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13.7.8.8 RESTART Statement

RESTART

This statement stops and restarts the MySQL server. It requires the SHUTDOWN privilege.

One use for RESTART is when it is not possible or convenient to gain command-line access to the MySQL server on the server host to restart it. For example, SET PERSIST_ONLY can be used at runtime to make configuration changes to system variables that can be set only at server startup, but the server must still be restarted for those changes to take effect. The RESTART statement provides a way to do so from within client sessions, without requiring command-line access on the server host.

Note

After executing a RESTART statement, the client can expect the current connection to be lost. If auto-reconnect is enabled, the connection will be reestablished after the server restarts. Otherwise, the connection must be reestablished manually.

A successful RESTART operation requires mysqld to be running in an environment that has a monitoring process available to detect a server shutdown performed for restart purposes:

  • In the presence of a monitoring process, RESTART causes mysqld to terminate such that the monitoring process can determine that it should start a new mysqld instance.

  • If no monitoring process is present, RESTART fails with an error.

These platforms provide the necessary monitoring support for the RESTART statement:

  • Windows, when mysqld is started as a Windows service or standalone. (mysqld forks, and one process acts as a monitor to the other, which acts as the server.)

  • Unix and Unix-like systems that use systemd or mysqld_safe to manage mysqld.

To configure a monitoring environment such that mysqld enables the RESTART statement:

  1. Set the MYSQLD_PARENT_PID environment variable to the value of the process ID of the process that starts mysqld, before starting mysqld.

  2. When mysqld performs a shutdown due to use of the RESTART statement, it returns exit code 16.

  3. When the monitoring process detects an exit code of 16, it starts mysqld again. Otherwise, it exits.

Here is a minimal example as implemented in the bash shell:

#!/bin/bash

export MYSQLD_PARENT_PID=$$

export MYSQLD_RESTART_EXIT=16

while true ; do
  bin/mysqld mysqld options here
  if [ $? -ne $MYSQLD_RESTART_EXIT ]; then
    break
  fi
done

On Windows, the forking used to implement RESTART makes determining the server process to attach to for debugging more difficult. To alleviate this, starting the server with --gdb suppresses forking, in addition to its other actions done to set up a debugging environment. In non-debug settings, --no-monitor may be used for the sole purpose of suppressing forking the monitor process. For a server started with either --gdb or --no-monitor, executing RESTART causes the server to simply exit without restarting.

The Com_restart status variable tracks the number of RESTART statements. Because status variables are initialized for each server startup and do not persist across restarts, Com_restart normally has a value of zero, but can be nonzero if RESTART statements were executed but failed.