Synchronizing a timer to prevent overlap

问题:

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).

回答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 you're 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....

回答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
}

回答3:

I prefer System.Threading.Timer for things like this, because I don't have to go through the event handling mechanism:

Timer UpdateTimer = new Timer(UpdateCallback, null, 30000, 30000);

object updateLock = new object();
void UpdateCallback(object state)
{
    if (Monitor.TryEnter(updateLock))
    {
        try
        {
            // do stuff here
        }
        finally
        {
            Monitor.Exit(updateLock);
        }
    }
    else
    {
        // previous timer tick took too long.
        // so do nothing this time through.
    }
}

You can eliminate the need for the lock by making the timer a one-shot and re-starting it after every update:

// Initialize timer as a one-shot
Timer UpdateTimer = new Timer(UpdateCallback, null, 30000, Timeout.Infinite);

void UpdateCallback(object state)
{
    // do stuff here
    // re-enable the timer
    UpdateTimer.Change(30000, Timeout.Infinite);
}

回答4:

instead of locking (which could cause all of your timed scans to wait and eventually stack up). You could start the scan/update in a thread and then just do a check to see if the thread is still alive.

Thread updateDBThread = new Thread(MyUpdateMethod);

...

private void timer_Elapsed(object sender, ElapsedEventArgs e)
{
    if(!updateDBThread.IsAlive)
        updateDBThread.Start();
}

回答5:

You could use the AutoResetEvent as follows:

// Somewhere else in the code
using System;
using System.Threading;

// In the class or whever appropriate
static AutoResetEvent autoEvent = new AutoResetEvent(false);

void MyWorkerThread()
{
   while(1)
   {
     // Wait for work method to signal.
        if(autoEvent.WaitOne(30000, false))
        {
            // Signalled time to quit
            return;
        }
        else
        {
            // grab a lock
            // do the work
            // Whatever...
        }
   }
}

A slightly "smarter" solution is as follow in pseudo-code:

using System;
using System.Diagnostics;
using System.Threading;

// In the class or whever appropriate
static AutoResetEvent autoEvent = new AutoResetEvent(false);

void MyWorkerThread()
{
  Stopwatch stopWatch = new Stopwatch();
  TimeSpan Second30 = new TimeSpan(0,0,30);
  TimeSpan SecondsZero = new TimeSpan(0);
  TimeSpan waitTime = Second30 - SecondsZero;
  TimeSpan interval;

  while(1)
  {
    // Wait for work method to signal.
    if(autoEvent.WaitOne(waitTime, false))
    {
        // Signalled time to quit
        return;
    }
    else
    {
        stopWatch.Start();
        // grab a lock
        // do the work
        // Whatever...
        stopwatch.stop();
        interval = stopwatch.Elapsed;
        if (interval < Seconds30)
        {
           waitTime = Seconds30 - interval;
        }
        else
        {
           waitTime = SecondsZero;
        }
     }
   }
 }

Either of these has the advantage that you can shutdown the thread, just by signaling the event.

---

Edit

I should add, that this code makes the assumption that you only have one of these MyWorkerThreads() running, otherwise they would run concurrently.

回答6:

I've used a mutex when I've wanted single execution:

    private void OnMsgTimer(object sender, ElapsedEventArgs args)
    {
        // mutex creates a single instance in this application
        bool wasMutexCreatedNew = false;
        using(Mutex onlyOne = new Mutex(true, GetMutexName(), out wasMutexCreatedNew))
        {
            if (wasMutexCreatedNew)
            {
                try
                {
                      //<your code here>
                }
                finally
                {
                    onlyOne.ReleaseMutex();
                }
            }
        }

    }

Sorry I'm so late...You will need to provide the mutex name as part of the GetMutexName() method call.