场景
Mutex主要用于有大量并发访问并存在cache过期的场合，如
- 首页top 10, 由数据库加载到memcache缓存n分钟；
- 微博中名人的content cache, 一旦不存在会大量请求不能命中并加载数据库；
- 需要执行多个IO操作生成的数据存在cache中, 比如查询db多次；
问题
在大并发的场合，当cache失效时，大量并发同时取不到cache，会同一瞬间去访问db并回设cache，可能会给系统带来潜在的超负荷风险。我们曾经在线上系统出现过类似故障。
解决方法
方法一
高并发时，增加data_lock信号标识，只充许一个用户update cache，在cache数据期间，其它并发用户等待，只到这个用户cache成功，当然这个地方需要设置最大等待时间，毕竟当很长时间cache不成功时，不能让用户一直等待:
<?php
function get_my_data2()
{
$cache_id = "mykey";
$data = $memcache_obj->get($cache_id);
if (!$data)
{
// check to see if someone has already set the lock
$data_lock = $memcache_obj->get($cache_id . '_qry_lock');
if ($data_lock)
{
$lock_counter = 0;
// loop until you find that the lock has been released. that implies that the query has finished
while ($data_lock)
{
// you may only want to wait for a specified period of time.
// one second is usually sufficient since your goal is to always have sub-second response time
// if you query takes more than 1 second, you should consider "warming" your cached data via a cron job
if ($lock_counter > $max_time_to_wait)
{
$lock_failed = true;
break;
}
// you really want this to be a fraction of a second so the user waits as little as possible
// for the simplicity of example, I'm using the sleep function.
sleep(1);
$data_lock = $memcache_obj->get($cache_id . '_qry_lock');
}
// if the loop is completed, that either means the user waited for too long
// or that the lock has been removed. try to get the cached data again; it should exist now
$data = $memcache_obj->get($cache_id);
if ($data)
{
return $data;
}
}
// set a lock for 2 seconds
$memcache_obj->set($cache_id . '_qry_lock', true, 2);
$data = get_data_from_db_function();
$memcache_obj->set($cache_id, $data, $sec_to_cache_for);
// don't forget to remove the lock
$memcache_obj->delete($cache_id . '_qry_lock');
}
return $data;
}
方法二
需要给数据增加一个cache过期时间标识，高并发时，在用户取数据时，检查cache是否快要过期，如果即将过期，则充许一个用户去更新cache，其它用户依然访问没有update的数据。
<?php
function get_my_data3()
{
$cache_id = "mykey";
$data = $memcache_obj->get($cache_id);
// if there is cached data and the expire timestamp has already expired or is within the next 2 minutes
// then we want the user to freshen up the cached data
if ($data && ($data['cache_expires_timestamp'] - time()) < 120)
{
// if the semaphore lock has already been set, just return the data like you normally would.
if ($memcache_obj->get($cache_id . '_expire_lock'))
{
return $data;
}
// now we want to set the lock and have the user freshen the data.
$memcache_obj->set($cache_id . '_expire_lock', true, 2);
// by unsetting the data it will cause the data gather logic below to execute.
unset($data);
}
if (!$data)
{
// be sure to include all of the semaphore logic from example 2
// set the _qry_lock for 2 seconds
$memcache_obj->set($cache_id . '_qry_lock', true, 2);
$raw_data = get_data_from_db_function();
$data['cache_expires_timestamp'] = time() + $sec_to_cache_for;
$data['cached_data'] = $raw_data;
$memcache_obj->set($cache_id, $data, $sec_to_cache_for);
// remove the _qry_lock
$memcache_obj->delete($cache_id . '_qry_lock');
// remove the _expires_lock
$memcache_obj->delete($cache_id . '_expires_lock');
}
return $data;
}
项目中的一个缓存类参考：
CacheModel.class.php
<?php
namespace framework;
use framework\Cache;
/**
* 缓存模型 - 业务逻辑模型
*
* @example
* setType($type) 主动设置缓存类型
* set($key, $value, $expire = 0) 设置缓存key=>value，expire表示有效时间，0表示永久
* get($key, $mutex = false) 获取缓存数据，支持mutex模式
* getList($prefix, $key) 批量获取指定前缀下的多个key值的缓存
* rm($key) 删除缓存
*/
class CacheModel
{
protected $config = array(); // 缓存配置文件
protected $handler = null; // 当前缓存操作对象
protected $type = ''; // 当前缓存类型
/**
* 取得缓存类实例
*
* @param array $config 缓存节点
* @return mixed 返回类实例
*/
public static function getInstance($connection = 'default')
{
static $_instance = null;
if (!isset($_instance))
{
$_instance = new self($connection);
}
return $_instance;
}
/**
* 初始化缓存模型对象，缓存类型
*
* @return void
*/
public function __construct($connection = '')
{
$this->_initHandler($connection);
}
/**
* 初始化配置文件
*/
private function _initHandler($connection = '')
{
// 获取缓存配置信息
$connection = $connection ? $connection : 'default';
if (!isset($this->config[$connection]))
{
$this->config[$connection] = array_merge(get_config('cache/__common__'), get_config('cache/' . $connection));
}
$this->type = strtolower($this->config[$connection]['type']);
$this->handler = Cache::getInstance($this->config[$connection]);
}
/**
* 链式设置缓存节点
*
* @param string $type 缓存类型
* @return object 缓存模型对象
*/
public function setConnection($connection = '')
{
$this->_initHandler($connection);
return $this;
}
/**
* 设置缓存
*
* @param string $key 缓存Key值
* @param mix $value 缓存Value值
* @param int $expire 有效时间（单位：秒，0表示永不过期）
* @param boolean 是否设置成功
*/
public function set($key, $value, $expire = 0)
{
$value = array(
'cache_data' => $value, // 缓存数据
'cache_mtime' => time(), // 缓存修改时间戳
'cache_expire' => is_null($expire) ? 0 : intval($expire) // 缓存有效时间
);
return $this->handler->set($key, $value);
}
/**
* 获取缓存操作，支持mutex模式
* mutex使用注意
* 1.设置缓存(set)时，需要设置有效时间
* 2.获取缓存(get)时，需要主动创建缓存
*
* @param string $key 缓存Key值
* @param boolean $mutex 是否启用mutex模式，默认启用
* @return mix 缓存数据
*/
public function get($key, $mutex = true)
{
// 静态缓存
$sc = get_static('cache_model_' . $key);
if (isset($sc))
{
return $sc;
}
// 获取缓存数据
$data = $this->handler->get($key);
// 未成功取到缓存
if ($data === false)
{
return false;
}
// 未过期
if (($data ['cache_expire'] === 0) || (($data ['cache_mtime'] + $data ['cache_expire']) > time ()))
{
return $this->_returnData($data['cache_data'], $key);
}
// 已过期
if ($mutex) // mutex模式开启
{
$data['cache_mtime'] = time();
$this->handler->set($key, $data);
// 返回false，让调用程序去主动更新缓存
set_static('cache_model_' . $key, null);
return false;
}
else // mutex模式没开启
{
$this->rm($key);
return false;
}
}
/**
* 删除缓存
*
* @param string $_key 缓存Key值
* @return boolean 是否删除成功
*/
public function rm($key)
{
set_static('cache_model_' . $key, null);
return $this->handler->rm($key);
}
/**
* 清除缓存
*
* @access public
* @return boolen
*/
public function clear()
{
return $this->handler->clear();
}
/**
* 根据某个前缀，批量获取多个缓存
*
* @param string $prefix 缓存前缀
* @param string $keys 缓存Keys值
* @return mix 缓存数据
*/
public function getList($prefix = '', $keys = array())
{
if ($this->type == 'memcache')
{
// Memcache有批量获取缓存的接口
$_data = $this->handler->getMulti($prefix, $keys);
if ($_data)
{
foreach ($_data as $key => $val)
{
$data[$key] = $this->_returnData($val['cache_data'], $prefix . $key);
}
}
}
else
{
foreach ($keys as $key)
{
$_k = $prefix . $key;
$data[$key] = $this->get($_k);
}
}
return $data;
}
/**
* 返回缓存数据操作，方法中，将数据缓存到静态缓存中
*
* @param mix $data 缓存数据
* @param string $key 缓存Key值
* @return mix 缓存数据
*/
private function _returnData($data, $key)
{
set_static('cache_model_' . $key, $data);
return $data;
}
}
参考：
memcached PHP semaphore & cache expiration handling
[Tim]Memcache mutex设计模式