I am making an invoicing system, with the support for multiple subsidaries which each have their own set of invoice numbers, therefore i have a table with a primary key of (Subsidiary, InvoiceNo)
I cannot use MySQL auto increment field, as then it will be constantly incrementing the same count for all subsidaries.
I don’t want to make seperate tables for each subsidiary as there will be new subsidaries added as need be…
I am currently using “Select Max (ID) Where Subsidiary = X”, from my table and adding the invoice according to this.
I am using nHibernate, and the Invoice insert, comes before the InvoiceItem insert, therefore if Invoice insert fails, InvoiceItem will not be carried out. But instead i will catch the exception, re-retrieve the Max(ID) and try again.
What is the problem with this approach? And if any, what is an alternative?
The reson for asking is because i read one of the answers on this question: Nhibernate Criteria: ‘select max(id)’
As you say, the problem with this approach is multiple sessions might try and insert the same invoice ID. You get a unique constraint violation, have to try again, that might fail as well, and so on.
I solve such problems by locking the subsiduary during the creation of new invoices. However, don’t lock the table, (a) if you are using InnoDB there are problems that a
lock tablecommand by default will commit the transaction. (b) There is no reason why invoices for two different subsiduaries shouldn’t be added at the same time as they have different independent invoice numbers.What I would do in your situation is:
SELECT .. FOR UPDATEcommand. This can be done usingLockMode.UPGRADEin NHibernate.This serializes all invoice inserts for one subsiduary (i.e. only one session can do such an insert at once, any second attempt will wait until the first is complete or has rolled back) but that’s what you want. You don’t want holes in your invoice numbers (e.g. if you insert invoice id 3485 and then it fails, then there are invoices 3484 and 3486 but no 3485).