避免多个用户使用事务插入相同的号码

Question

I have couple insert queries which are merged into transaction scope. First of that insert is to create new product article number which is built by incrementing of the highest in table by one. Unfortunately I just noticed that mostly during tests if for instance two users from two different applications clicks button which trigger my transaction's method they could get same new product number and insert it. How can I avoid that situation?

Part of my transaction query below:

Public Sub ProcessArticle(ByRef artikel As ArticlesVariations)
        Dim strcon = New AppSettingsReader().GetValue("ConnectionString", GetType(System.String)).ToString()
        Using connection As New SqlConnection(strcon)
            connection.Open()
            Using transaction = connection.BeginTransaction()
                Try

                            Using cmd As New SqlCommand("INSERT INTO tbArtikel (Nummer) VALUES (@Nummer);Select Scope_Identity()", transaction.Connection)
                                cmd.CommandType = CommandType.Text
                                cmd.Connection = connection
                                cmd.Transaction = transaction

                                 'Get next product number from table tbArtikel  (this will be new product number)'
                                 Dim NewArtNummer as String =  New DALArtikel().GetNewArtikelNumber(transaction)
                                 art.Nummer = NewArtNummer

                                cmd.Parameters.AddWithValue("@Nummer", art.Nummer)
'Get inserted product id for other diffrent inserts below'
 newArticleRowId = CInt(cmd.ExecuteScalar()) 

'....
other INSERTs queries to other tables ...
...'

       transaction.Commit()
                Catch ex As Exception
                    transaction.Rollback()
                    Throw 'Rethrow exception.'
                End Try
            End Using
        End Using
    End Sub

Article number is varchar column (I know it's bad etc) but it is how its implemented in current scope without possibility to change it.

Note that I cannot do any changes on the database level like new tables, trigger etc. (one exception is to create unique index on number column). I have to achieve that from application level.

I got three options on top of my mind and would like to take your advice which to choose:

1st option:

a) Make this column as unique column so when couple users inserting only one will insert the number but all of rest users will get error from catch about unique key violation so i could manage it somehow like make information to retry again.

b) Same as a) but also with isolation level set to ReadUncommited - from my point of view this will give me that when next users read newnumber they have chance to read from dirty numbers (so also from first user insertion) so there is chance they get next number.

2nd option:

Just to make transaction isolation to Read uncommitted - but this will not guaranty all same line users would get new number (probably worst out of all)

3rd option:

Heard something about TABLOCK but have no idea whether could be considered in this situation.

...something else?

At the moment I am considering to use 1st option -catch error and inform users.

Answer 1

I followed some of your older questions... This is obviously not so easy to solve. Therefore I have a completely different idea:

As your ProductNumber is a VARCHAR you could write there whatever you want?

Would it work to set a GETDATE() as string-literal (ISO 8601) into this column on insert and execute an UPDATE statement later, which will re-set the value to the correct numbering according to the order of insertion?

Strange and ugly anyway...