Design for task scheduler in .NET 4 (Task Parallel Library)

Question

I'm trying to create a service that executes scheduled Tasks in an asynchronous (and parallel) way using TPL.

The basic requirement is that for a bunch of different tasks, each with their own scheduled rates (some to be executed every second, others 30 seconds, other 5 mins etc) to be executed concurrently. And I'm not sure of the best way to go about this, especially considering the ConcurrentBag (which I was considering as a holder of all future tasks) contains no methods with which to select collections of tasks that need to be executed.

It also means that I can't use WaitAny or WaitAll, as these short-running tasks need to finish and requeue themselves independently.

How should I proceed with this?

Edit:

Ok basically my design is thus:

A ScheduledTask, which is a wrapper for Task with a Scheduled DateTime property. A bunch of these are stored in a ConcurrentBag

A Controller that polls the ConcurrentBag (currently just a while(true) loop, but could be a Timer or similar), removing any that are scheduled, and Start()'s them.

Each ScheduledTask holds a reference to the ConcurrentBag, and enqueues a new instance of itself when it completes, with a new ScheduledTime.

This design seems to work so far, but there is something about each Task holding a reference to the ConcurrentBag that doesn't sit well with me. Any design comments or suggestions would be appreciated.

Answer 1

Have you considered using the EventLoopScheduler from RX?

Rx has lots of different scheduler implementations, but EventLoopScheduler sounds like the right one for you.

To create a repeating task with RX, you'd just use Observable.Interval(timespan, scheduler).Subscribe(action) .

Answer 2

You can't use a concurrentbag since you need to remove specific items.

One way to do it is to let each task look like

MyTask SomeAction() {
    DateTime now = DateTime.Now;
    DoSomeTask();
    return new MyTask { StartTime = now.AddMinutes(1), DoSomething = SomeAction }
}

the scheduler will look something like

List<MyTask> tasklist = new List<MyTask>();

public void Scheduler() {
    for (;;)
        DateTime now = DateTime.Now;

        List<MyTask> tasksToRun;
        lock (taskList) 
            taskToRun = taskList.Where(x => x.StartTime <= now)
                                .ToList();

        var tasks = tasksToRun.Select(x => RunTask(x))
                              .ToArray();
        Thread.Sleep(1000);
    }
}

private Task<MyTask> RunTask(MyTask myTask) {
    lock (taskList)
        tasklist.Remove(myTask);

    return Task<MyTask>.Factory.StartNew(myTask.DoSomething())
                               .ContinueWith(t => {
                                                      if (t.Result != null)
                                                          lock (taskList)
                                                              taskList.Add(t.Result);
                                                  });
}

Answer 3

如果您不想自己动手，可以使用http://quartznet.sourceforge.net/ 。

Answer 4

I wrote a synchronous task scheduler for a school project using the thread pool (not TPL though). I never got around to making it async; you could run the tasks on a separate background thread and have it execute a callback delegate. See here on SourceForge.