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. In this case, the library tries once to reconnect to the server and send the statement again.
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 to disable
auto-reconnect. This can be done explicitly by calling
mysql_options()
with the
MYSQL_OPT_RECONNECT
option:
my_bool reconnect = 0; mysql_options(&mysql, MYSQL_OPT_RECONNECT, &reconnect);
In MySQL 5.5, auto-reconnect is disabled by default.
If the connection has gone down, the
mysql_ping()
function performs a
reconnect if auto-reconnect is enabled. If auto-reconnect is
disabled, mysql_ping()
returns an
error instead.
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()
, and then call
mysql_thread_id()
again to see
whether the identifier has 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 and your application will not know about it. 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 variables are reinitialized to the values of the
corresponding variables. This also affects 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.
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()
.
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