情景：假设数据库有个值为10，有三个脚本进程同时读取了10，A进程+10保存到数据库，B进程+20保存到数据库，C进程+30保存到数据，最终数据库中的值为40！实际上我们想要的值是10+10+20+30=70，也就是说一个进程读取出来的值要依赖于另外一个进程的执行结果，而不能是同时读取，所以这里必须是以同步方式运行三个进程（A -> B -> C或者其他顺序也行，看调度）。
解决方法：使用锁机制
<?php
/医院
* LockSystem.php
*
* php锁机制
*
* Copy right (c) 2016 http://blog.csdn.net/CleverCode
*
* modification history:
* --------------------
* 2016/9/10, by CleverCode, Create
*
*/
class LockSystem
{
const LOCK_TYPE_DB = 'SQLLock';
const LOCK_TYPE_FILE = 'FileLock';
const LOCK_TYPE_MEMCACHE = 'MemcacheLock';
private $_lock = null; //$type对应的锁实例
private static $_supportLocks = array('FileLock', 'SQLLock', 'MemcacheLock');
public function __construct($type, $options = array())
{
if(false == empty($type))
{
$this->createLock($type, $options);
}
}
/*
* 功能：创建$type对应的锁实例
*/
public function createLock($type, $options=array())
{
if (false == in_array($type, self::$_supportLocks))
{
throw new Exception("没有这种类型<${type}>的锁！");
}
$this->_lock = new $type($options);
}
/*
* 功能：获取锁
*/
public function getLock($key, $timeout = ILock::EXPIRE)
{
if (false == $this->_lock instanceof ILock)
{
throw new Exception('该锁没有继承 ILock 接口！');
}
$this->_lock->getLock($key, $timeout);
}
/*
* 功能：释放锁
*/
public function releaseLock($key)
{
if (false == $this->_lock instanceof ILock)
{
throw new Exception('该锁没有继承 ILock 接口！');
}
$this->_lock->releaseLock($key);
}
}
interface ILock
{
const EXPIRE = 5;
public function getLock($key, $timeout=self::EXPIRE);
public function releaseLock($key);
}
/*
* 文件锁
*/
class FileLock implements ILock
{
private $_fp;
private $_single;
public function __construct($options)
{
if (isset($options['path']) && is_dir($options['path']))
{
$this->_lockPath = $options['path'].'/';
}
else
{
$this->_lockPath = '/tmp/';
}
$this->_single = isset($options['single'])?$options['single']:false;
}
public function getLock($key, $timeout=self::EXPIRE)
{
$file = md5(__FILE__.$key);
is_file($this -> _lockPath . $file . '.lock') || touch($_SERVER['DOCUMENT_ROOT'] . $this -> _lockPath . $file . '.lock');
$this->fp = fopen($_SERVER['DOCUMENT_ROOT'] . $this->_lockPath.$file.'.lock', "w+");
if (true || $this->_single)
{
$op = LOCK_EX + LOCK_NB;
}
else
{
$op = LOCK_EX;
}
if (false == flock($this->fp, $op, $a))
{
throw new Exception('添加文件锁失败！');
}
return true;
}
public function releaseLock($key)
{
flock($this->fp, LOCK_UN);
fclose($this->fp);
}
}
/*
* sql锁
*/
class SQLLock implements ILock
{
public function __construct($options)
{
$this->_db = new mysql();
}
public function getLock($key, $timeout=self::EXPIRE)
{
$sql = "SELECT GET_LOCK('".$key."', '".$timeout."')";
$res = $this->_db->query($sql);
return $res;
}
public function releaseLock($key)
{
$sql = "SELECT RELEASE_LOCK('".$key."')";
return $this->_db->query($sql);
}
}
/*
* Memcache锁
*/
class MemcacheLock implements ILock
{
public function __construct($options)
{
$this->memcache = new Memcache();
$this -> memcache -> addServer('127.0.0.1', 11211);
}
public function getLock($key, $timeout=self::EXPIRE)
{
//$waittime, $totalWaittime, $time的单位都是useconds；$timeout的单位是秒
$waitime = 20000;
$totalWaitime = 0;
$time = $timeout*1000000;
//动作：在等待锁没有超时并且添加失败时，进入休眠状态
while ($totalWaitime < $time && false == $this->memcache->add($key, 1, $timeout))
{
usleep($waitime);
$totalWaitime += $waitime;
}
if ($totalWaitime >= $time)
throw new Exception('等待锁超时，超时时间为：'.$timeout.'秒');
}
public function releaseLock($key)
{
$this->memcache->delete($key);
}
}缺点：会阻塞其他进程的运行，因此需要：使用缓存加快锁内部分的数据读写，优化sql，适当添加索引等！
