Web applications and multi-threading

Question

I'm working on porting a desktop application (WinForm) to a web application (Java/Spring/JPA). The problems are many and I'm struggling a bit...

Now the problem is threading!

In the original application, that performs the export of certain data from the DB, there is a progress-bar indicating the progress of the process.

I want to port this progress-bar in the new web application. To do this I thought of using AJAX and use a separate thread to run the data export.

The main concerns are:

• Am I following the right approach? Are there problems using multi-threading in web applications?
• If during the export process F5 or refresh button are pressed what exactly happens? How can I stop the process?
• How do I update the progress bar periodically? Do I have to make calls via ajax to the server?

Answer 1

I'm primarily an ASP.Net developer but from what I know of the HTTP protocol this just isn't the way to go about it. I've seen a lot of fairly clever solutions for this but in the end what becomes clear is that the HTTP protocol simply isn't designed to work like this.

Obviously you're aware that a flash or silverlight app would be able to do this but that comes with it's own set of issues.

Myself I prefer to keep all the weirdness on the server. In the past I've had to come up with a way to deliver several thousand emails through a web application and update the user on how it's coming along. I designed a set of tables to act as a queue. The web application would simply place any delivery requests in this queue and the progress bar would be determined by a request that checks the status of the items in the queue. Running in the background was a windows service which would also check this queue and was actually responsible for delivering the mail and setting the status of each item as it completed or failed.

It was a bit difficult to develop since windows services can be tricky but once it was up and running it was extremely smooth and reliable. Depending on your circumstances perhaps a simple scheduled task set to run every few minutes would do the trick for you.