Question

The below script is called every 5 seconds. The issue is that if the server is responding slow, one entry in “blog” can get selected twice in a row because the server hasn’t had time to set “done” to “1” yet. Is there an industry standard (or whatever you call it) way to prevent this from happening?

$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
    }
}

I think I could change it to

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

But will that really fix the problem? I would imagine there would be a better way that really prevents it from happening without a shadow of a doubt.

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
    }
}