• @var array $slaves.Slave database connection(Read) config array.
• 配置符合 CDbConnection.
• @example
• 'components'=>array(
• 'db'=>array(
• 'connectionString'=>'MysqL://',
• 'slaves'=>array(
• array('connectionString'=>'MysqL://'),
• array('connectionString'=>'MysqL://'),
• )
• )
• )
• */
  public $slaves = array();
  /**
• 从数据库状态 false 则只用主数据库
• @var bool $enableSlave
• */
  public $enableSlave = true;

/**

• @var slavesWrite 紧急情况主数据库无法连接 切换从服务器（读写）.
  */
  public $slavesWrite = false;

/**

• @var masterRead 紧急情况从主数据库无法连接 切换从住服务器（读写）.
  */
  public $masterRead = false;

/**

• @var _slave
  */
  private $_slave;

/**

• @var _disableWrite 从服务器（只读）.
  */
  private $_disableWrite = true;

/**

• 重写 createCommand 方法,1.开启从库 2.存在从库 3.当前不处于一个事务中 4.从库读数据
• @param string $sql
• @return CDbCommand
• */
  public function createCommand($sql = null) {
  if ($this->enableSlave && !emptyempty($this->slaves) && is_string($sql) && !$this->getCurrentTransaction() && self::isReadOperation($sql) && ($slave = $this->getSlave())
  ) {
  return $slave->createCommand($sql);
  } else {
  if (!$this->masterRead) {
  if ($this->_disableWrite && !self::isReadOperation($sql)) {

throw new CDbException("Master db server is not available now!Disallow write operation on slave server!");
}
}
return parent::createCommand($sql);
}
}

/**

• 获得从服务器连接资源
• @return CDbConnection
• */
  public function getSlave() {
  if (!isset($this->_slave)) {
  shuffle($this->slaves);
  foreach ($this->slaves as $slaveConfig) {
  if ($this->_isDeadServer($slaveConfig['connectionString'])) {
  continue;
  }
  if (!isset($slaveConfig['class']))
  $slaveConfig['class'] = 'CDbConnection';

$slaveConfig['autoConnect'] = false;
try {
if ($slave = Yii::createComponent($slaveConfig)) {
Yii::app()->setComponent('dbslave',$slave);
$slave->setAttribute(PDO::ATTR_TIMEOUT,$this->timeout);
$slave->setAttribute(PDO::MysqL_ATTR_USE_BUFFERED_QUERY,true);
$slave->setActive(true);
$this->_slave = $slave;
break;
}
} catch (Exception $e) {
$this->_markDeadServer($slaveConfig['connectionString']);
Yii::log("Slave database connection Failed!ntConnection string:{$slaveConfig['connectionString']}",'warning');

continue;
}
}

if (!isset($this->_slave)) {
$this->_slave = null;
$this->enableSlave = false;
}
}
return $this->_slave;
}

public function setActive($value) {
if ($value != $this->getActive()) {
if ($value) {
try {
if ($this->_isDeadServer($this->connectionString)) {
throw new CDbException('Master db server is already dead!');
}
//PDO::ATTR_TIMEOUT must set before pdo instance create
$this->setAttribute(PDO::ATTR_TIMEOUT,$this->timeout);
$this->open();
} catch (Exception $e) {
$this->_markDeadServer($this->connectionString);
$slave = $this->getSlave();
Yii::log($e->getMessage(),CLogger::LEVEL_ERROR,'exception.CDbException');
if ($slave) {
$this->connectionString = $slave->connectionString;
$this->username = $slave->username;
$this->password = $slave->password;
if ($this->slavesWrite) {
$this->_disableWrite = false;
}
$this->open();
} else { //Slave also unavailable
if ($this->masterRead) {
$this->connectionString = $this->connectionString;
$this->username = $this->username;
$this->password = $this->password;
$this->open();
} else {
throw new CDbException(Yii::t('yii','CDbConnection Failed to open the DB connection.'),(int) $e->getCode(),$e->errorInfo);
}
}
}
} else {
$this->close();
}
}
}

/**

• 检测读操作 sql 语句
• 关键字： SELECT,DECRIBE,SHOW ...
• 写操作:UPDATE,INSERT,DELETE ...
• */
  public static function isReadOperation($sql) {
  $sql = substr(ltrim($sql),10);
  $sql = str_ireplace(array('SELECT','SHOW','DESCRIBE','PRAGMA'),'^O^',$sql); //^O^,magic smile
  return strpos($sql,'^O^') === 0;
  }

/**

• 检测从服务器是否被标记 失败.
  */
  private function _isDeadServer($c) {
  $cache = Yii::app()->{$this->cacheID};
  if ($cache && $cache->get('DeadServer::' . $c) == 1) {
  return true;
  }
  return false;
  }

/**

• 标记失败的slaves.
  */
  private function _markDeadServer($c) {
  $cache = Yii::app()->{$this->cacheID};
  if ($cache) {
  $cache->set('DeadServer::' . $c,1,$this->markDeadSeconds);
  }
  }
  }