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MySQL PHP API  /  ...  /  mysqlnd_ms_set_user_pick_server

7.8.10 mysqlnd_ms_set_user_pick_server

Copyright 1997-2014 the PHP Documentation Group.

  • mysqlnd_ms_set_user_pick_server

    Sets a callback for user-defined read/write splitting


bool mysqlnd_ms_set_user_pick_server(string function);

Sets a callback for user-defined read/write splitting. The plugin will call the callback only if pick[]=user is the default rule for server picking in the relevant section of the plugins configuration file.

The plugins built-in read/write query split mechanism decisions can be overwritten in two ways. The easiest way is to prepend the query string with the SQL hints MYSQLND_MS_MASTER_SWITCH, MYSQLND_MS_SLAVE_SWITCH or MYSQLND_MS_LAST_USED_SWITCH. Using SQL hints one can control, for example, whether a query shall be send to the MySQL replication master server or one of the slave servers. By help of SQL hints it is not possible to pick a certain slave server for query execution.

Full control on server selection can be gained using a callback function. Use of a callback is recommended to expert users only because the callback has to cover all cases otherwise handled by the plugin.

The plugin will invoke the callback function for selecting a server from the lists of configured master and slave servers. The callback function inspects the query to run and picks a server for query execution by returning the hosts URI, as found in the master and slave list.

If the lazy connections are enabled and the callback chooses a slave server for which no connection has been established so far and establishing the connection to the slave fails, the plugin will return an error upon the next action on the failed connection, for example, when running a query. It is the responsibility of the application developer to handle the error. For example, the application can re-run the query to trigger a new server selection and callback invocation. If so, the callback must make sure to select a different slave, or check slave availability, before returning to the plugin to prevent an endless loop.



The function to be called. Class methods may also be invoked statically using this function by passing array($classname, $methodname) to this parameter. Additionally class methods of an object instance may be called by passing array($objectinstance, $methodname) to this parameter.

Return Values

Host to run the query on. The host URI is to be taken from the master and slave connection lists passed to the callback function. If callback returns a value neither found in the master nor in the slave connection lists the plugin will fallback to the second pick method configured via the pick[] setting in the plugin configuration file. If not second pick method is given, the plugin falls back to the build-in default pick method for server selection.



mysqlnd_ms_set_user_pick_server is available with PECL mysqlnd_ms < 1.1.0. It has been replaced by the user filter. Please, check the Change History for upgrade notes.


Example 7.105 mysqlnd_ms_set_user_pick_server example

master[] = localhost
slave[] =
slave[] =
pick[] = user

function pick_server($connected, $query, $master, $slaves, $last_used)
 static $slave_idx = 0;
 static $num_slaves = NULL;
 if (is_null($num_slaves))
  $num_slaves = count($slaves);

 /* default: fallback to the plugins build-in logic */
 $ret = NULL;

 printf("User has connected to '%s'...\n", $connected);
 printf("... deciding where to run '%s'\n", $query);

 $where = mysqlnd_ms_query_is_select($query);
 switch ($where)
   printf("... using master\n");
   $ret = $master[0];
   /* SELECT or SQL hint for using slave */
   if (stristr($query, "FROM table_on_slave_a_only"))
    /* a table which is only on the first configured slave  */
    printf("... access to table available only on slave A detected\n");
    $ret = $slaves[0];
    /* round robin */
    printf("... some read-only query for a slave\n");
    $ret = $slaves[$slave_idx++ % $num_slaves];
   printf("... using last used server\n");
   $ret = $last_used;

 printf("... ret = '%s'\n", $ret);
 return $ret;


$mysqli = new mysqli("myapp", "root", "root", "test");

if (!($res = $mysqli->query("SELECT 1 FROM DUAL")))
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);

if (!($res = $mysqli->query("SELECT 2 FROM DUAL")))
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);

if (!($res = $mysqli->query("SELECT * FROM table_on_slave_a_only")))
 printf("[%d] %s\n", $mysqli->errno, $mysqli->error);


The above example will output:

User has connected to 'myapp'...
... deciding where to run 'SELECT 1 FROM DUAL'
... some read-only query for a slave
... ret = 'tcp://'
User has connected to 'myapp'...
... deciding where to run 'SELECT 2 FROM DUAL'
... some read-only query for a slave
... ret = 'tcp://'
User has connected to 'myapp'...
... deciding where to run 'SELECT * FROM table_on_slave_a_only'
... access to table available only on slave A detected
... ret = 'tcp://'

See Also

Filter concept
user filter

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