PDO with multi server support

Database.php

<?php
/**
 * This file is a part of KK Framework
 * Author: kookxiang <r18@ikk.me>
 */
namespace Core;

class Database {
	const MASTER = 'MASTER';
	const SLAVE = 'SLAVE';
	private static $inTransaction = false;
	private static $ConnectionPool = array(self::MASTER => array(), self::SLAVE => array());
	private static $SelectedConnection = array();

	/**
	 * Register a new database into connection pool
	 * @link http://php.net/manual/pdo.construct.php
	 * @param string $dsn      Data Source Name, contains the information required to connect to the database.
	 * @param string $username Username for the DSN string
	 * @param string $password Password for the DSN string
	 * @param array  $options  A key=>value array of driver-specific connection options.
	 * @param string $type     Connection type (MASTER / SLAVE)
	 */
	public static function register($dsn, $username = null, $password = null, $options = array(), $type = self::MASTER){
		self::$ConnectionPool[$type][] = array(
			'dsn' => $dsn,
			'username' => $username,
			'password' => $password,
			'options' => $options,
		);
	}

	/**
	 * Get a database connection
	 * @param string $type Connection type (MASTER / SLAVE)
	 * @return \PDO PDO Connection
	 * @throws Error
	 */
	public static function GetServer($type = self::MASTER){
		if(empty(self::$ConnectionPool[self::SLAVE])){
			$type = self::MASTER;
		}
		if(self::$SelectedConnection[$type]){
			return self::$SelectedConnection[$type];
		}
		$offset = array_rand(self::$ConnectionPool[$type]);
		$config = self::$ConnectionPool[$type][$offset];
		if(!$config){
			throw new Error('No available database connection');
		}
		return self::$SelectedConnection[$type] = new \PDO($config['dsn'], $config['username'], $config['password'], $config['options']);
	}

	/**
	 * Select a database connection automatically
	 * @param $statement string SQL statement
	 * @return \PDO PDO Connection
	 * @throws Error
	 */
	public static function AutoDecideServer($statement){
		if(self::$inTransaction){
			return self::GetServer(self::MASTER);
		}
		static $keywords = array('INSERT', 'REPLACE', 'UPDATE', 'DELETE', 'CREATE', 'ALTER', 'DROP');
		list($operation) = explode(' ', strtoupper(trim($statement)), 2);
		if(in_array($operation, $keywords)){
			return self::GetServer(self::MASTER);
		}else{
			return self::GetServer(self::SLAVE);
		}
	}

	/**
	 * Prepares a statement for execution and returns a statement object
	 * @link http://php.net/manual/pdo.prepare.php
	 * @param string $statement      This must be a valid SQL statement for the target database server.
	 * @param array  $driver_options Please refer to the document of PDO prepare method
	 * @return \PDOStatement
	 */
	public static function prepare($statement, array $driver_options = array()){
		$connection = self::AutoDecideServer($statement);
		return $connection->prepare($statement, $driver_options);
	}

	/**
	 * Execute an SQL statement and return the number of affected rows
	 * @link http://php.net/manual/pdo.exec.php
	 * @param string $statement The SQL statement to prepare and execute.
	 * @return int <b>PDO::exec</b> returns the number of rows that were modified or deleted by the SQL statement
	 * you issued. If no rows were affected, <b>PDO::exec</b> returns 0.
	 */
	public static function exec($statement){
		$connection = self::AutoDecideServer($statement);
		return $connection->exec($statement);
	}

	/**
	 * Executes an SQL statement, returning a result set as a PDOStatement object
	 * @link http://php.net/manual/pdo.query.php
	 * @param string $statement The SQL statement to prepare and execute.
	 * @return \PDOStatement <b>PDO::query</b> returns a PDOStatement object, or <b>FALSE</b> on failure.
	 */
	public static function query($statement){
		$connection = self::AutoDecideServer($statement);
		return $connection->query($statement);
	}

	/**
	 * Returns the ID of the last inserted row or sequence value
	 * @link http://php.net/manual/pdo.lastinsertid.php
	 * @param string $name Name of the sequence object from which the ID should be returned
	 * @return string <p>If a sequence name was not specified for the <i>name</i> parameter, <b>PDO::lastInsertId</b>
	 * returns a string representing the row ID of the last row that was inserted into the database.</p><p>If
	 * a sequence name was specified for the <i>name</i> parameter, <b>PDO::lastInsertId</b> returns a string
	 * representing the last value retrieved from the specified sequence object.</p>
	 */
	public static function lastInsertId($name = null){
		$connection = self::GetServer(self::MASTER);
		return $connection->lastInsertId($name);
	}

	/**
	 * Initiates a transaction
	 * @link http://php.net/manual/pdo.begintransaction.php
	 * @return bool <b>TRUE</b> on success or <b>FALSE</b> on failure.
	 */
	public static function beginTransaction(){
		$connection = self::GetServer(self::MASTER);
		$result = $connection->beginTransaction();
		if($result){
			self::$inTransaction = true;
		}
		return $result;
	}

	/**
	 * Commits a transaction
	 * @link http://php.net/manual/pdo.commit.php
	 * @return bool <b>TRUE</b> on success or <b>FALSE</b> on failure.
	 */
	public static function commit(){
		$connection = self::GetServer(self::MASTER);
		$result = $connection->commit();
		if($result){
			self::$inTransaction = true;
		}
		return $result;
	}

	/**
	 * Rolls back a transaction
	 * @link http://php.net/manual/pdo.rollback.php
	 * @return bool <b>TRUE</b> on success or <b>FALSE</b> on failure.
	 */
	public static function rollBack(){
		$connection = self::GetServer(self::MASTER);
		$result = $connection->rollBack();
		if($result){
			self::$inTransaction = true;
		}
		return $result;
	}

	/**
	 * Checks if inside a transaction
	 * @link http://php.net/manual/pdo.intransaction.php
	 * @return bool <b>TRUE</b> if a transaction is currently active, and <b>FALSE</b> if not.
	 */
	public static function inTransaction(){
		return self::$inTransaction;
	}
}