Oracle 5.0 Reference Manual page 2583

Mysqlnd replication and load balancing plugin (mysqlnd_ms)
state of a connection, see the
decides to switch connections for load balancing, the application could be given a connection which
has a different state. Applications must be made aware of this.
Example 20.222. Plugin config with one slave and one master
{
"myapp": {
"master": {
"master_0": {
"host": "localhost",
"socket": "\/tmp\/mysql.sock"
}
},
"slave": {
"slave_0": {
"host": "192.168.2.27",
"port": "3306"
}
}
}
}
Example 20.223. Pitfall: connection state and SQL user variables
<?php
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
/* Of course, your error handling is nicer... */
die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));
/* Connection 1, connection bound SQL user variable, no SELECT thus run on master */
if (!$mysqli->query("SET @myrole='master'")) {
printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
}
/* Connection 2, run on slave because SELECT */
if (!($res = $mysqli->query("SELECT @myrole AS _role"))) {
printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
} else {
$row = $res->fetch_assoc();
$res->close();
printf("@myrole = '%s'\n", $row['_role']);
}
$mysqli->close();
?>
The above example will output:
@myrole = ''
The example opens a load balanced connection and executes two statements. The first statement
does not begin with the string SELECT. Therefore the plugin does not recognize
@myrole='master'
it as a read-only query which shall be run on a slave. The plugin runs the statement on the connection
to the master. The statement sets a SQL user variable which is bound to the master connection. The
state of the master connection has been changed.
The next statement is
SELECT @myrole AS
query and sends it to the slave. The statement is run on a connection to the slave. This second
connection does not have any SQL user variables bound to it. It has a different state than the first
connection pooling and switching
_role. The plugin does recognize it as a read-only
2563
concepts documentation. If the plugin
SET