Frank
From: Mufaddal Khumri <[EMAIL PROTECTED]> Reply-To: "Tomcat Users List" <[EMAIL PROTECTED]> To: "Tomcat Users List" <[EMAIL PROTECTED]> Subject: Re: Design question .. Date: Mon, 21 Jun 2004 14:47:52 -0700
Hi Frank
I understand this approach. The problem is that the course table has a field called "maxregistration" if this field is set to -1 it means that there is no limit. If there is a value in this field, it will be >= 1. I have another table called Enrollments. This table has courseId, userId information. It keeps track of what user is enrolled in what course. I have a waitinglist table which has userId, courseId, timestamp. It keeps track of what users are waiting to be enrolled in a course that has met its capacity. The instructor can give override to users in waitinglist for a table. An override transfers a user from the WaitingList table to the Enrollment table. This means that at any given time the enrollment table could have more than "course.maxregistration" users enrolled for a course.
Could you give me an example of a constraint that I could put on my tables so that it does not allow registrations more than "course.maxregistration"?
On Jun 21, 2004, at 12:56 PM, Frank Zammetti wrote:
Probably the easiest way to handle this is simply to have a constraint on your database that says a record cannot be added if some field is 0 (or a unique constraint, depending on how your table is keyed). Then, just catch the exception in your code and check to see if it's a violation of your rule, then return a message to the user saying the class has filled up.
Simply put, don't worry about them putting the class in the shopping cart. Make sure you have a note on the site that says they are NOT actually registered until the shopping cart is processed. Then, let the database handle the concurrency issues (which they are very good at!) and you don't have to complicate your code any.
I generally like staying away from database-level rules like this, just in case you tie yourself to a particular vendor, but something like this is pretty safe, and is tailor-made for such a mechanism.
Frank
From: Mufaddal Khumri <[EMAIL PROTECTED]> Reply-To: "Tomcat Users List" <[EMAIL PROTECTED]> To: "Tomcat Users List" <[EMAIL PROTECTED]> Subject: Design question .. Date: Mon, 21 Jun 2004 12:37:04 -0700
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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