PHP method execution only from one client at a time - php

Question

I wrote a class that syncs the db from an xml file and reports through email any alerts.
The xml contains product prices and stock.
The execution of the method only occurs only if the xml filetime is newer than the last one synced.
Here is the first problem. I suspect that server (randomly) changes the filetime for some reason, becuse the sync method runs although no new xml file produced.
The xml file is exported from a local server and uploads to the remote server through an ftp client
(SyncBack)
Second problem is that on heavy traffic hours, the do_sync method runs more than once because i get the alerts more than once into my email.
I understand why it is called many times, so i created a flag syncing_now, to prevent the execution.
The mistake is that the flag is stored into db and since the first call has to update the db, all other call can run the method.
<?php class Sync extends Model{
public function __construct(){
parent::__construct();
$this->syncing_now = $this->db->get($syncing_now);
}//END constructor
public function index(){
if($this->determine_sync()){
$this->do_sync();
}else{
return FALSE;
}
}
public function determine_sync(){
if( filemtime($file) <= $this->db->last_sync() or !$this->$syncing_now){
return FALSE;
}else{
return TRUE;
}
}
public function do_sync(){
$this->db->update('syncing_now', TRUE);
//the sync code works fine..
$this->db->update('syncing_now', FALSE);
}
}
So what can i do to run the method only once and how can track down why the filetime change occurs?
Thanks all any help appreciated.

Answer 1

I suggest you use a Table that stores your Synchronisations.
id | md5_of_xml_file | synched_date
Now use LOCK_TABLES in order to ensure that only one process at a time may process your sync files.
Lock the synchronisations table. If locking them fails, just quit.
if (!mysqli_query('LOCK TABLES synchronisations WRITE')) {
die();//quit;
}
If an entry with the hash of the XML Sync File already exists, just quit.
$md5Hash = md5_file('yourXmlSyncFile.xml');
$result = null;
$stmt= $mysqli->prepare("SELECT md5_of_xml_file FROM synchronisations
WHERE md5_of_xml_file=?");
$stmt->bind_param("s", $md5Hash);
$stmt->execute();
$stmt->bind_result($result);
$stmt->fetch();
$stmt->close();
if ($result == $md5Hash) {
die();//quit;
}
Else, attempt to sync the file. If that works, add an entry, storing when you did this and a hash of the file used for synchronisation.