PHP / MySQL - how to prevent two requests *Update

Question

I have some question ... example: a user will buy something for his USD

1. Check his USD Balance
2. Deduct the USD from his account
3. Make an Order -

Answer 1

Use TRANSACTION and if it fails you can rollback.

For example, assume the current balance is $20.

Connection A               Connection B
=======================    ===========================
BEGIN TRANSACTION         
                           BEGIN TRANSACTION
SELECT AccountBalance  
                           SELECT AccountBalance
--returns $20
--sufficient balance,
--proceed with purchase
                           --returns $20
                           --sufficient balance,
                           --proceed with purchase

                            --update acquires exclusive lock
                           UPDATE SET AccountBalance
                              = AccountBalance - 20
--update blocked due
UPDATE SET AccountBalance
  = AccountBalance - 20

                           --order complete
                           COMMIT TRANSACTION

--update proceeds

--database triggers
--constraint violation
--"AccountBalance >= 0"

ROLLBACK TRANSACTION

Answer 2

First off, you have to use transactions, but that's not enough. In your transaction, you can use SELECT FOR UPDATE.

It's basically saying, "I'm going to update the records I'm selecting", so it's setting the same locks that an UPDATE would set. But remember this has to happen inside a transaction with autocommit turned off.

Answer 3

you need to use TRANSACTION at the SERIALIZABLE isolation level.

Answer 4

This is how I used to do it many years ago..

results = query("UPDATE table SET value=value-5 WHERE value>=5 AND ID=1")
if (results == 1) YEY!

(Is this still a reliable method?)

Answer 5

You need to use Data revision for MySQL UPDATE.