PHP concurrent locking case analysis

This article introduces an example of php concurrent locking. The data is locked and only one user is allowed to operate at a time. At this time, the lock needs to be used. Friends who need it can learn about it.

In the work project, you will encounter some PHP concurrent access to modify a data problem. If the data is not locked, it will cause data errors. Next I will analyze a financial payment lock problem. I hope to be helpful.

1 No application lock mechanism

1.1 Financial payment simplified version code

<!--?php 
/** 
 * pay.php 
 * 
 * 支付没有应用锁
 * 
 * Copy right (c) 2016 
 * 
 * modification history: 
 * -------------------- 
 * 2016/9/10, by CleverCode, Create 
 * 
 */ 
//用户支付
function pay($userId,$money)
{
  
 if(false == is_int($userId) || false == is_int($money))
 {
  return false;
 } 
  
 //取出总额
 $total = getUserLeftMoney($userId);
  
 //花费大于剩余
 if($money --> $total)
 {
  return false; 
 }
  
 //余额
 $left = $total - $money;
  
 //更新余额
 return setUserLeftMoney($userId,$left);
 
}
 
//取出用户的余额
function getUserLeftMoney($userId)
{
 if(false == is_int($userId))
 {
  return 0;
 }
 $sql = "select account form user_account where userid = ${userId}";
  
 //$mysql = new mysql();//mysql数据库
 return $mysql->query($sql);
}
 
//更新用户余额
function setUserLeftMoney($userId,$money)
{
 if(false == is_int($userId) || false == is_int($money))
 {
  return false;
 }  
  
 $sql = "update user_account set account = ${money} where userid = ${userId}";
  
 //$mysql = new mysql();//mysql数据库
 return $mysql->execute($sql);
}
?>

1.2 Problem Analysis

If there are two operators (p and m), both use user number 100 accounts, Log in to the PC and mobile terminals at the same time. The total balance of the 100 account is 1,000. The p operator spends 200 and the m operator spends 300. The concurrent process is as follows.

pOperator:

1. Withdraw the user’s balance of 1,000.

2. Remaining 800 = 1000 - 200 after payment.

3. The updated account balance is 800.

mOperator:

1. Withdraw the user’s balance of 1,000.

2. The remaining 700 after payment = 1000 - 300.

3. The account balance after payment is 700.

After two payments, the balance in the account is still 700. It should be that after spending 500, the account balance is 500. The root cause of this phenomenon is that during concurrency, the balance data obtained by p and m at the same time is both 1000.

2 Lock design

The lock operation generally only has two steps, one is to obtain the lock (getLock); the other is to release the lock (releaseLock) . However, there are many ways to implement locks in reality, including file implementation, SQL implementation, and Memcache implementation. According to this scenario, we consider using the strategy mode.

2.1 The class diagram design is as follows

##2.2 The php source code design is as follows

LockSystem.php

<!--?php 
/** 
 * LockSystem.php 
 * 
 * php锁机制
 * 
 * Copy right (c) 2016
 * 
 * modification history: 
 * -------------------- 
 * 2016/9/10, by CleverCode, Create 
 * 
 */ 
class LockSystem
{
 const LOCK_TYPE_DB = &#39;SQLLock&#39;;
 const LOCK_TYPE_FILE = &#39;FileLock&#39;;
 const LOCK_TYPE_MEMCACHE = &#39;MemcacheLock&#39;;
  
 private $_lock = null;
 private static $_supportLocks = array(&#39;FileLock&#39;, &#39;SQLLock&#39;, &#39;MemcacheLock&#39;); 
  
 public function __construct($type, $options = array()) 
 {
  if(false == empty($type))
  {
   $this--->createLock($type, $options);
  }
 } 
 public function createLock($type, $options=array())
 {
  if (false == in_array($type, self::$_supportLocks))
  {
   throw new Exception("not support lock of ${type}");
  }
  $this->_lock = new $type($options);
 }  
 public function getLock($key, $timeout = ILock::EXPIRE)
 {
  if (false == $this->_lock instanceof ILock) 
  {
   throw new Exception(&#39;false == $this->_lock instanceof ILock&#39;);   
  } 
  $this->_lock->getLock($key, $timeout); 
 }
  
 public function releaseLock($key)
 {
  if (false == $this->_lock instanceof ILock) 
  {
   throw new Exception(&#39;false == $this->_lock instanceof ILock&#39;);   
  } 
  $this->_lock->releaseLock($key);   
 } 
}
interface ILock
{
 const EXPIRE = 5;
 public function getLock($key, $timeout=self::EXPIRE);
 public function releaseLock($key);
}
 
class FileLock implements ILock
{
 private $_fp;
 private $_single;
 
 public function __construct($options)
 {
  if (isset($options[&#39;path&#39;]) && is_dir($options[&#39;path&#39;]))
  {
   $this->_lockPath = $options[&#39;path&#39;].&#39;/&#39;;
  }
  else
  {
   $this->_lockPath = &#39;/tmp/&#39;;
  }
  
  $this->_single = isset($options[&#39;single&#39;])?$options[&#39;single&#39;]:false;
 }
 public function getLock($key, $timeout=self::EXPIRE)
 {
  $startTime = Timer::getTimeStamp();
 
  $file = md5(__FILE__.$key);
  $this->fp = fopen($this->_lockPath.$file.&#39;.lock&#39;, "w+");
  if (true || $this->_single)
  {
   $op = LOCK_EX + LOCK_NB;
  }
  else
  {
   $op = LOCK_EX;
  }
  if (false == flock($this->fp, $op, $a))
  {
   throw new Exception(&#39;failed&#39;);
  }
  
  return true;
 }
 public function releaseLock($key)
 {
  flock($this->fp, LOCK_UN);
  fclose($this->fp);
 }
}
 
class SQLLock implements ILock
{
 public function __construct($options)
 {
  $this->_db = new mysql(); 
 }
 
 public function getLock($key, $timeout=self::EXPIRE)
 {  
  $sql = "SELECT GET_LOCK(&#39;".$key."&#39;, &#39;".$timeout."&#39;)";
  $res = $this->_db->query($sql);
  return $res;
 }
 
 public function releaseLock($key)
 {
  $sql = "SELECT RELEASE_LOCK(&#39;".$key."&#39;)";
  return $this->_db->query($sql);
 }
}
 
class MemcacheLock implements ILock
{
 public function __construct($options)
 {
   
  $this->memcache = new Memcache();
 }
 
 public function getLock($key, $timeout=self::EXPIRE)
 {  
  $waitime = 20000;
  $totalWaitime = 0;
  $time = $timeout*1000000;
  while ($totalWaitime < $time && false == $this->memcache->add($key, 1, $timeout)) 
  {
   usleep($waitime);
   $totalWaitime += $waitime;
  }
  if ($totalWaitime >= $time)
   throw new Exception(&#39;can not get lock for waiting &#39;.$timeout.&#39;s.&#39;);
 
 }
 
 public function releaseLock($key)
 {
  $this->memcache->delete($key);
 }
}

3 Application lock mechanism

##3.1 Payment system application lock

<!--?php
 
/** 
 * pay.php 
 * 
 * 支付应用锁
 * 
 * Copy right (c) 2016 
 * 
 * modification history: 
 * -------------------- 
 * 2016/9/10, by CleverCode, Create 
 * 
 */ 
//用户支付
function pay($userId,$money)
{
  
 if(false == is_int($userId) || false == is_int($money))
 {
  return false;
 } 
  
 try
 {
  //创建锁(推荐使用MemcacheLock)
  $lockSystem = new LockSystem(LockSystem::LOCK_TYPE_MEMCACHE);    
   
  //获取锁
  $lockKey = &#39;pay&#39;.$userId;
  $lockSystem--->getLock($lockKey,8);
   
  //取出总额
  $total = getUserLeftMoney($userId);
   
  //花费大于剩余
  if($money > $total)
  {
   $ret = false; 
  }
  else
  { 
   //余额
   $left = $total - $money;
    
   //更新余额
   $ret = setUserLeftMoney($userId,$left);
  }
   
  //释放锁
  $lockSystem->releaseLock($lockKey); 
 }
 catch (Exception $e)
 {
  //释放锁
  $lockSystem->releaseLock($lockKey);  
 }
 
}
//取出用户的余额
function getUserLeftMoney($userId)
{
 if(false == is_int($userId))
 {
  return 0;
 }
 $sql = "select account form user_account where userid = ${userId}";
  
 //$mysql = new mysql();//mysql数据库
 return $mysql->query($sql);
}
 
//更新用户余额
function setUserLeftMoney($userId,$money)
{
 if(false == is_int($userId) || false == is_int($money))
 {
  return false;
 }  
  
 $sql = "update user_account set account = ${money} where userid = ${userId}";
  
 //$mysql = new mysql();//mysql数据库
 return $mysql->execute($sql);
}
?>

3.2 Lock analysis

pOperator:

Get the lock: pay100
Take out the user’s balance of 1000.
The remaining amount after payment is 800 = 1000 - 200.
The updated account balance is 800.
Release lock: pay100

5. 
   

m Operator:

Waiting for lock: pay100
Get the lock: pay100
Get the balance: 800
The remaining 500 after payment = 800-300.
The account balance after payment is 500.
Release lock: pay100

6. 
   

After two payments, the balance is 500. It perfectly solves the access problem of critical section resources caused by concurrency.

Summary: The above is the entire content of this article, I hope it will be helpful to everyone's study.

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