Question

I’m currently building a Windows Service which needs to process a queue of messages that are sat in a database table. This queue could vary in length and could take anything from 5 seconds to 55 seconds to execute against all rows in the database (I’m currently using a test data set of 500,000 records)

The Windows Service is configured to run on a 30 second timer so I have tried, unsuccessfully, to ensure that when the timer delegate runs that it is not able to run again until the previous request to the method has completed successfully

I have the following code in my Windows Service OnStart method:

     AutoResetEvent autoEvent = new AutoResetEvent(false);
     TimerCallback timerDelegate = new TimerCallback(MessageQueue.ProcessQueue);

     Timer stateTimer = new Timer(timerDelegate, autoEvent, 1000, Settings.Default.TimerInterval); // TimerInterval is 30000

     autoEvent.WaitOne();

And the following code in MessageQueue.ProcessMessage:

      Trace.Write("Starting ProcessQueue");
      SmtpClient smtp = new SmtpClient("winprev-01");

      AutoResetEvent autoEvent = (AutoResetEvent)stateObject;

      foreach (MessageQueue message in AllUnprocessed)
      {
          switch (message.MessageType)
          {
              case MessageType.PlainText:
              case MessageType.HTML:
                  SendEmail(smtp, message);

                  break;

              case MessageType.SMS:
                  SendSms(message);

                  break;

              default:
                  break;
          }
      }

      autoEvent.Set();
      Trace.Write("Ending ProcessQueue");

I’m using DebugView to analyse the view the Trace statements as the Service runs and I can see multiple instances of “Starting ProcessQueue” which occur every 30 seconds which is what I am trying to avoid happening

In summary: I want to call ProcessQueue and ensure that it is not executed again unless it has completed its work (this enables me to prevent the same messages in the queue being processed multiple times

I’m sure I’m missing something pretty obvious here so any help would be much appreciated 🙂

Dave

Answer 1

Why don’t you have your delegate disable the timer and then re-enable it (or continue working, if timer would expire immediately) once it’s through working. Provided the latency between timer firing and your delegate waking up is < 30 seconds, this should be watertight.

while (true)
{
  Trace.Write("Starting ProcessQueue")
  stateTimer.Enabled = false;
  DateTime start = DateTime.Now;

  // do the work

  // check if timer should be restarted, and for how long
  TimeSpan workTime = DateTime.Now - start;
  double seconds = workTime.TotalSeconds;
  if (seconds > 30)
  {
    // do the work again
    continue;
  }
  else
  {
     // Restart timer to pop at the appropriate time from now
     stateTimer.Interval = 30 - seconds;
     stateTimer.Enabled = true;
     break;
  }
}