One means of restricting client 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, setting
max_user_connections does not enable management of individual accounts. Both types of control are of interest to MySQL administrators.
To address such concerns, MySQL permits limits for individual accounts on use of these server resources:
- The number of queries an account can issue per hour
- The number of updates an account can issue per hour
- The number of times an account can connect to the server per hour
- The number of simultaneous connections to the server by an account
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 corresponds to a row in the
mysql.user table. That is, a connection is assessed against the
Host values in the
user table row that applies to the connection. For example, an account
'usera'@'%.example.com' corresponds to a row in the
user table that has
Host values of
%.example.com, to permit
usera to connect from any host in the
example.com domain. In this case, the server applies resource limits in this row collectively to all connections by
usera from any host in the
example.com domain because all such connections use the same account.
Before MySQL 5.0.3, an “account” was assessed against the actual host from which a user connects. This older method of accounting may be selected by starting the server with the
--old-style-user-limits option. In this case, if
usera connects simultaneously from
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.
To establish resource limits for an account at account-creation time, use the
CREATE USER statement. To modify the limits for an existing account, use
ALTER USER. (Before MySQL 5.7.6, use
GRANT, for new or existing accounts.) Provide a
WITH clause that names each resource to be limited. The default value for each limit is zero (no limit). For example, to create a new account that can access the
customer database, but only in a limited fashion, issue these statements:
CREATE USER 'francis'@'localhost' IDENTIFIED BY 'frank'->
WITH MAX_QUERIES_PER_HOUR 20->
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. For
MAX_USER_CONNECTIONS, the limit is an integer representing the maximum number of simultaneous connections by the account. If this limit is set to zero, the global
max_user_connections system variable value determines the number of simultaneous connections. If
max_user_connections is also zero, there is no limit for the account.
To modify limits for an existing account, use an
ALTER USER statement. The following statement changes the query limit for
francis to 100:
ALTER USER '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:
ALTER USER 'francis'@'localhost' WITH MAX_CONNECTIONS_PER_HOUR 0;
As mentioned previously, the simultaneous-connection limit for an account is determined from the
MAX_USER_CONNECTIONS limit and the
max_user_connections system variable. Suppose that the global
max_user_connections value is 10 and three accounts have individual resource limits specified as follows:
ALTER USER 'user1'@'localhost' WITH MAX_USER_CONNECTIONS 0; ALTER USER 'user2'@'localhost' WITH MAX_USER_CONNECTIONS 5; ALTER USER 'user3'@'localhost' WITH MAX_USER_CONNECTIONS 20;
user1 has a connection limit of 10 (the global
max_user_connections value) because it has a
MAX_USER_CONNECTIONS limit of zero.
user3 have connection limits of 5 and 20, respectively, because they have nonzero
The server stores resource limits for an account in the
user table row corresponding to the account. The
max_connectionscolumns store the per-hour limits, and the
max_user_connections column stores the
MAX_USER_CONNECTIONS limit. (See Section 6.2.2, “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, the server rejects further connections for the account until that hour is up. Similarly, if the account reaches its limit on the number of queries or updates, the server rejects further queries or updates until the hour is up. In all such cases, the server issues appropriate error messages.
Resource counting occurs 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:
- To reset the current counts to zero for all accounts, issue a
FLUSH USER_RESOURCESstatement. The counts also can be reset by reloading the grant tables (for example, with a
FLUSH PRIVILEGESstatement or a mysqladmin reload command).
- The counts for an individual account can be reset to zero by setting any of its limits again. Specify a limit value equal to the value currently assigned to the account.
Per-hour counter resets do not affect the
All counts begin at zero when the server starts. Counts do not carry over through server restarts.
MAX_USER_CONNECTIONS limit, an edge case can occur if the account currently has open the maximum number of connections permitted to it: A disconnect followed quickly by a connect can result in an error (
ER_USER_LIMIT_REACHED) if the server has not fully processed the disconnect by the time the connect occurs. When the server finishes disconnect processing, another connection will once more be permitted.
Source: MySQL online documentation