Windows Azure - Troubleshooting & Debugging

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Asynchronous Parallel Block Blob Transfers with Progress Change Notification

Asynchronous Parallel Block Blob Transfers with Progress Change Notification

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UPDATE March 6, 2013:

This code has been significantly updated and the new version can be found at http://blogs.msdn.com/b/kwill/archive/2013/03/06/asynchronous-parallel-block-blob-transfers-with-progress-change-notification-2-0.aspx. 

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Have you ever wanted to asynchronously upload or download blobs from a client application and be able to show progress change notifications to the end user?  I started off using the System.Net.WebClient class and calling UploadFileAsync or DownloadFileAsync and subscribing to the ProgressChanged event handlers.  This works fairly well for most scenarios, but there are a couple problems with this approach:  UploadFileAsync will throw an OutOfMemoryException on large files due to the fact that it tries to read the entire file into a buffer prior to sending it, and the WebClient transfers are slow compared to taking advantage of transferring a blob using multiple parallel blocks.

You could also use the CloudBlockBlob class and call BeginUploadFromStream or BeginDownloadFromStream, but you don’t get progress change notifications or parallel block uploads.  You could wrap the storage client stream in a ProgressStream to get the progress change notifications, but then you still don’t get the speed of parallel block uploads.  If you don’t need the extra speed of the parallel block uploads, but you still want progress change notifications and an easy programming model I would suggest checking out the code from http://appfabriccat.com/2011/02/exploring-windows-azure-storage-apis-by-building-a-storage-explorer-application/.

To get all of the features I wanted I ended up writing my own BlobTransfer class which gives me the following benefits:

  • Fast uploads and downloads by using parallel block blob transfers.  Check out http://azurescope.cloudapp.net/Default.aspx for some of the performance benefits.
  • Asynchronous programming model
  • Progress change notifications

 

BlobTransfer.cs

using System;
using System.Text;
using System.ComponentModel;
using System.Windows.Forms;
 
using System.Collections.Generic;
using System.Threading;
using System.Runtime.Remoting.Messaging;
using Microsoft.WindowsAzure;
using Microsoft.WindowsAzure.StorageClient;
using Microsoft.WindowsAzure.StorageClient.Protocol;
using System.IO;
using System.Net;
using System.Security.Cryptography;
using System.Linq;
 
namespace BlobTransferUI
{
    class BlobTransfer
    {
        // Async events and properties
        public event AsyncCompletedEventHandler TransferCompleted;
        public event EventHandler<BlobTransferProgressChangedEventArgs> TransferProgressChanged;
        private delegate void BlobTransferWorkerDelegate(MyAsyncContext asyncContext, out bool cancelled, AsyncOperation async);
        private bool TaskIsRunning = false;
        private MyAsyncContext TaskContext = null;
        private readonly object _sync = new object();
 
        // Used to calculate download speeds
        Queue<long> timeQueue = new Queue<long>(100);
        Queue<long> bytesQueue = new Queue<long>(100);
        DateTime updateTime = System.DateTime.Now;
 
        // BlobTransfer properties
        private string m_FileName;
        private CloudBlockBlob m_Blob;
 
        public TransferTypeEnum TransferType;
 
        public void UploadBlobAsync(CloudBlob blob, string LocalFile)
        {   
            TransferType = TransferTypeEnum.Upload;
            //attempt to open the file first so that we throw an exception before getting into the async work
            using (FileStream fs = new FileStream(LocalFile, FileMode.Open, FileAccess.Read)) { }
 
            m_Blob = blob.ToBlockBlob;
            m_FileName = LocalFile;
 
            BlobTransferWorkerDelegate worker = new BlobTransferWorkerDelegate(UploadBlobWorker);
            AsyncCallback completedCallback = new AsyncCallback(TaskCompletedCallback);
 
            lock (_sync)
            {
                if (TaskIsRunning)
                    throw new InvalidOperationException("The control is currently busy.");
 
                AsyncOperation async = AsyncOperationManager.CreateOperation(null);
                MyAsyncContext context = new MyAsyncContext();
                bool cancelled;
 
                worker.BeginInvoke(context, out cancelled, async, completedCallback, async);
 
                TaskIsRunning = true;
                TaskContext = context;
            }
        }
 
        public void DownloadBlobAsync(CloudBlob blob, string LocalFile)
        {
            TransferType = TransferTypeEnum.Download;
            m_Blob = blob.ToBlockBlob;
            m_FileName = LocalFile;
 
 
            BlobTransferWorkerDelegate worker = new BlobTransferWorkerDelegate(DownloadBlobWorker);
            AsyncCallback completedCallback = new AsyncCallback(TaskCompletedCallback);
 
            lock (_sync)
            {
                if (TaskIsRunning)
                    throw new InvalidOperationException("The control is currently busy.");
 
                AsyncOperation async = AsyncOperationManager.CreateOperation(null);
                MyAsyncContext context = new MyAsyncContext();
                bool cancelled;
 
                worker.BeginInvoke(context, out cancelled, async, completedCallback, async);
 
                TaskIsRunning = true;
                TaskContext = context;
            }
        }
 
        public bool IsBusy
        {
            get { return TaskIsRunning; }
        }
 
        public void CancelAsync()
        {
            lock (_sync)
            {
                if (TaskContext != null)
                    TaskContext.Cancel();
            }
        }
 
        private void UploadBlobWorker(MyAsyncContext asyncContext, out bool cancelled, AsyncOperation async)
        {
            cancelled = false;
 
            ParallelUploadFile(asyncContext, async);
 
            // check for Cancelling
            if (asyncContext.IsCancelling)
            {
                cancelled = true;
            }
 
        }
 
        private void DownloadBlobWorker(MyAsyncContext asyncContext, out bool cancelled, AsyncOperation async)
        {
            cancelled = false;
 
            ParallelDownloadFile(asyncContext, async);
 
            // check for Cancelling
            if (asyncContext.IsCancelling)
            {
                cancelled = true;
            }
 
        }
 
        private void TaskCompletedCallback(IAsyncResult ar)
        {
            // get the original worker delegate and the AsyncOperation instance
            BlobTransferWorkerDelegate worker = (BlobTransferWorkerDelegate)((AsyncResult)ar).AsyncDelegate;
            AsyncOperation async = (AsyncOperation)ar.AsyncState;
 
            bool cancelled;
 
            // finish the asynchronous operation
            worker.EndInvoke(out cancelled, ar);
 
            // clear the running task flag
            lock (_sync)
            {
                TaskIsRunning = false;
                TaskContext = null;
            }
 
            // raise the completed event
            AsyncCompletedEventArgs completedArgs = new AsyncCompletedEventArgs(null, cancelled, null);
            async.PostOperationCompleted(delegate(object e) { OnTaskCompleted((AsyncCompletedEventArgs)e); }, completedArgs);
        }
 
        protected virtual void OnTaskCompleted(AsyncCompletedEventArgs e)
        {
            if (TransferCompleted != null)
                TransferCompleted(this, e);
        }
 
        private double CalculateSpeed(long BytesSent)
        {
            double speed = 0;
 
            if (timeQueue.Count == 80)
            {
                timeQueue.Dequeue();
                bytesQueue.Dequeue();
            }
 
            timeQueue.Enqueue(System.DateTime.Now.Ticks);
            bytesQueue.Enqueue(BytesSent);
 
            if (timeQueue.Count > 2)
            {
                updateTime = System.DateTime.Now;
                speed = (bytesQueue.Max() - bytesQueue.Min()) / TimeSpan.FromTicks(timeQueue.Max() - timeQueue.Min()).TotalSeconds;
            }
 
            return speed;
        }
 
        protected virtual void OnTaskProgressChanged(BlobTransferProgressChangedEventArgs e)
        {
            if (TransferProgressChanged != null)
                TransferProgressChanged(this, e);
        }
 
        // Blob Upload Code
        // 200 GB max blob size
        // 50,000 max blocks
        // 4 MB max block size
        // Try to get close to 100k block size in order to offer good progress update response.
        private int GetBlockSize(long fileSize)
        {
            const long KB = 1024;
            const long MB = 1024 * KB;
            const long GB = 1024 * MB;
            const long MAXBLOCKS = 50000;
            const long MAXBLOBSIZE = 200 * GB;
            const long MAXBLOCKSIZE = 4 * MB;
 
            long blocksize = 100 * KB;
            //long blocksize = 4 * MB;
            long blockCount;
            blockCount = ((int)Math.Floor((double)(fileSize / blocksize))) + 1;
            while (blockCount > MAXBLOCKS - 1)
            {
                blocksize += 100 * KB;
                blockCount = ((int)Math.Floor((double)(fileSize / blocksize))) + 1;
            }
 
            if (blocksize > MAXBLOCKSIZE)
            {
                throw new ArgumentException("Blob too big to upload.");
            }
 
            return (int)blocksize;
        }
 
        private void ParallelUploadFile(MyAsyncContext asyncContext, AsyncOperation asyncOp)
        {
            BlobTransferProgressChangedEventArgs eArgs = null;
            object AsyncUpdateLock = new object();
 
            // stats from azurescope show 10 to be an optimal number of transfer threads
            int numThreads = 10;
            var file = new FileInfo(m_FileName);
            long fileSize = file.Length;
 
            int maxBlockSize = GetBlockSize(fileSize);
            long bytesUploaded = 0;
            int blockLength = 0;
 
            // Prepare a queue of blocks to be uploaded. Each queue item is a key-value pair where
            // the 'key' is block id and 'value' is the block length.
            Queue<KeyValuePair<int, int>> queue = new Queue<KeyValuePair<int, int>>();
            List<string> blockList = new List<string>();
            int blockId = 0;
            while (fileSize > 0)
            {
                blockLength = (int)Math.Min(maxBlockSize, fileSize);
                string blockIdString = Convert.ToBase64String(ASCIIEncoding.ASCII.GetBytes(string.Format("BlockId{0}", blockId.ToString("0000000"))));
                KeyValuePair<int, int> kvp = new KeyValuePair<int, int>(blockId++, blockLength);
                queue.Enqueue(kvp);
                blockList.Add(blockIdString);
                fileSize -= blockLength;
            }
 
            m_Blob.DeleteIfExists();
 
            BlobRequestOptions options = new BlobRequestOptions()
            {
                RetryPolicy = RetryPolicies.RetryExponential(RetryPolicies.DefaultClientRetryCount, RetryPolicies.DefaultMaxBackoff),
                Timeout = TimeSpan.FromSeconds(90)
            };
 
            // Launch threads to upload blocks.
            List<Thread> threads = new List<Thread>();
 
            for (int idxThread = 0; idxThread < numThreads; idxThread++)
            {
                Thread t = new Thread(new ThreadStart(() =>
                {
                    KeyValuePair<int, int> blockIdAndLength;
 
                    using (FileStream fs = new FileStream(file.FullName, FileMode.Open, FileAccess.Read))
                    {
                        while (true)
                        {
                            // Dequeue block details.
                            lock (queue)
                            {
                                if (asyncContext.IsCancelling)
                                    break;
 
                                if (queue.Count == 0)
                                    break;
 
                                blockIdAndLength = queue.Dequeue();
                            }
 
                            byte[] buff = new byte[blockIdAndLength.Value];
                            BinaryReader br = new BinaryReader(fs);
 
                            // move the file system reader to the proper position
                            fs.Seek(blockIdAndLength.Key * (long)maxBlockSize, SeekOrigin.Begin);
                            br.Read(buff, 0, blockIdAndLength.Value);
 
                            // Upload block.
                            string blockName = Convert.ToBase64String(BitConverter.GetBytes(
                                blockIdAndLength.Key));
                            using (MemoryStream ms = new MemoryStream(buff, 0, blockIdAndLength.Value))
                            {
                                string blockIdString = Convert.ToBase64String(ASCIIEncoding.ASCII.GetBytes(string.Format("BlockId{0}", blockIdAndLength.Key.ToString("0000000"))));
                                string blockHash = GetMD5HashFromStream(buff);
                                m_Blob.PutBlock(blockIdString, ms, blockHash, options);
                            }
 
                            lock (AsyncUpdateLock)
                            {
                                bytesUploaded += blockIdAndLength.Value;
 
                                int progress = (int)((double)bytesUploaded / file.Length * 100);
 
                                // raise the progress changed event
                                eArgs = new BlobTransferProgressChangedEventArgs(bytesUploaded, file.Length, progress, CalculateSpeed(bytesUploaded), null);
                                asyncOp.Post(delegate(object e) { OnTaskProgressChanged((BlobTransferProgressChangedEventArgs)e); }, eArgs);
                            }
                        }
                    }
                }));
                t.Start();
                threads.Add(t);
            }
 
            // Wait for all threads to complete uploading data.
            foreach (Thread t in threads)
            {
                t.Join();
            }
 
            if (!asyncContext.IsCancelling)
            {
                // Commit the blocklist.
                m_Blob.PutBlockList(blockList, options);
            }
 
        }
 
        /// <summary>
        /// Downloads content from a blob using multiple threads.
        /// </summary>
        /// <param name="blob">Blob to download content from.</param>
        /// <param name="numThreads">Number of threads to use.</param>
        private void ParallelDownloadFile(MyAsyncContext asyncContext, AsyncOperation asyncOp)
        {
            BlobTransferProgressChangedEventArgs eArgs = null;
 
            int numThreads = 10;
            m_Blob.FetchAttributes();
            long blobLength = m_Blob.Properties.Length;
 
            int bufferLength = GetBlockSize(blobLength);  // 4 * 1024 * 1024;
            long bytesDownloaded = 0;
 
            // Prepare a queue of chunks to be downloaded. Each queue item is a key-value pair 
            // where the 'key' is start offset in the blob and 'value' is the chunk length.
            Queue<KeyValuePair<long, int>> queue = new Queue<KeyValuePair<long, int>>();
            long offset = 0;
            while (blobLength > 0)
            {
                int chunkLength = (int)Math.Min(bufferLength, blobLength);
                queue.Enqueue(new KeyValuePair<long, int>(offset, chunkLength));
                offset += chunkLength;
                blobLength -= chunkLength;
            }
 
            int exceptionCount = 0;
 
            FileStream fs = new FileStream(m_FileName, FileMode.OpenOrCreate, FileAccess.Write, FileShare.Read);
 
            using (fs)
            {
                // Launch threads to download chunks.
                List<Thread> threads = new List<Thread>();
                for (int idxThread = 0; idxThread < numThreads; idxThread++)
                {
                    Thread t = new Thread(new ThreadStart(() =>
                    {
                        KeyValuePair<long, int> blockIdAndLength;
 
                        // A buffer to fill per read request.
                        byte[] buffer = new byte[bufferLength];
 
                        while (true)
                        {
                            if (asyncContext.IsCancelling)
                                return;
 
                            // Dequeue block details.
                            lock (queue)
                            {
                                if (queue.Count == 0)
                                    break;
 
                                blockIdAndLength = queue.Dequeue();
                            }
 
                            try
                            {
                                // Prepare the HttpWebRequest to download data from the chunk.
                                HttpWebRequest blobGetRequest = BlobRequest.Get(m_Blob.Uri, 60, null, null);
 
                                // Add header to specify the range
                                blobGetRequest.Headers.Add("x-ms-range", string.Format(System.Globalization.CultureInfo.InvariantCulture, "bytes={0}-{1}", blockIdAndLength.Key, blockIdAndLength.Key + blockIdAndLength.Value - 1));
 
                                // Sign request.
                                StorageCredentials credentials = m_Blob.ServiceClient.Credentials;
                                credentials.SignRequest(blobGetRequest);
 
                                // Read chunk.
                                using (HttpWebResponse response = blobGetRequest.GetResponse() as
                                    HttpWebResponse)
                                {
                                    using (Stream stream = response.GetResponseStream())
                                    {
                                        int offsetInChunk = 0;
                                        int remaining = blockIdAndLength.Value;
                                        while (remaining > 0)
                                        {
                                            int read = stream.Read(buffer, offsetInChunk, remaining);
                                            lock (fs)
                                            {
                                                fs.Position = blockIdAndLength.Key + offsetInChunk;
                                                fs.Write(buffer, offsetInChunk, read);
                                            }
                                            offsetInChunk += read;
                                            remaining -= read;
                                            Interlocked.Add(ref bytesDownloaded, read);
                                        }
 
                                        int progress = (int)((double)bytesDownloaded / m_Blob.Attributes.Properties.Length * 100);
 
                                        // raise the progress changed event
                                        eArgs = new BlobTransferProgressChangedEventArgs(bytesDownloaded, m_Blob.Attributes.Properties.Length, progress, CalculateSpeed(bytesDownloaded), null);
                                        asyncOp.Post(delegate(object e) { OnTaskProgressChanged((BlobTransferProgressChangedEventArgs)e); }, eArgs);
                                    }
                                }
                            }
                            catch (Exception ex)
                            {
                                // Add block back to queue
                                queue.Enqueue(blockIdAndLength);
 
                                exceptionCount++;
                                // If we have had more than 100 exceptions then break
                                if (exceptionCount == 100)
                                {
                                    throw new Exception("Received 100 exceptions while downloading. Cancelling download. " + ex.ToString());
                                }
                                if (exceptionCount >= 100)
                                {
                                    break;
                                }
                            }
                        }
                    }));
                    t.Start();
                    threads.Add(t);
                }
 
 
                // Wait for all threads to complete downloading data.
                foreach (Thread t in threads)
                {
                    t.Join();
                }
            }
        }
 
        private string GetMD5HashFromStream(byte[] data)
        {
            MD5 md5 = new MD5CryptoServiceProvider();
            byte[] blockHash = md5.ComputeHash(data);
            return Convert.ToBase64String(blockHash, 0, 16);
        }
 
        internal class MyAsyncContext
        {
            private readonly object _sync = new object();
            private bool _isCancelling = false;
 
            public bool IsCancelling
            {
                get
                {
                    lock (_sync) { return _isCancelling; }
                }
            }
 
            public void Cancel()
            {
                lock (_sync) { _isCancelling = true; }
            }
        }
 
 
        public class BlobTransferProgressChangedEventArgs : ProgressChangedEventArgs
        {
            private long m_BytesSent = 0;
            private long m_TotalBytesToSend = 0;
            private double m_Speed = 0;
 
            public long BytesSent
            {
                get { return m_BytesSent; }
            }
 
            public long TotalBytesToSend
            {
                get { return m_TotalBytesToSend; }
            }
 
            public double Speed
            {
                get { return m_Speed; }
            }
 
            public TimeSpan TimeRemaining
            {
                get
                {
                    TimeSpan time = new TimeSpan(0, 0, (int)((TotalBytesToSend - m_BytesSent) / (m_Speed == 0 ? 1 : m_Speed)));
                    return time;
                }
            }
 
            public BlobTransferProgressChangedEventArgs(long BytesSent, long TotalBytesToSend, int progressPercentage, double Speed, object userState)
                : base(progressPercentage, userState)
            {
                m_BytesSent = BytesSent;
                m_TotalBytesToSend = TotalBytesToSend;
                m_Speed = Speed;
            }
        }
    }
 
    public enum TransferTypeEnum
    {
        Download,
        Upload
    }
}

 

Simple Console Client

Calling the upload or download method from BlobTransfer is a pretty simple matter of obtaining a CloudBlob reference to the blob of interest, subscribing to the TransferProgressChanged and TransferCompleted eventargs, and then calling UploadBlobAsync or DownloadBlobAsync.  The following console app shows a simple example.

  1. Create a new console application
  2. Add a reference to System.Web (you will need to change the project’s Target Framework property to .NET Framework 4 instead of .NET Framework 4 Client Library) and Microsoft.WindowsAzure.StorageClient.
  3. Add BlobTransfer.cs
  4. Add the following code to Program.CS and change the const members to valid values.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
using Microsoft.WindowsAzure;
using Microsoft.WindowsAzure.StorageClient;
 
namespace ConsoleApplication1
{
    class Program
    {
        const string ACCOUNTNAME = "ENTER ACCOUNT NAME";
        const string ACCOUNTKEY = "ENTER ACCOUNT KEY";
        const string LOCALFILE = @"ENTER LOCAL FILE";
        const string CONTAINER = "temp";
 
        private static CloudStorageAccount AccountFileTransfer;
        private static CloudBlobClient BlobClientFileTransfer;
        private static CloudBlobContainer ContainerFileTransfer;
 
        private static bool Transferring;
 
        static void Main(string[] args)
        {
            System.Net.ServicePointManager.DefaultConnectionLimit = 35;
 
            AccountFileTransfer = CloudStorageAccount.Parse("DefaultEndpointsProtocol=http;AccountName=" + ACCOUNTNAME + ";AccountKey=" + ACCOUNTKEY);
            if (AccountFileTransfer != null)
            {
                BlobClientFileTransfer = AccountFileTransfer.CreateCloudBlobClient();
                ContainerFileTransfer = BlobClientFileTransfer.GetContainerReference(CONTAINER);
                ContainerFileTransfer.CreateIfNotExist();
            }
 
            // Upload the file
            CloudBlob blobUpload = ContainerFileTransfer.GetBlobReference(CONTAINER + "/" + System.IO.Path.GetFileName(LOCALFILE));
            BlobTransfer transferUpload = new BlobTransfer();
            transferUpload.TransferProgressChanged += new EventHandler<BlobTransfer.BlobTransferProgressChangedEventArgs>(transfer_TransferProgressChanged);
            transferUpload.TransferCompleted += new System.ComponentModel.AsyncCompletedEventHandler(transfer_TransferCompleted);
            transferUpload.UploadBlobAsync(blobUpload, LOCALFILE);
 
            Transferring = true;
            while (Transferring)
            {
                Console.ReadLine();
            }
 
            // Download the file
            CloudBlob blobDownload = ContainerFileTransfer.GetBlobReference(CONTAINER + "/" + System.IO.Path.GetFileName(LOCALFILE));
            BlobTransfer transferDownload = new BlobTransfer();
            transferDownload.TransferProgressChanged += new EventHandler<BlobTransfer.BlobTransferProgressChangedEventArgs>(transfer_TransferProgressChanged);
            transferDownload.TransferCompleted += new System.ComponentModel.AsyncCompletedEventHandler(transfer_TransferCompleted);
            transferDownload.DownloadBlobAsync(blobDownload, LOCALFILE + ".copy");
 
            Transferring = true;
            while (Transferring)
            {
                Console.ReadLine();
            }
        }
 
        static void transfer_TransferCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
        {
            Transferring = false;
            Console.WriteLine("Transfer completed. Press any key to continue.");
        }
 
        static void transfer_TransferProgressChanged(object sender, BlobTransfer.BlobTransferProgressChangedEventArgs e)
        {
            Console.WriteLine("Transfer progress percentage = " + e.ProgressPercentage + " - " + (e.Speed / 1024).ToString("N2") + "KB/s");
        }
    }
}

 

UI Client

For a more full featured sample check out the BlobTransferUI project for simple UI showing progress bars for multiple simultaneous uploads and downloads.

image

 

Edit June 19, 2011

  • Fixed some issues in BlobTransfer to change some int’s into long’s in order to fix a bug when transferring a >2GB file.  Thanks to Jeff Baxter with our HPC team for helping me discover the issue.
  • Thanks very much for the project, it works nicely!

  • This is good project. I have couple of issues.

    1) we have noticed couple of timeout while putblock is called. This is not happening for all the blocks. Please provide your advise here.

    2)  how to handle the situation in case the file got modified while uploading to storage?  

    possible solutions would be lock the file or copy to working directory and then upload.

    Please suggest.

  • Hi kiran, glad you are getting some value out of this.

    1. This is usually caused by transferring a large file over a slower connection.  The block size will be calculated as 4 MB, and the code will try to transfer 10 blocks at a time.  So basically you are trying to transfer roughly 40 MB during the storage client's timeout window.  To resolve this you can try to increase the timeout in the BlobRequestOptions, or you can reduce the number of parallel threads.  Ideally you would detect the timeout and dynamically scale down the number of threads until you reach the highest bandwidth possible without timing out the operation.

    2. Either of your options (locking the file or copying it to a working directory) would work.  I would probably lock the file because that is a much cleaner approach.  

  • Any update on the fix for the error described at the top of the post?

  • Hi Kevin,

    This is great project. While I was using this code I came across a small bug in the code where you divide file into blocks and add them into queue. You divide file into blocks of 100KB. The code which divides file into blocks of 100 KB creates the last block also with size 100KB where as it could be less than that. Following code helped me to fix the issue

    var kvp = fileSize > blockLength

                                 ? new KeyValuePair<int, int>(blockId++, blockLength)

                                 : new KeyValuePair<int, int>(blockId++, (int) fileSize);

  • In ParallelDownloadFile, you use a lock when picking items from the queue. If there's an error performing the http request, you re-enqueue the item on the queue using Enqueue. But when you do this, you don't use any locking. The Enqueue method is not thread safe on the queue you are using. Shouldn't you either use a lock, or use ConcurrentQueue?  

    The fact that Enqueue is not thread safe is mentioned in its documentation, and I was able to confirm that items are lost when enqueueing items in the same queue from multiple threads at once in a standalone program.

    We have had some issues with large downloads using this code. The download completes, but the local file is corrupt. I suspect that this is due to the retry-logic loosing items on the queue. Of course, you have to be a bit unlucky, but it will happen on large uploads if there's a network issue.

    Or am I missing something?

  • Looks like a similar threading issue could happen with exceptionCount to. Two threads could update the count to 100 and 101 before exceptionCount==100 is evaluated. When this happens, all threads will return but no error will be reported since the exception is never thrown, leading to a file which may be of proper size but some chunks in it are not filled (corrupt file)

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