<?php
/**
* Mutexes are used to avoid concurrent access to your shared
* variables.
*
* This demo shows you:
* - why you should use mutexes
* - how to use mutex-style locks with Sync
*
* Should be run using PHP Cli
*/
require("Sync.php");
$sync = new Sync("/tmp/demo.sync");
// Master process (the one you launched)
if (isset($argv[1]) === false)
{
$sync->withoutMutexAlreadyCalculated = false;
$sync->withoutMutex = 1;
$sync->withMutexAlreadyCalculated = false;
$sync->withMutex = 1;
// Execute 5 daemons
// see http://stackoverflow.com/questions/12341421
$command = sprintf("/usr/bin/php %s demo > /dev/null 2>&1 &", escapeshellarg($argv[0]));
exec($command);
exec($command);
exec($command);
exec($command);
exec($command);
// Wait for all daemons to end
sleep(3);
// Display results
echo sprintf("Without mutex, 1 + 1 = %d\n", $sync->withoutMutex); // random between 2 and 6
echo sprintf("With a mutex, 1 + 1 = %d\n", $sync->withMutex); // 2
}
// Without mutex...
if (isset($argv[1]) === true)
{
// for all processes, this condition is true
if ($sync->withoutMutexAlreadyCalculated === false)
{
// a "long" calculation
sleep(1);
$sync->withoutMutex = $sync->withoutMutex + 1;
$sync->withoutMutexAlreadyCalculated = true;
}
}
// With a mutex...
if (isset($argv[1]) === true)
{
$sync->lock();
// Only one process at once can pass between lock and unlock.
// So this condition is true only once.
if ($sync->withMutexAlreadyCalculated === false)
{
// a "long" calculation
sleep(1);
$sync->withMutex = $sync->withMutex + 1;
$sync->withMutexAlreadyCalculated = true;
}
$sync->unlock();
}
/*
* Explaination : access to $sync->withoutMutexAlreadyCalculated and
* $sync->withoutMutex is concurrent : it means, before those variables has
* been set by a process, they have been read with the same value several times.
* Result of our calculation becomes unexepected.
*/
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