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MySQL 5.7 Reference Manual
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B.4.2.6 Too many connections

If clients encounter Too many connections errors when attempting to connect to the mysqld server, all available connections are in use by other clients.

The permitted number of connections is controlled by the max_connections system variable. The default value is 151 to improve performance when MySQL is used with the Apache Web server. To support more connections, set max_connections to a larger value.

mysqld actually permits max_connections + 1 client connections. The extra connection is reserved for use by accounts that have the SUPER privilege. By granting the privilege to administrators and not to normal users (who should not need it), an administrator who also has the PROCESS privilege can connect to the server and use SHOW PROCESSLIST to diagnose problems even if the maximum number of unprivileged clients are connected. See Section 13.7.5.29, “SHOW PROCESSLIST Statement”.

For more information about how the server handles client connections, see Section 8.12.5.1, “How MySQL Handles Client Connections”.