Question

I have a simple power system as users can borrow to each other:

mysql_query("INSERT INTO power (sender, receiver, amount)
    VALUES ('$sender', '$receiver', '$amount')");
mysql_query("UPDATE users SET power=power-$amount WHERE user_id='$sender'");
mysql_query("UPDATE users SET power=power+$amount WHERE user_id='$receiver'");

I check if the sender has enough credit to transfer by a SELECT query before running the above-mentioned set of queries.

Issues 1: I think, it is better to put all these four queries into a Transaction; as InnoDB ACID will guarantee a safer performance. What will be the best transaction to do so?

Issue 2: power is unsigned int. If by any chance (even unlikely), user does not have enough credit power-$amount will not set 0; instead it will be cycled through the largest value of int(), which is 4294967295. This means that the user will be granted almost unlimited power (credit).

Answer 1

First of all, don’t bother with this:

I check if the sender has enough credit to transfer by a SELECT query before running the above-mentioned set of queries.

That leaves you open to a race condition that you’ll have to deal with anyway. Instead, set up constraints in your database to make it impossible to get into an invalid state.

Issue 2: power is unsigned int.

Don’t do that either, there’s no need. A 4 byte signed integer should have more than enough space on the positive side; if it doesn’t, you can switch to a signed 8 byte integer. The reason you want a signed value is that it makes integrity checking fairly easy: if a balance drops below zero then something is wrong. If you use an unsigned value, you’d have to reserve 4294967295-n (for some n) to detect underflow and overflow instead of a simple < 0.

As far as constraining your data is concerned, normally you’d use a CHECK constraint like this:

check (power >= 0)

but MySQL doesn’t support CHECK constraints. However, you can write a BEFORE INSERT OR UPDATE trigger that can check that new.power >= 0 and raise an exception if it isn’t.

Now you have a users table that doesn’t allow invalid power values so we’re done with issue 2.

On to issue 1: transactions. Yes, you absolutely do want to use a transaction for the transfer. You’ll want a sequence like this:

1. start transaction
2. INSERT INTO power (sender, receiver, amount) VALUES ('$sender', '$receiver', '$amount')
3. UPDATE users SET power=power-$amount WHERE user_id='$sender'
4. UPDATE users SET power=power+$amount WHERE user_id='$receiver'
5. Have there been any errors from the “check trigger”?
   • If there have then send a rollback to the database and tell the user what went wrong.
   • If there haven’t then send a commit to the database.

The transaction will ensure that all three operations will either succeed or fail as a single unit and the CHECK constraint (implemented as a trigger) ensures that no one can give away more power than they have.