.com, to allow usera to connect from any host in the example.com domain. If usera connects simultaneously from host1.example.com and host2.example.com, the server applies the account resource limits separately to each connection. If usera connects again from host1.example.com, the server applies the limits for that connection together with the existing connection from that host.

The server limits account resources based on the resource-related columns of the user table in the mysql database: max_questions, max_updates, max_connections, and max_user_connections. If your user table does not have these columns, it must be upgraded; see Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.

To set resource limits for an account, use the GRANT statement (see Section 12.4.1.2, “GRANT Syntax”). Provide a WITH clause that names each resource to be limited. 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;

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).

To modify existing limits for an 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;

The statement modifies only the limit value specified and leaves the account otherwise unchanged.

To remove a 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;

The server stores resource limits for an account in the user table row corresponding to the account. The max_questions, max_updates, and max_connections columns store the per-hour limits. (See Section 5.5.2, “Privilege System Grant Tables”.)

Resource-use counting takes place when any account has a nonzero 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.

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

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