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.
In MySQL 5.1, auto-reconnect is disabled by default.
If it is important for your application to know that the
connection has been dropped (so that is 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
my_bool reconnect = 0; mysql_options(&mysql, MYSQL_OPT_RECONNECT, &reconnect);
Some client programs might provide the capability of controlling
automatic reconnection. For example, mysql
reconnects by default, but the
option can be used to suppress this behavior.
If an automatic reconnection does occur (for example, as a result
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.
TEMPORARY tables are closed (and
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
User variable settings are lost.
Prepared statements are released.
HANDLER variables are closed.
The value of
is reset to 0.
Locks acquired with
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
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