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28.7.28 C API Automatic Reconnection Control

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The MySQL client library can perform an automatic reconnection to the server if it finds that the connection is down when you attempt to send a statement to the server to be executed. If auto-reconnect is enabled, the library tries once to reconnect to the server and send the statement again.

Auto-reconnect is disabled by default.

If it is important for your application to know that the connection has been dropped (so that it can exit or take action to adjust for the loss of state information), be sure that auto-reconnect is disabled. To ensure this, call mysql_options() with the MYSQL_OPT_RECONNECT option: 

bool reconnect = 0;
mysql_options(&mysql, MYSQL_OPT_RECONNECT, &reconnect);

If the connection has gone down, the effect of mysql_ping() depends on the auto-reconnect state. If auto-reconnect is enabled, mysql_ping() performs a reconnect. Otherwise, it returns an error.

Some client programs might provide the capability of controlling automatic reconnection. For example, mysql reconnects by default, but the --skip-reconnect option can be used to suppress this behavior.

If an automatic reconnection does occur (for example, as a result of calling mysql_ping()), there is no explicit indication of it. To check for reconnection, call mysql_thread_id() to get the original connection identifier before calling mysql_ping(), then call mysql_thread_id() again to see whether the identifier changed.

Automatic reconnection can be convenient because you need not implement your own reconnect code, but if a reconnection does occur, several aspects of the connection state are reset on the server side and your application will not be notified.

The connection-related state is affected as follows:

  • Any active transactions are rolled back and autocommit mode is reset.

  • All table locks are released.

  • All TEMPORARY tables are closed (and dropped).

  • Session system variables are reinitialized to the values of the corresponding global system variables, including system variables that are set implicitly by statements such as SET NAMES.

  • User variable settings are lost.

  • Prepared statements are released.

  • HANDLER variables are closed.

  • The value of LAST_INSERT_ID() is reset to 0.

  • Locks acquired with GET_LOCK() are released.

  • The association of the client with the Performance Schema threads table row that determines connection thread instrumentation is lost. If the client reconnects after a disconnect, the session is associated with a new row in the threads table and the thread monitoring state may be different. See Section 26.12.17.5, “The threads Table”.

If reconnection occurs, any SQL statement specified by calling mysql_options() with the MYSQL_INIT_COMMAND option is re-executed.

If the connection drops, it is possible that the session associated with the connection on the server side will still be running if the server has not yet detected that the client is no longer connected. In this case, any locks held by the original connection still belong to that session, so you may want to kill it by calling mysql_kill().


Rechercher dans le manuel MySQL

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Document créé le 26/06/2006, dernière modification le 26/10/2018
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