Choosing which application instance will do the updates when dealing with multiple workstations w/o server involved?

Question

So, I've got an interesting design problem and I honestly don't know how to tackle this. I have an application that will be ran on a dozen of workstations. Of those workstations, it is impossible to know at any given time how many of them will be on or using my application. Every 4 hours of local time if any of the applications are on, they would check database and perform some changes. The problem is that if more than one tries to do that, there will be some major complications. So I need a way for only one application to perform the updates if more than one are running.

I was thinking of creating a file that would reside on the network drive where applications would append their workstation addresses as they're turned on and remove as they're turned off. Every 4 hours of local time, applications would check the file and whoever is at the top would be the one to run the update. The problem with that is that if an application is shut down in the way that it can't remove itself from the top of the list, no updates will appear from there on.

I don't have an option to put any applications to run on the server itself, besides storing files there, so that's out of question. Does anyone know of a solution to this problem? I can't seem to wrap my brain around it.

Edit: What I have implemented right now is this:

        int updateTimerOffset = (Convert.ToInt32(DateTime.Now.ToString("mm")) * 60000) - 120000;

        updateTimer = new System.Timers.Timer(3600000 - updateTimerOffset);
        updateTimer.AutoReset = true;
        updateTimer.Enabled = true;

        updateTimer.Elapsed += new System.Timers.ElapsedEventHandler(PerformUpdates);

and then down in my PerformUpdates I have some code that looks like this:

        int currentHr = Convert.ToInt32(DateTime.Now.ToString("HH"));

        if (currentHr % 4 >= 0 && currentHr >= fileNum)
        {
            // change fileNum in the file to currentHr + 4 if currentHr is not 24,
            // if it is, then make the value equal to 4
            // Update database stuff
        }

fileNum is a number that is stored in a file on the server.

This is done on every application's start up. So, every application, unless started absolutely at the exact same second will have a difference of 2 minutes (120,000 milliseconds) plus some seconds that I didn't account for between their start ups.

What I can do is have a simple file holding a number for the target hour (fileNum). So that when the application checks it, it looks at the number to do comparison.

So let's say that it's 16:00, in which case 16 % 4 gives us 0 and it's time for an update. That application checks the file and sees that the value is 12. 16 is equal to or greater than 12. So it changes the value to current hour (16) + 4, which is 20.

The next application's timer elapses a minute or so later, and it has 0 for its hour mod value. So, it looks at the file and sees that value is 20, which is greater than 16, so it doesn't do anything.

What do you guys think of this solution? It wouldn't work if there were hundreds of workstations, since chances of having both applications trying to do this at the same time are higher, but with a dozen or so workstations I can pull this off? Unless someone has a better solution?

Answer 1

Put an agent program on the server, and allow your client programs to initiate an update through the agent, rather than updating the database directly. This will give you the ability to manage the clients in a way that they don't clash with each other.

For example, if one client requests an update cycle, you can set a flag on the agent program to accept update requests from other clients, but simply ignore them. You can even send status back to the second client, saying "Thanks, but this update is already in progress."

You can use a Windows Service and WCF to build the agent, and talk to it as a WCF service from the clients.