Master-Slave database in CodeIgniter

config.app.php

<?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');

//Create a new config file

/*
|--------------------------------------------------------------------------
| Number of databases in use
|--------------------------------------------------------------------------
|
| Using a Master and Slave. The Slave doubles as a read-only database.
| 
| If we find mid-campaign that the slave is too out of sync, we can 
| shave this down to 1 database so everything reads/write from Master,
| putting more load on Master but keeping database integrity.
|
*/
$config['number_databases'] = 2;

config.production.database.php

<?php

$active_group = 'write';
$active_record = TRUE;

//Master DB IP: 11.11.111.111 [WRITE, READ primary]
//Slave DB IP: 22.22.222.222 [READ, secondary]

//Note master and slave have same username/database/password names

$db['write']['hostname'] = '11.11.111.111';
$db['write']['username'] = 'production_user';
$db['write']['password'] = 'production_pass';
$db['write']['database'] = 'production_db';
$db['write']['dbdriver'] = 'mysql';
$db['write']['dbprefix'] = '';
$db['write']['pconnect'] = TRUE;
$db['write']['db_debug'] = TRUE;
$db['write']['cache_on'] = FALSE;
$db['write']['cachedir'] = '';
$db['write']['char_set'] = 'utf8';
$db['write']['dbcollat'] = 'utf8_general_ci';
$db['write']['swap_pre'] = '';
$db['write']['autoinit'] = TRUE;
$db['write']['stricton'] = FALSE;

//50/50 odds of reading from either server to distribute load from READing database
$db['read']['hostname'] = (mt_rand(1, 100) > 50) ? '11.11.111.111' : '22.22.222.222';
$db['read']['username'] = 'production_user';
$db['read']['password'] = 'production_pass';
$db['read']['database'] = 'production_db';
$db['read']['dbdriver'] = 'mysql';
$db['read']['dbprefix'] = '';
$db['read']['pconnect'] = TRUE;
$db['read']['db_debug'] = TRUE;
$db['read']['cache_on'] = FALSE;
$db['read']['cachedir'] = '';
$db['read']['char_set'] = 'utf8';
$db['read']['dbcollat'] = 'utf8_general_ci';
$db['read']['swap_pre'] = '';
$db['read']['autoinit'] = TRUE;
$db['read']['stricton'] = FALSE;

some_model.php

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

/**
*    Application logic needs to be aware if its reading or writing. 
*
*    Note that this needs to be done carefully. If you read and write at the same time, its
*     better to do both with the WRITE database so you don't WRITE to master and then READ from
*     slave before the data has time to sync between databases.
*/

class SomeModel extends CI_Model {

	private $READ;
	private $WRITE;

	public function __construct(){
       parent::__construct();
      $numDB = ($this->config->item('number_databases')) ? $this->config->item('number_databases') : 1;
		if($numDB > 1) {
			$this->WRITE = $this->load->database('write', TRUE);
			$this->READ = $this->load->database('read', TRUE);
		} else {
			$this->WRITE = $this->READ = $this->load->database('write', TRUE);
		}
	}//End __construct

	public function verifyUser($username, $password) {
		$q = $this->READ->select()
						->from('users')
						->where('username', $username)
						->where('password', md5($password))
						->limit(1)
						->get();
		
		if($q->num_rows() > 0) {
			return $q->row();
		}
		return FALSE;
	}

        public function createUser($username, $password) {
		$q = $this->WRITE->INSERT('users', array('user' => $username, 'password' => $password));
                return $q;
	}
}

Not the best solution！！！

Do you have a better one? Like to hear it, please.