防止两次调用相同的脚本来选择相同的mysql行

Question

下面的脚本每5秒调用一次。问题是，如果服务器响应较慢，“博客”中的一个条目可以连续选中两次，因为服务器还没有时间将“完成”设置为“1”。是否有行业标准（或任何你称之为）来防止这种情况发生？

$result = mysql_query("SELECT * FROM blogs WHERE done=0 LIMIT 1"); 
$rows = mysql_num_rows($result); //If there are no entries in with done set to 0, that means we've done them all; reset all entries to 0. 
if($rows == 0) 
{ 
    mysql_query("UPDATE blogs SET done=0 WHERE done=1"); 
} 
else 
{ 
    while($row = mysql_fetch_array($result)) 
    { 
     mysql_query("UPDATE blogs SET done=1 WHERE id=$row[id]"); 
     // Do stuff 
    } 
} 

我认为我可以将其更改为

while($row = mysql_fetch_array($result)) 
{ 
     if($row['done'] == 1){ die; } 
     mysql_query("UPDATE blogs SET done=1 WHERE id=$row[id]"); 
     //Do stuff 
} 

但将真正解决这一问题？我会想象会有一个更好的方式，真正防止它发生，毫无疑问的影子。

Answer 1

$result = mysql_query("SELECT * FROM blogs WHERE done=0 LIMIT 1"); 
$rows = mysql_num_rows($result); //If there are no entries in with done set to 0, that means we've done them all; reset all entries to 0. 
if($rows == 0) 
{ 
    mysql_query("UPDATE blogs SET done=0 WHERE done=1"); 
} 
else 
{ 
    while($row = mysql_fetch_array($result)) 
    { 
     mysql_query("UPDATE blogs SET done=1 WHERE id=$row[id] AND done=0"); 
     if(mysql_affected_rows() != 1) 
      die(); 
     // Do stuff 
    } 
} 

Answer 2

我本来想去交易。以下是另一个示例StackOverflow question

只是一个问题：如果服务器更慢会发生什么？例如，选择statament需要很长时间（如5秒），一旦它完成（返回0行），新的选择执行（返回1个或多个行）

MySQL documentation

Answer 3

我认为最好的方法以防止选择相同的行使用SELECT GET_LOCK("lock_name");和SELECT RELEASE_LOCK("lock_name");。当你从MySQL服务器获得锁定时，其他处理尝试获取锁定将等待锁定被释放。下面是一个示例实现：

<?php 
function getLock($lockName, $dbc) { 
    $query = "SELECT GET_LOCK('".$lockName."', 0)"; 
    $result = mysql_query($query, $dbc); 
    $lockResult = mysql_fetch_row($result); 
    $lockResult = $lockResult[0]; 
    return $lockResult == 1 ? true : false; 
} 

function releaseLock($lockName, $dbc) { 
    $query = "SELECT RELEASE_LOCK('".$lockName."')"; 
    $result = mysql_query($query, $dbc); 
} 

// CONNECT TO DATABASE 
$dbc = mysql_connect('localhost', 'root', ''); 
mysql_select_db('test', $dbc); 

$loopQueue = true; 
$rowsProcessed = 0; 

// MAIN QUEUE LOOP 
while ($loopQueue) { 

    // TRY UNTIL GETTING A LOCK 
    $queueLockName = 'queue_lock_1'; 
    while (getLock($queueLockName, $dbc) === true) { 

     // WE GOT THE LOCK, GET A QUEUE ROW WITH PENDING STATUS 
     $query = 'SELECT * FROM test WHERE status = 0 ORDER BY ID ASC LIMIT 1'; 
     $result = mysql_query($query, $dbc); 

     if (mysql_num_rows($result) < 1) { 
      // SINCE WE DON"T HAVE ANY QUEUE ROWS, RELEASE THE LOCK 
      releaseLock($queueLockName, $dbc); 
      // WE DONT NEED TO LOOP THE MAIN QUEUE ANYMORE SINCE WE DONT HAVE ANY QUEUE ROWS PENDING 
      $loopQueue = false; 
      // BREAK THIS LOOP 
      break; 
     } 

     // WE GOT THE QUEUE ROW, CONVERT IT TO ARRAY 
     $queueRowArray = mysql_fetch_assoc($result); 

     // UPDATE QUEUE ROW STATUS TO SENDING 
     $query = 'UPDATE test SET status = 1 WHERE id = '.$queueRowArray['id']; 
     mysql_query($query); 

     // RELEASE THE LOCK SO OTHER JOBS CAN GET QUEUE ROWS 
     releaseLock($queueLockName, $dbc); 

     // DO STUFF ... 

     // UPDATE QUEUE ROW STATUS TO PROCESSED 
     $query = 'UPDATE test SET status = 2 WHERE id = '.$queueRowArray['id']; 
     mysql_query($query); 

     $rowsProcessed++; 
    } 
} 

echo "\n\n".'process finished ('.$rowsProcessed.')'."\n\n";