Automatic countdown timer to adjust database record

Question 1

Have you considered Ajax? are you storing the last updated time on manually setting the flag? You can use Ajax request to simultaneously run using jquery Set interval. which will trigger the ajax request every 2 minutes. Find the last time it was updated, if that is passed 20 minutes then remove one from the database, your return would be the new number and jquery can update that number for you.

Quite a simple process actually but need more detail on the underlying data.

Here is how I can see it working from your question

In Controller

public ActionResult ajaxUpdate()
        {
            //open connection
            dbcontext db = new dbcontext();
            db.Connection.Open();

            // get the last updated record in the database.
            var entry = db.Entry.OrderByDecending(m=> m.LastUpdatedDate).FirstOrDefault();

            //clean up
            db.Connection.Close();
            db.Dispose();

            //return -1 as error
            if(entry == null){

                return Json(-1,JsonRequestBehavior.AllowGet);

            }

            // get current number of people in queue
            Int32 numberOfPeople = entry.QueueNumber;

            TimeSpan span = DateTime.Now.Subtract(entry.LastUpdatedDate);

            if(span.Minutes >= 20){

                // if 20 mins have passed assume a person has been completed since manual update
                numberOfPeople--;

            }

            //this returns a number, alternatively you can return a Partial
            return Json(numberOfPeople, JsonRequestBehavior.AllowGet);
        }

Jquery and Ajax

$(document).ready(function () {

    // run function every x minutes
    setInterval(function () {
        UpdateQueue();
    }, 100000);





});
    function UpdateQueue() {

    $.ajax({
        cache: true,
        type: 'POST',
        url: "/ControllerName/ajaxUpdate",
        async: false,
        dataType: "json",
        success: function (result) {
            // on success result will be the number returned

            // -1 is error
            if (result == -1) {
                return;

            }

            // check the -- didn't return a negative
            if (result < 0) {

                result = 0;

            }

            //find your element in the HTML to update
            $('#NumberElement').text().replaceWith(result);


        }

    });


}

You must ensure you include your jquery libraries before you include this code or you will have Jquery not defined.

Question 2

I have made up for you server side solution with a little bit threading. Hope I am correct on critical sections locks.

It has an advantage that admin of your application does not have to hang on the page to get number of current customers downcounted (like he should with ajax requests).

How it works

On 'number of customers' update it is starting (if necessary) new counting-down thread, which waits (sleeps) for predefined interval and then decreases the number.

public class CustomerAdminService
{
    // time in milliseconds it will take to decrease number of waiting customers 
    const int sleepTime = 10000;
    // current number of customers (just for simplicity - you can have it in db or somewhere else)
    static int numberOfCustomers;

    static Thread updaterThread;

    // object lock
    static readonly object locker = new Object();

    public int GetNumberOfCustomers()
    {
        return numberOfCustomers;
    }

    public void UpdateCustomers(int value)
    {
        lock (locker)
        {
            if (updaterThread == null)
            {
                //start new downcounting thread
                updaterThread = new Thread(new ThreadStart(UpdateWorker));
                updaterThread.Start();
            }
            SetNumberOfWaitingCustomers(value);
        }
    }

    private void SetNumberOfWaitingCustomers(int value)
    {
        numberOfCustomers = value;
    }

    // downcounting thread method
    private void UpdateWorker()
    {      
        while (true)
        {
            // sleep for predefined time
            Thread.Sleep(sleepTime);
            lock (locker)
            {              
                var number = GetNumberOfCustomers();             
                if (number <= 1)
                {
                    // if number of currents customers is now zero - end the downcounting thread
                    SetNumberOfWaitingCustomers(0);
                    updaterThread = null;
                    return;
                }
                SetNumberOfWaitingCustomers(number - 1);
            }
        }
    }
}

Comment: You can consider using jQuery for some timer down-counting script. Showing something like: You can be served in 40 minutes ;-)

Question 3

Yes Ajax is the key. It can be used by your website to communicate with your server unnoticeably.

Question 4

An alternative approach would be to not update the count in the database but simply use a query to determine the number of customers within a certain time period. You can do this by modifying the model so that instead of QueueNumber it uses an arrival time and changing the controller so that it inserts a new Data record.

{
    public class Data
    {
        public int ID { get; set; }
        public DateTime Arrival_Time { get; set; }
    }

    public class DataDBContext : DbContext
    {
        public DbSet<Data> Queue { get; set; }
    } 
}

This way, as others have suggested you can use AJAX to poll for the number of people in the queue with a controller action that might look something like this:

[HttpGet]
public ActionResult NumberOfPeopleInQueue()
{
    var result = db.NumberOfCustomersSince(DateTime.Now.AddMinutes(-20));
    return Json(result);
}

The nice thing about this approach is that should haircuts start to take longer (say 30 minutes) you can simply change the query and the application continues to work.

Automatic countdown timer to adjust database record

Controller

Model code