Question

I wrote a multi user app in c# some time age using SQL Server 2005 express as back-end.

I have a Orders collection. In order to use this class you would need to instantiate it and just call the Load(CustomerCode) method in order to populate the collection with the specified customers`s orders.

My question:

How do I enforce concurrency, so that only 1 user can request a Orders collection for a specific customer? When the user is done with the object(when the object is set to null), I will need to make it available again.

Answer 1

You need to implement the Pessimistic Offline Lock pattern.

Essentially you have a table that you put records in that represent a ‘lock’ on records in other tables. When you want to edit a record, you check to see if there’s a lock in the lock table first, and react accordingly in your domain logic/UI.

It doesn’t have to be a database, it could be an in-memory cache. When I say ‘table’ in my example, I mean a logical table, not necessarily a database one.

Pessimistic Offline Lock prevents conflicts by avoiding them altogether. It forces a business transaction to acquire a lock on a piece of data before it starts to use it, so that, most of the time, once you begin a business transaction you can be pretty sure you’ll complete it without being bounced by concurrency control.