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because WordPress is not thread-safe

WordPress is no longer just a blogging platform. It's a framework. And like all mature frameworks it drastically needs a lock API.

Example

Consider the following user balance topup function that is susceptible to a race condition:

// topup function that is not thread-safe
public function topup_user_balance( $user_id, $topup ) {
	$balance = get_user_meta( $user_id, 'balance', true );
	$balance = $balance + $topup;
	update_user_meta( $user_id, 'balance', $balance );
	return $balance;
}

Try to call the above code 100 times in 16 threads. The balance will be less than it is supposed to be.

// A thread-safe version of the above topup function.
public function topup_user_balance( $user_id, $topup ) {
	$user_balance_lock = new WP_Lock( "$user_id:meta:balance" );
	$user_balance_lock->acquire( WP_Lock::WRITE );

	$balance = get_user_meta( $user_id, 'balance', true );
	$balance = $balance + $topup;
	update_user_meta( $user_id, 'balance', $balance );

	$user_balance_lock->release();

	return $balance;
}

The above code is thread safe.

Lock levels

WP_Lock::READ - other processes can acquire READ but not WRITE until the original lock is released. A shared read lock.
WP_Lock::WRITE (default) - other processes can't acquire READ or WRITE locks until the original lock is released. An exclusive read-write lock

Usage

Require via Composer composer require soulseekah/wp-lock in your plugin.

use soulseekah\WP_Lock;

require 'vendor/autoload.php';

$lock = new WP_Lock\WP_Lock( 'my-first-lock' );

Caveats

In highly concurrent setups you may get Deadlock errors from MySQL. This is normal. The library handles these gracefully and retries the query as needed.

soulseekah / wp-lock

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