MACROMEDIA ColdFusion MX, Develop Manual

Page: 334 / 860

334 Chapter 15: Using Persistent Data and Locking
Sample locking scenarios
The following examples present scenarios in which you need to lock ColdFusion code. These
scenarios show only two of the circumstances where locking is vital.
Reading and writing a shared variable
If you have an application-wide value, such as a counter of the total number of tickets sold, you
might have code such as the following on a login page:
<cfset Application.totalTicketsSold = Application.totalTicke
ticketOrder>
When ColdFusion executes this code, it performs the following operations:
1
Retrieves the current value of Application.totalTicketsSold from temporary storage.
2
Increments this value.
3
Stores the result back in the Application scope.
Suppose that ColdFusion processes two ticket orders at approximately the same time, and that the
value of Application.totalTicketsSold is initially 160. The following sequence might happen:
1
Order 1 reads the total tickets sold as 160.
2
Order 2 reads the total tickets sold as 160.
3
Order 1 adds an order of 5 tickets to 160 to get 165.
4
Order 2 adds an order of 3 tickets to 160 to get 163.
5
Order 1 saves the value 165 to Application.totalTicketsSold
6
Order 2 saves the value 163 to Application.totalTicketsSold
The application now has an inaccurate count of the tickets sold, and is in danger of selling more
tickets than the auditorium can hold.
To prevent this from happening, lock the code that increments the counter, as follows:
<cflock scope="Application" timeout="10" type="Exclusive">
<cfset Application.totalTicketsSold = Application.totalTicke
ticketOrder>
</cflock>
The
cflock
tag ensures that while ColdFusion performs the processing in the tag body, no other
threads can access the Application scope. As a result, the second transaction is not processed until
the first one completes. The processing sequence looks something like the following:
1
Order 1 reaches the lock tag, which gets an Application scope lock.
2
Order 1 reads the total tickets sold as 160.
3
Order 2 reaches the lock tag. Because there is an active Application scope lock, ColdFusion
waits for the lock to free.
4
Order 1 adds an order of 5 tickets to 160 to get 165.
5
Order 1 saves the value 165 to Application.totalTicketsSold.
6
Order 1 exits the lock tag. The Application scope lock is now free.
7
Order 2 gets the Application scope lock and can begin processing.
8
Order 2 reads the total tickets sold as 165.
9
Order 2 adds an order of 3 tickets to 165 to get 168.
10
Order 2 saves the value 168 to Application.totalTicketsSold.