Have you thought about doing something like a soft reservation? As the
person adds courses to their shopping cart, the available space is
decremented. If the transaction isn't finalized either because of an
error or timeout, just release the seat for someone else. When they do
finalize the transaction, make the reservation a hard, permanent one.
Just a thought.
--David
Mufaddal Khumri wrote:
Hi,
I am in the process of writing a webapp that allows users to make a
payment and register for a course. Using Apache -- Tomcat --- MySQL.
The question is a design question i guess, unless there is something
in Tomcat that I can leverage, which i might not know. I know that
this is a design question , but since I am using Tomcat as my
JSP/Servlet container and since the participants here are experts in
this field , I thought that I might get some good pointers here.
Lets take this scenario:
User 1 and User 2 are trying to register for a course called
"Taekwando". The fee for registering is X amount. Also there is
another course called "Majagutamba" and it costs Y amount. Both theses
courses have exactly 1 seat remaining.
Now lets say user 1 adds both these courses in his shopping cart, and
user 2 does the same, since user 1 has not completed his transaction
and paid the enrollment table wont have an entry for user1. (The
Enrollment table keeps track of which user is enrolled in which
course). Therefore both users have both those courses in their
shopping carts. Now both of them proceed to checkout. They enter their
credit card information and say submit. Both those users make payments
and get enrolled for both those courses!!! Which is wrong , since both
those courses could only enroll 1 more person, instead two new users
were just added.
To avoid the above problem one could implement a singleton
synchronized Transaction object that would process shopping cart
checkout in a queue. The problem with this approach are:
1. If anything goes wrong with any one transaction, it would hold up
the entire queue. (Well we can have some sort of timeouts and take
care of that.)
2. Since this is a syncrhonized singleton and if the traffic for
registering for the courses is high, this would be a slow process for
which the user will have to wait.
Is there a better solution, algorithm, to do this ? Any help is
appreciated.
Thanks,
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