Google App Engine PHP - Semaphores or alternatives for an Internet Banking

[Mark Millan via StackOverflow]

I am developing an Internet Banking, running on Google App Engine and developed in PHP.

Race condition is a big concern. Users can't ever have less than $0.00 (I know that I have other problems to worry about, but let's focus just on this one right now).

Pseudo-code:

<?php

$user_id =         $_GET['user_id'];
$withdraw_amount = $_GET['withdraw_amount'];

if(getUserBalance($user_id) - $withdraw_amount >= 0){
    setUserBalance($user_id, getUserBalance($user_id) - $withdraw_amount);
    sendMoneyToUser($user_id, $withdraw_amount);
    echo 'Success';
}
else{
    echo 'Insufficient funds';
}

?>

Google Cloud SQL Database:

USER_ID     BALANCE
   1         10.00
   2         20.00

The above database is a very simplistic version of the actual database. SQL lock tables/rows would not work for me in the real world.

How can I prevent race conditions?

Remember, the code is running on Google App Engine and I don't have access to PHP Semaphores extension.

Please DO NOT REPLY directly to this email but go to StackOverflow:
http://stackoverflow.com/questions/44016197/google-app-engine-php-semaphores-or-alternatives-for-an-internet-banking

[Mark Millan via StackOverflow]

I am developing an Internet Banking, running on Google App Engine and developed in PHP.

Race condition is a big concern. Users can't ever have less than $0.00 (I know that I have other problems to worry about, but let's focus just on this one right now).

Pseudo-code:

<?php

$user_id =         $_GET['user_id'];
$withdraw_amount = $_GET['withdraw_amount'];

if(getUserBalance($user_id) - $withdraw_amount >= 0){
    setUserBalance($user_id, getUserBalance($user_id) - $withdraw_amount);
    sendMoneyToUser($user_id, $withdraw_amount);
    echo 'Success';
}
else{
    echo 'Insufficient funds';
}

?>

Google Cloud SQL Database:

USER_ID     BALANCE
   1         10.00
   2         20.00

The above database is a very simplistic version of the actual database. SQL lock tables/rows would not work for me in the real world.

Race Condition case:

GAE instance #1              GAE instance #2

user_id = 1                  user_id = 1
balance = 10                 balance = 10
withdraw_amount = 10         withdraw_amount = 10
10 - 10 >= 0    (true)       10 - 10 >= 0    (true)
sendMoneyToUser(1, 10)       sendMoneyToUser(1, 10)

User gets $10                User gets $10

The user now has $20 in his hands, but he had just $10 in the bank!!!