FileStream (Simultaneous Writing)

[pedrodepacos]

Hello,

I have a file that is being written to just before a customer goes to my payment processing company to pay for their order.

The file logs the order details and items that were in the customer's shopping cart in case something happens in the Payment Notification routine that is called after the customer pays for their order. The shopping cart is stored as a session variable so this is a way to find the shopping cart details if something goes wrong.

My question is about simultaneous writes to the file and what I should do to prevent problems when two people checkout at the same time. This is my code - I'm using ASP.NET (I'm not too familiar with writing to files so any help would be appreciated)...

 Try
   'the following filestream will open the file if it exists or create a new file
   'if it doesn't exist
   Dim fs As New FileStream(strPath, FileMode.OpenOrCreate)
   Dim sw As New StreamWriter(fs)
   'the next line of code is needed in order for the streamwriter to append the
   'lines written to the end of the file
   sw.BaseStream.Seek(0, SeekOrigin.End)
   'write the pending order information to the file
   sw.WriteLine(str)
   'flush and close the file
   sw.Flush()
   sw.Close()
  Catch DefaultExc As Exception
   Err.Clear()
  End Try

[plumsauce]

this is the reply i posted to a very similar
question in another forum:

korkus2000,
If i recall, you are on asp.net, if that is so,
then everytime you want to write to the file, grab
an application object lock. That will effectively
serialize all writes to the file. Any request to
write to the file just queues up, so that page will
take longer to be returned to the client but will
eventually succeed. For reading, just don't bother
grabbing the lock.

Make sure you open the file in write/shared read mode.
(see docs)

+++

[duckhunter]

You could also use a new log file per user. This would prevent you from having a very large single file. You could run regular scripts to clean up files dated older than "n" days.

[pedrodepacos]

Thanks for both of your input. I have already implemented Duckhunter's method as I had completely forgot about the Application Lock.

[webdevsf]

STandard way to do this is to use a new thread class that has a queue in it. You don't actually write to a file- you write to the queue, and the thread class will write to the file for you. Very performant.

[pedrodepacos]

Webdefsf,

Can you please provide me an example of the code that you would use to implement this. I'm really interested in this Thread class option.

[mattur]

Apologies if I've misunderstood, but isn't a database the easiest way to handle this?

Manually coding a locking handler can be complex and difficult, but RDBMS like SQL Server will (generally) take care of all the locking issues for you transparently.

[pedrodepacos]

You haven't misunderstood... you are right, but since I'm using Access as the database, I don't really want to connect to the database unless I really need to. I've implemented caching the data in the database as much as possible to lower the amount of connections to the database and I don't want to create another connection just to add some temporary data that, hopefully, 99% of the time will not be needed.