A random Twitterizer developer that emailed me was attempting to pull data for multiple users all in background threads, but noticed that his requests started to be rejected by Twitter. I proposed that it was possibly a spam countermeasure by the Twitter API, and that he needed to throttle his requests. He had no idea how to do that. Seeing an excellent challenge, I jumped into Visual Studio and created the following console application.

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namespace ThrottledThreads
{
    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Threading;

    class Program
    {
        // Configuration stuff
        static int maxThreads = 10000;
        static int maxActiveThreads = 4;
        static int maxWaitTime = 10 * 1000;

        static bool verbose = false;

        // The rest doesn't need to be modified.
        public static int currentlyActive = 0;
        public static Queue<BackgroundWorker> threadQueue = new Queue<BackgroundWorker>();
        public static AutoResetEvent resetEvent = new AutoResetEvent(false);
        static object currentlyActiveLock = new object();

        static Random rnd = new Random();

        /// <summary>
        /// The entry point for the application
        /// </summary>
        /// <param name="args">The args.</param>
        /// <remarks></remarks>
        static void Main(string[] args)
        {
            Console.WriteLine("[Main] Queueing threads");

            // Add the background threads to the queue.
            for (int i = 0; i < maxThreads; i++)
            {
                BackgroundWorker worker = new BackgroundWorker();
                worker.DoWork += new DoWorkEventHandler(backgroundWorker_DoWork);
                worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(backgroundWorker_RunWorkerCompleted);

                threadQueue.Enqueue(worker);
            }

            int threadNumber = 1;

            Console.WriteLine("[Main] Entering main loop.");

            // Loop through the queue and process new workers as they complete
            while (threadQueue.Count > 0)
            {
                lock (currentlyActiveLock)
                {
                    for (int index = currentlyActive; index <= maxActiveThreads; index++)
                    {
                        Console.WriteLine("[Main] Running {0}", threadNumber);
                        threadQueue.Dequeue().RunWorkerAsync(threadNumber);
                        currentlyActive++;
                        threadNumber++;
                    }
                }

                if (verbose)
                    Console.WriteLine("[Main] Waiting ...");
                resetEvent.WaitOne();
            }

            Console.WriteLine("[Main] All threads completed");
            Console.WriteLine("Press any key to exit");
            Console.ReadKey();
        }

        /// <summary>
        /// Handles the DoWork event of the backgroundWorker control.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The <see cref="System.ComponentModel.DoWorkEventArgs"/> instance containing the event data.</param>
        /// <remarks></remarks>
        private static void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker worker = sender as BackgroundWorker;

            if (verbose)
                Console.WriteLine("[{0}] Starting", e.Argument);

            int time = rnd.Next(500, maxWaitTime);

            // This is where our actual work is done. For the example, we'll just sleep for a while.
            Thread.Sleep(time);

            Console.WriteLine("[{0}] Finished in {1:0.00}s", e.Argument, time / 1000d);

            e.Result = e.Argument;
        }


        /// <summary>
        /// Handles the RunWorkerCompleted event of the backgroundWorker control.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The <see cref="System.ComponentModel.RunWorkerCompletedEventArgs"/> instance containing the event data.</param>
        /// <remarks></remarks>
        private static void backgroundWorker_RunWorkerCompleted(
            object sender, RunWorkerCompletedEventArgs e)
        {
            BackgroundWorker worker = sender as BackgroundWorker;

            lock (currentlyActiveLock)
            {
                currentlyActive--;
            }

            Console.WriteLine("[{0}] Completed", e.Result);

            // Signal that the thread has finished and a new thread can be processed.
            resetEvent.Set();
        }
    }
}

The code uses the generic Queue<T> class to queue a large set of BackgroundWorker objects. It will execute a set number of threads, then wait for a worker to complete. When a worker completes, it sends a signal to trigger a new worker to be executed from the queue. It’s pretty cool to watch.

References