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I've been trying to make a transaction using queries; I've done that like so, first I check and execute a query to check if an user has enough balance, secondly I deduct his balance and process the transaction.

Problem with this is if you download a tool, or use a macro that can click like 200 times per second (I think) the signals are sent quicker than the query can handle therefore it will still think the user has enough balance which he then ends up not having and his balance will become negative.

Here's the quick code

var processTransaction = function(userid, cost){
    database.query('SELECT `balance` FROM `user` WHERE `id` = ' + database.pool.escape(userid), function(err, row){
        if(err){
            return;
        }

        if(!row.length){
            return;
        }

        var userBalance = row[0].balance;
        if(userBalance >= cost){
            /* User has enough, process */

            addBalance(userid, -cost); //deduct query
        }
    });
}

is there any mistake I'm making here, do I approach this differently?

Query Function

var query = function(sql, callback) {
  if (typeof callback === 'undefined') {
    callback = function() {};
  }
  pool.getConnection(function(err, connection) {
    if(err) return callback(err);
    connection.query(sql, function(err, rows) {
      if(err) return callback(err);
      connection.release();
      return callback(null, rows);
    });
  });
};

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Utilizing a "queuing" architecture would also allow for you to handle concurrent requests more readily. The concept is all the requests are pushed into a queue. There is a worker set to "poll" this queue, likely on a cron of some sort, to read from the queue and disburse work. When items are received by the worker, it will issue your update/create request more sequentially and prevent race conditions. With this type of behavior, you will need to be more equipped to handle asynchronous events.

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What about making UPDATE without SELECT?

UPDATE user_balance 
SET balance = balance - :value 
WHERE user_id = :user_id AND balance >= :value

I believe this approach and transactions themselves should help to avoid reaching negative balance.

[UPD] and for detecting if balance has been actually updated or not(because of invalid balance for sure) you can read amount of rows affected(depending on lib used it could be separated method or result of UPDATE query)

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Comments
  • in mysql you can use select ... for update to lock row
  • @IłyaBursov 'lock row' ?
  • yes, lock row to prevent update in another thread
  • You can also try to fit everything into one query, something like UPDATE user SET balance=balance-x WHERE id=y AND balance>=x.
  • @riv I currently have a lot of sections using queries in the same way, it'll take me a long while to change everything to fit in one query, sadly.
  • When do I unlock the table, what if the query returns an error?
  • You still need to unlock. You can also wrap everything inside a SQL transaction, so that if an error occurs you can issue a ROLLBACK and it's as if nothing happened. Otherwise you COMMIT. Added the example from the MySQL manual.
  • LOCK/UNLOCK TABLES is not for use by InnoDB. You are using InnoDB, aren't you!?!
  • @RickJames sorry, but how is LOCK/UNLOCK tables "not for use by InnoDB"? The manual seems to say the opposite. It's true that you can set things up so you needn't use LOCK/UNLOCK tables, but there are scenarios where you have little other choice.
  • @LSerni - OK, LOCK TABLES may work with InnoDB. It was invented for MyISAM, where there is no transactional locking. For InnoDB, it is a sledgehammer chasing a fly.
  • while I think this idea is good, I want to mention that would make both BE and FE code more complicated since user still wants to see result of his/her action. and with this approach reaching that goal would become much harder
  • @skyboyer agreed, the tradeoff is handling asynchronous operations. Many queue techs, like redis, have operations baked in to view state of tasks.