Preventing Users from Working on the Same Row

Question

I have a web application at work that is similar to a ticket working system. Some users enter new issues. Other workers choose and resolve issues. All of the data is maintained in MS SQL server 2005.

The users working to resolve issues go to a page where they can view open issues. Because up to twenty people can be looking at this page at the same time, one potential problem I had to address was what happens if someone picks an issue that someone else picked just after their page loaded.

To address this, I did two things. First, the gridview displaying the issues to select uses an AJAX timer to update every second. Once an issue has been selected, it disappears one second later at most. In case they select one within this second, they get a message asking them to choose another.

The problem is that the AJAX part of this is sending too many updates (this is what I am assuming) and it is affecting the performance of the page and database. In addition, the updates are not performing every second. I find the timer to be unreliable when working to trigger stored procedures.

There has to be a better way, but I can't seem to find one. Does anyone have experience with a situation like this or have suggestions to keep multiple users from selecting the same record to maintain? I really do not want to disable the AJAX part entirely because I feel the message alone would make the application frustrating to use.

Thanks,

Answer 1

Have you tried increasing the time between refreshes. I would expect that once per 30 seconds would be sufficient. 40 requests/minute is a lot less load than 1200/minute. Your users may not even notice the difference.

If they do, how about providing a refresh button on the page so the users can manually refresh the list just prior to selecting an item to avoid the annoying message if they choose.

Answer 2

If possible limit the system so that they just get the next open issue off the work queue as opposed having them be able choose from all open issues.

If that isn't possible, I suppose you could check upon the choosing of an issue to see if it is still available. If it's not available, then make it disappear after the user clicks on it. This way you are only requesting when they actually click on something as opposed to constant polling of the data.

Answer 3

Put a lock timestamp field on the row in the database. Write a stored proc that returns true or false if the expiration timsetamp is older than a specific time. Set your sessions on your web app to expire in the same time, a minute or two. When a user select a row they hit the stored proc which helps the app to decide if it should let the user to modify it.

Hope that makes sense....

Answer 4

I did something similar where once a user opened a ticket (row) it assigned that ticket to that user and set a value on that record, like and FK to that particular user, so if anyone else tried to open that ticket (row) it would let them know it has already been assigned to someone else.

Answer 5

I'm missing to see the issue, specially after you mentioned you are already flagging tickets as in progress/being maintained and have a timestamp/version of the item.

Isn't the following enough:

1. User browses the tickets and sees a list of available tickets i.e. this excludes ones that are in the db as in progress. If you want the users to also see tickets in progress, you indicate it clearly in the ticket status and disable the option to take it.
2. User either flags a ticket as in progress explicitly or implicitly by opening the ticket (depends on the user experience / how its presented to the users).
3. User explicitly moves the ticket to a different status i.e. completed, invalid, awaiting for feedback, etc.

When the items are retrieved at 1, you include a timestamp/version. When 2 happens, you use a optimistic concurrency approach to make sure that if 2 persons try to update the take the ticket at the same time only the first one will be successful.

What will happen is that for the second person, the update ... where ... timestamp = @timestamp will not find any records to update and you will report back that the ticket was already taken.

If you want, you can build on top of the above to update the UI as tickets are grabbed. This could be by just doing a full refresh of the current page of tickets after x time (maybe alerting/prompting the user), or even by retrieving a list of tickets changed for the page of tickets being showed with ajax. You still have the earlier steps in place, as this modification its just a convenience for the users.