Synchronize and pause Thread in PHP - php

Question

I am running 2 threads at the same time, but I have critical section where I need to put something in a MySql DB. The problem is that they can put the same thing in at the same time.
I have done some calculations that shows that for indexing 20000 different news pages, the indexes are from 20000 to 20020. (So 0 to 20 are duplicates)
How do I pause one thread while the other is accessing the database?
-----thread.php
class Process extends Thread {
public function __construct($website_url){
$this->website_url = $website_url;
}
public function run() {
work($this->website_url);
}
}
-------------- work
function work($website_url) {
while(condition) {
some work...
if(something->check){ // if this exist in base
mysqli->query("INSERT something IN db...");
prepare bind exec...
}
// between check and insert, second thread can put that element
// critical section is really small but sometimes occurs ...
}
}
------ main.php
$job1 = new Process($website_url,$trigger);
$job2 = new Process($website_url,$trigger);
$job1->start();
$job2->start();

Answer 1

Mutual Exclusion
The simplest way of achieving what you want here is by the use of a single Mutex:
<?php
class Process extends Thread {
public function __construct($url, $mutex) {
$this->url = $url;
$this->mutex = $mutex;
}
public function run() {
work($this->url, $this->mutex);
}
protected $url;
protected $mutex;
}
function work($url, $mutex) {
while (1) {
/* some work */
/* failing to check the return value of calls to acquire
or release mutex is bad form, I haven't done so for brevity */
Mutex::lock($mutex);
{
/* critical section */
printf("working on %s\n", $url);
/* sleeping here shows you that the critical section is
not entered by the second thread, this is obviously not needed */
sleep(1);
}
Mutex::unlock($mutex);
/* breaking here allows the example code to end, not needed */
break;
}
}
$website = "stackoverflow.com";
$lock = Mutex::create();
$jobs = [
new Process($website, $lock),
new Process($website, $lock)
];
foreach ($jobs as $job)
$job->start();
foreach ($jobs as $job)
$job->join();
/* always destroy mutex when finished with them */
Mutex::destroy($lock);
?>
This code should explain itself, I have added a few comments to guide you through it.