Contention Control

So we have not discussed how DP4 behaves when two programs attempt to update the same record at the same time, or how to prevent this situation from arising.

Database systems can take three approaches to contention:

• They can be single user, and only allow one program access to the database at a time.
• They can use a locking scheme that force programs to lock any records they wish to update.
• They can use optimistic contention (or concurrency) control. In this scheme programs are allowed to update any record without needing to lock it, but if two programs try to update the same record at the same time one will encounter an error condition.

DP4 supports all three approaches to contention some extent. The preferred approach is usually the third. DP4 was one of the first databases to use optimistic concurrency control. You can find out more about each approach to contention control on the following pages:

• Using Exclusive Access to Prevent Contention
• Record Locking in DP4
• Optimistic Contention Control