Handling multiple updates to a singe db field - entity-framework

Question

To give a bit of background to my issue, I've got a very basic banking system. The process at the moment goes:
A transaction is added to an Azure Service Bus
An Azure Webjob picks up this message and creates the new row in the SQL DB.
The balance (total) of the account needs to be updated with the value in the message (be it + or -).
So for example if the field is 10 and I get two updates (10, -5) the field needs to be 15 (10 + 10 - 5), it isn't a case of just updating the value, it needs to do some arithmetic.
Now I'm not too sure how to handle the update of the balance as there could be many requests come in so need to update accordingly.
I figured one way is to do the update on the SQL side rather than the web job, but that doesn't help with concurrent updates.
Can I do some locking with the field? But what happens to an update when it is blocked because an update is already in progress? Does it wait or fail? If it waits then this should be OK. I'm using EF.
I figured another way round this is to have another WebJob that will run on a schedule and will add up all the amounts and update the value once, and so this will be the only thing touching that field.
Thanks

Answer 1

One way or another, you will need to serialize write access to account balance field (actually to the whole row).
Having a separate job that picks up "pending" inserts, and eventually updates balance will be ok in case writes are more frequent on your system than reads, or you don't have to always return most recent balance. Otherwise, to get the current balance you will need to do something like
SELECT balance +
ISNULL((SELECT SUM(transaction_amount)
FROM pending_insert pi WHERE pi.user_id = ac.user_id
),0) as actual_balance
FROM account ac
WHERE ac.user_id = :user_id
That is definitely more expensive from performance perspective , but for some systems it's perfectly fine. Another pitfall (again, it may or may not be relevant to your case) is enforcing, for instance, non-negative balance.
Alternatively, you can consistently handle banking transactions in the following way :
begin database transaction
find and lock row in account table
validate total amount if needed
insert record into banking_transaction
update user account, i.e. balance = balance +transasction_amount
commit /rollback
If multiple user accounts are involved, you have to always lock them in the same order to avoid deadlocks.
That approach is more robust, but potentially worse from concurrency point of view (again, it depends on the nature of updates in your application - here the worst case is many concurrent banking transactions for one user, updates to multiple users will go fine).
Finally, it's worth mentioning that since you are working with SQLServer, beware of deadlocks due to lock escalation. You may need to implement some retry logic in any case

Answer 2

You would want to use a parameter substitution method in your sql. You would need to find out how to do that based on the programming language you are using in your web job.
$updateval = -5;
Update dbtable set myvalue = myvalue + $updateval
code example:
int qn = int.Parse(TextBox3.Text)
SqlCommand cmd1 = new SqlCommand("update product set group1 = group1 + #qn where productname = #productname", con);
cmd1.Parameters.Add(new SqlParameter("#productname", TextBox1.Text));
cmd1.Parameters.Add(new SqlParameter("#qn", qn));
then execute.