One 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, setting
max_user_connections does not enable management of individual accounts. Both
types of control are of interest to many MySQL administrators, particularly those working for Internet Service
In MySQL 5.7, you can limit use of the following server resources for individual accounts:
The number of queries that an account can issue per hour
The number of updates that 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
table row that applies to the connection. For example, an account
'usera'@'%.example.com' corresponds to a row in the
user table that has
%.example.com, to permit
usera to connect from any host in the
domain. In this case, the server applies resource limits in this row collectively to all connections by
usera from any host in the
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 accounting may be selected by starting the server with
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
the server applies the limits for that connection together with the existing connection from that host.
To set resource limits for an account, use the
GRANT statement (see Section 126.96.36.199, "
GRANT Syntax"). 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
CREATE USER 'francis'@'localhost' IDENTIFIED BY 'frank';mysql>
GRANT ALL ON customer.* TO 'francis'@'localhost'->
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 existing limits for an account, use a
USAGE statement at the global level (
ON *.*). The following statement
changes the query limit for
francis to 100:
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:
GRANT USAGE ON *.* TO '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 resource limits
GRANT ... TO 'user1'@'localhost' WITH MAX_USER_CONNECTIONS 0;GRANT ... TO 'user2'@'localhost' WITH MAX_USER_CONNECTIONS 5;GRANT ... TO 'user3'@'localhost' WITH MAX_USER_CONNECTIONS 20;
user1 has a connection limit of 10 (the global
max_user_connections value) because it has a 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_connections columns store the
per-hour limits, and the
max_user_connections column stores the
MAX_USER_CONNECTIONS limit. (See Section
6.2.2, "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.
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:
To reset the current counts to zero for all accounts, issue a
FLUSH USER_RESOURCES statement. The counts also can be reset by reloading
the grant tables (for example, with a
PRIVILEGES statement or a mysqladmin reload command).
The counts for an individual account can be set to zero by re-granting it any of
its limits. To do this, use
GRANT USAGE as described earlier and specify a limit value equal to
the value that the account currently has.
Counter resets do not affect the
All counts begin at zero when the server starts; counts are not carried over through a restart.
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.