Queuing using the Database or MSMQ?

Question

A part of the application I'm working on is an swf that shows a test with some 80 questions. Each question is saved in SQL Server through WebORB and ASP.NET.

If a candidate finishes the test, the session needs to be validated. The problem is that sometimes 350 candidates finish their test at the same moment, and the CPU on the web server and SQL Server explodes (350 validations concurrently).

Now, how should I implement queuing here? In the database, there's a table that has a record for each session. One column holds the status. 1 is finished, 2 is validated.

I could implement queuing in two ways (as I see it, maybe you have other propositions):

• A process that checks the table for records with status 1. If it finds one, it validates the session. So, sessions are validated one after one.
• If a candidate finishes its session, a message is sent to a MSMQ queue. Another process listens to the queue and validates sessions one after one.

Now:

• What would be the best approach?
• Where do you start the process that will validate sessions? In your global.asax (application_start)? As a windows service? As an exe on the root of the website that is started in application_start?

To me, using the table and looking for records with status 1 seems the easiest way.

Answer 1

What is validating? Before working on your queuing strategy, I would try to make validating as fast as possible, including making it set based if it isn't already so.

Answer 2

The MSMQ approach decouples your web-facing application from the validation logic service and the database.

This brings many advantages, a few of which:

• It would be easier to handle situations where the validation logic can handle 5 sessions per second, and it receives 300 all at once. Otherwise you would have to handle copmlicated timeouts, re-attempts, etc.

• It would be easier to do maintanance on the validation service, without having to interrupt the rest of the application. When the validation service is brought down, messages would queue up in MSMQ, and would get processed again as soon as it is brought up.

• The same as above applies for database maintanance.

Answer 3

If you don't have experience using MSMQ and no infrastructrure set up, I would advice against it. Sure, it might be the "proper" way of doing queueing on the Microsoft platform, but it is not very straight-forward and has quite a learning curve.

The same goes for creating a Windows Service; don't do it unless you are familiar with it. For simple cases such as this I would argue that the pain is greater than the rewards.

The simplest solution would probably be to use the table and run the process on a background thread that you start up in global.asax. You probably also want to create an admin page that can report some status information about the process (number of pending jobs etc) and maybe a button to restart the process if it for some reason fails.