SQL row locks and transactions

Question

I’m really new to relational databases. I’m working on a project that involves finances and so I want any actions that affect balance not to occur at the same time and I want to achieve that using locks, however I’m not sure how to use them. Vision I have now: I want to have a separate table for each action and a balance field in users table, value of which would be derived from all the relevant tables. That being sad I’m never actually going to update existing records - only adding them. I want to make sure only one record for each user is being inserted at a time in these tables. For instance: 3 transactions occur at the same time and so 3 records are about to be added to any relevant tables. Two of the records have the same userid, a foreign key to my users table, and the other one has a different one. I want my records with the same foreign keys to be pipelined, and the other one can be done whenever. How do I achieve this? Are there any better ways to approach this?

Answer 1

I want any actions that affect balance not to occur at the same time

Why?

I want to achieve that using locks

Why?

To give you a counter example. Let's say you want to avoid having negative account balances. When a user withdraws 500$, how can you model that without locks.

UPDATE accounts
   SET balance = balance - 500
 WHERE accountholderid = 42
   AND balance >= 500

This works without any explicit locks and is safe for concurrent access. You will have to check the update count, if it is 0 you would have overdrawn the account.

(I'm aware MySQL will still acquire a row lock)

It still makes sense to have a ledger but even there the need for locks is not obvious to me.

Answer 2

1. Use ENGINE=InnoDB for all your tables.

2. Use transactions:

   BEGIN;
   do all the work for a single action
   COMMIT;
   

The classic example of a single action is to remove money from one account and add it to another account. The removing would include a check for overdraft, in which case you would have code to ROLLBACK instead of COMMIT.

The locks you get assure that everything for the single action is either completely done, or nothing at all is done. This even applies if the system crashes between the BEGIN and COMMIT.

Without begin and commit, but with autocommit=ON, each statement is implicitly surrounded by begin and commit. That is the UPDATE example in a previous answer is 'atomic'. However, if the money deducted from the one account needs to be added to another account, what will happen if a crash occurs just after the UPDATE? The money vanishes. So, you really need

 BEGIN;
 if not enough funds, ROLLBACK and exit
 UPDATE to take money from one account
 UPDATE to add that money to another account
 INSERT into some log or audit trail to track all transactions
 COMMIT;

Check after each step -- ROLLBACK and take evasive action on any unexpected error.

What happens if 2 (or more) actions happen at the "same time"?

• One waits for the other.
• There is a deadlock and a ROLLBACK is forced on it.

But, in no case, will the data be messed up.

A further note... In some cases you need FOR UPDATE:

BEGIN;
SELECT some stuff from a row FOR UPDATE;
test the stuff, such as account balance
UPDATE that same row;
COMMIT;

The FOR UPDATE says to other threads "Keep your hands off this row, I'm likely to change it; please wait until I am finished." Without FOR UPDATE, another thread could sneak in and drain the account of the money you thought was there.

Comments on some of your thoughts:

• One table is usually sufficient for many users and their account. It would contain the "current" balance for each account. I mentioned a "log"; that would be a separate table; it would contain a "history" (as opposed to just the "current" info).
• FOREIGN KEYs are mostly irrelevant in this discussion. They serve 2 purposes: Verify that another table has a row that should be there; and implicitly create an INDEX to make that check faster.
• Pipelining? If you are not doing more than a hundred 'transactions' per second, the BEGIN..COMMIT logic is all you need to worry about.
• "Same time" and "simultaneously" are misused terms. It is very unlikely that two users will hit the database at the "same time" -- consider browser delays, network delays, OS delays, etc. Plus the fact that most of those steps force activity to go single-file. The network forces one message to get there before another. Meanwhile, if one of your 'transactions' takes 0.01 second, who cares if the "simultaneous" request has to wait for it to finish. The point is that what I described will force the "wait" if needed to avoid messing up the data.

All that said, there still can be some "at the same time" -- If transactions don't touch the same rows, then the few milliseconds it takes from BEGIN to COMMIT could overlap. Consider this timeline of two transactions that came in almost simultaneously:

BEGIN;  -- A
pull money from Alice  -- A
      BEGIN;   -- B
      pull money from Bobby  -- B
give Alice's money to Alan  -- A
      give Bobby's money to Betty  --B
COMMIT;   --A
      COMMIT;  --B