PHP Prevent simultaneous function execution (throttling limits via PHP)

Question

I have a function that send HTTP request via CURL to www.server.com

My task is to make sure that www.server.com gets no more than one request every 2 seconds.

Possible solution:

Create a function checktime() that will store current call time in database and check with database on every next call and make system pause for 2 seconds:

$oldTime = $this->getTimeFromDatabase();
if ($oldTime < (time() - 2) ) { // if its been 2 seconds
  $this->setNewTimeInDatabase(); 
  return true;
} else {
  sleep(2);
  return false;
}

The problem/question:

Lets say, the last request to www.server.com was on 1361951000. Then 10 other users attempt to do request on 1361951001 (1 seconds later). checktime() Function will be called.

As since it only has been 1 second, the function will return false. All 10 users will wait 2 seconds. Does it means that on 1361951003 there are 10 requests will be sent simultaneously? And is it possible that the time of last request will not be changed in database, because of the missed call of $this->setNewTimeInDatabase() in checktime()?

Thank you!

UPDATE:

I have just been told that using a loop might solve the problem:

for($i=0;$i<300;$i++)
{
   $oldTime = $this->getTimeFromDatabase();
   if ($oldTime < (time() - 2) ) { // if its been 2 seconds
   $this->setNewTimeInDatabase(); 
   return true;
   } else {
    sleep(2);
    return false;
   }
}

But i don't really see logic in it.

Answer 1

I believe you need some implementation of a semaphore. The database could work, as long as you can guarantee that only one thread gets to write to the db and then make the request.

For example, you might use an update request to the db and then check for the updated rows (in order to check whether the update actually happened). If the update was succesful you can assume you got the mutex lock and then make the request (assuming the time is right to make it). Something like this:

$oldTime = $this->getTimeFromDatabase();
if ($oldTime < (time() - 2) && $this->getLock()) { // if its been 2 seconds
  $this->setNewTimeInDatabase(); 
  $this->releaseLock();
  return true;
} else {
  sleep(2);
  return false;
}  

function getLock()
{
    return $mysqli->query('UPDATE locktable set locked = 1 WHERE locked = 0');
}

function releaseLock()
{
     $mysqli->query('UPDATE locktable set locked = 0');
}

I'm not sure about the mysql functions, but I believe it's ok to get the general idea.

Answer 2

Watch out with using a database. For example MySQL is not always 100% in sync with its sessions, and for that reason it is not safe to rely on that for locking purposes.

You could use a file-lock through the method flock, where you would save the access time in. Then you could be sure to lock the file, so no two or more processes would ever access it at the same time.

It would probably go something like this:

$filepath = "lockfile_for_source";
touch($filepath);
$fp = fopen("lockfile_for_resource", "r") or die("Could not open file.");

while(true){
  while(!flock($fp, LOCK_EX)){
    sleep(0.25); //wait to get file-lock.
  }

  $time = file_get_contents($filepath);
  $diff = time() - $time;
  if ($diff >= 2){
    break;
  }else{
    flock($fp, LOCK_UN);
  }
}

//Following code would never be executed simultaneously by two scripts.
//You should access and use your resource here.

fwrite($fp, time());
fflush($fp);
flock($fp, LOCK_UN); //remove lock on file.
fclose($fp);

Please be aware that I have not tested the code.