Synchronizing a timer to prevent overlap

Question

I'm writing a Windows service that runs a variable length activity at intervals (a database scan and update). I need this task to run frequently, but the code to handle isn't safe to run multiple times concurrently.

How can I most simply set up a timer to run the task every 30 seconds while never overlapping executions? (I'm assuming System.Threading.Timer is the correct timer for this job, but could be mistaken).

Answer 1

You could do it with a Timer, but you would need to have some form of locking on your database scan and update. A simple lock to synchronize may be enough to prevent multiple runs from occurring.

That being said, it might be better to start a timer AFTER your operation is complete, and just use it one time, then stop it. Restart it after your next operation. This would give you 30 seconds (or N seconds) between events, with no chance of overlaps, and no locking.

Example :

System.Threading.Timer timer = null;

timer = new System.Threading.Timer((g) =>
  {
      Console.WriteLine(1); //do whatever

      timer.Change(5000, Timeout.Infinite);
  }, null, 0, Timeout.Infinite);

Work immediately .....Finish...wait 5 sec....Work immediately .....Finish...wait 5 sec....

Answer 2

I'd use Monitor.TryEnter in your elapsed code:

if (Monitor.TryEnter(lockobj))
{
  try
  {
    // we got the lock, do your work
  }
  finally
  {
     Monitor.Exit(lockobj);
  }
}
else
{
  // another elapsed has the lock
}