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24.4.2 Event Scheduler Configuration

Inhoudsopgave

Events are executed by a special event scheduler thread; when we refer to the Event Scheduler, we actually refer to this thread. When running, the event scheduler thread and its current state can be seen by users having the PROCESS privilege in the output of SHOW PROCESSLIST, as shown in the discussion that follows.

The global event_scheduler system variable determines whether the Event Scheduler is enabled and running on the server. It has one of these 3 values, which affect event scheduling as described here. The default is ON.

  • ON: The Event Scheduler is started; the event scheduler thread runs and executes all scheduled events.

    When the Event Scheduler is ON, the event scheduler thread is listed in the output of SHOW PROCESSLIST as a daemon process, and its state is represented as shown here:

    1. mysql> SHOW PROCESSLIST\G
    2. *************************** 1. row ***************************
    3.      Id: 1
    4.    User: root
    5.    Host: localhost
    6.      db: NULL
    7. Command: Query
    8.    Time: 0
    9.   State: NULL
    10.    Info: show processlist
    11. *************************** 2. row ***************************
    12.      Id: 2
    13.    User: event_scheduler
    14.    Host: localhost
    15.      db: NULL
    16. Command: Daemon
    17.    Time: 3
    18.   State: Waiting for next activation
    19.    Info: NULL
    20. 2 rows in set (0.00 sec)

    Event scheduling can be stopped by setting the value of event_scheduler to OFF.

  • OFF: The Event Scheduler is stopped. The event scheduler thread does not run, is not shown in the output of SHOW PROCESSLIST, and no scheduled events are executed.

    When the Event Scheduler is stopped (event_scheduler is OFF), it can be started by setting the value of event_scheduler to ON. (See next item.)

  • DISABLED: This value renders the Event Scheduler nonoperational. When the Event Scheduler is DISABLED, the event scheduler thread does not run (and so does not appear in the output of SHOW PROCESSLIST). In addition, the Event Scheduler state cannot be changed at runtime.

If the Event Scheduler status has not been set to DISABLED, event_scheduler can be toggled between ON and OFF (using SET). It is also possible to use 0 for OFF, and 1 for ON when setting this variable. Thus, any of the following 4 statements can be used in the mysql client to turn on the Event Scheduler:

  1. SET GLOBAL event_scheduler = ON;
  2. SET @@GLOBAL.event_scheduler = ON;
  3. SET GLOBAL event_scheduler = 1;
  4. SET @@GLOBAL.event_scheduler = 1;

Similarly, any of these 4 statements can be used to turn off the Event Scheduler:

  1. SET GLOBAL event_scheduler = OFF;
  2. SET @@GLOBAL.event_scheduler = OFF;
  3. SET GLOBAL event_scheduler = 0;
  4. SET @@GLOBAL.event_scheduler = 0;

Although ON and OFF have numeric equivalents, the value displayed for event_scheduler by SELECT or SHOW VARIABLES is always one of OFF, ON, or DISABLED. DISABLED has no numeric equivalent. For this reason, ON and OFF are usually preferred over 1 and 0 when setting this variable.

Note that attempting to set event_scheduler without specifying it as a global variable causes an error:

  1. mysql< SET @@event_scheduler = OFF;
  2. ERROR 1229 (HY000): Variable 'event_scheduler' is a GLOBAL
  3. variable and should be set with SET GLOBAL
Important

It is possible to set the Event Scheduler to DISABLED only at server startup. If event_scheduler is ON or OFF, you cannot set it to DISABLED at runtime. Also, if the Event Scheduler is set to DISABLED at startup, you cannot change the value of event_scheduler at runtime.

To disable the event scheduler, use one of the following two methods:

  • As a command-line option when starting the server: 

    --event-scheduler=DISABLED

  • In the server configuration file (my.cnf, or my.ini on Windows systems), include the line where it will be read by the server (for example, in a [mysqld] section): 

    event_scheduler=DISABLED

To enable the Event Scheduler, restart the server without the --event-scheduler=DISABLED command-line option, or after removing or commenting out the line containing event-scheduler=DISABLED in the server configuration file, as appropriate. Alternatively, you can use ON (or 1) or OFF (or 0) in place of the DISABLED value when starting the server.

Note

You can issue event-manipulation statements when event_scheduler is set to DISABLED. No warnings or errors are generated in such cases (provided that the statements are themselves valid). However, scheduled events cannot execute until this variable is set to ON (or 1). Once this has been done, the event scheduler thread executes all events whose scheduling conditions are satisfied.

Starting the MySQL server with the --skip-grant-tables option causes event_scheduler to be set to DISABLED, overriding any other value set either on the command line or in the my.cnf or my.ini file (Bug #26807).

For SQL statements used to create, alter, and drop events, see Section 24.4.3, “Event Syntax”.

MySQL provides an EVENTS table in the INFORMATION_SCHEMA database. This table can be queried to obtain information about scheduled events which have been defined on the server. See Section 24.4.4, “Event Metadata”, and Section 25.10, “The INFORMATION_SCHEMA EVENTS Table”, for more information.

For information regarding event scheduling and the MySQL privilege system, see Section 24.4.6, “The Event Scheduler and MySQL Privileges”.


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Bron van het afgedrukte document:https://www.gaudry.be/nl/mysql-rf-events-configuration.html

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