PHP non-blocking exclusive lock - php

Question

I would like to use exclusive/shared, both blocking and non-blocking locks like these found in flock(). How can this be achieved using semaphores?

Answer 1

It depends on the use case.
If you are on only one server, use a lock file.
function doSomething() {
$file = /temp/path/somthing.lock;
if (file_exists($file)) {
return false;
}
touch($file);
// Safely mess with things
unlink($file);
}
If you have multiple web servers, e.g. behind a load balancer, the same thing can be accomplished by using a table in mysql.
function doSomething() {
$query = "SELECT * FROM locks WHERE name='something'");
$res = mysqli_query($query);
if (mysql_num_rows($res) > 0) {
return false;
}
$query = "INSERT INTO locks (name) VALUES ('something')";
mysqli_query($query);
// Safely mess with things
$query = "DELETE FROM locks WHERE name='something'");
mysqli_query($query);
}
Memcache is another obvious candidate with multiple machine support.
You should not use ACP, because it is intended for caching only. This means you do not have control when ACP storage gets deleted, it could happen at any time.
You can also use semaphores, however the same caveats as with lock files apply if you use more than one server.
I would recommend creating lock($key), is_locked($key) and release($key) functions, and then religiously use them throughout your project. That way you can start with lock files (quick to implement), but then upgrade to something better later without editing the rest of your code. If you want to get really fancy, you can implement them as methods of a logger object that you put in a known location of your code.