Why does my async action hang while downloading large files?

Question

I am using an async Action to download large files (>500MB). My below code hangs my application until the file download is complete.

public async Task<ActionResult> Download(string filePath, string fileName)
    {
        try
        {
            if(!string.IsNullOrEmpty(filePath))
            {
                filePath = Path.Combine(Code.Config.UploadFilesPath(), filePath);
                string contentType = MimeMapping.GetMimeMapping(fileName);

                if (System.IO.File.Exists(filePath))
                {
                    await GetLargeFile(filePath, fileName);
                }
                else
                {
                    return Content("Requested File does not exist");
                }
            }
        }
        catch(Exception ex) { }
        return Content("");
    }

private async Task GetLargeFile(string fullPath, string outFileName)
{
  System.IO.Stream iStream = null;

        // Buffer to read 10K bytes in chunk:
        byte[] buffer = new Byte[10000];

        // Length of the file:
        int length;

        // Total bytes to read:
        long dataToRead;

        // Identify the file to download including its path.
        string filepath = fullPath;

        // Identify the file name.
        string filename = System.IO.Path.GetFileName(filepath);

        try
        {
            // Open the file.
            iStream = new System.IO.FileStream(filepath, System.IO.FileMode.Open,
                        System.IO.FileAccess.Read, System.IO.FileShare.Read);


            // Total bytes to read:
            dataToRead = iStream.Length;

            Response.Clear();
            Response.ContentType = MimeMapping.GetMimeMapping(filename);
            Response.AddHeader("content-disposition", "attachment; filename=\"" + outFileName + "\"");
            Response.AddHeader("Content-Length", iStream.Length.ToString());

            // Read the bytes.
            while (dataToRead > 0)
            {
                // Verify that the client is connected.
                if (Response.IsClientConnected)
                {
                    // Read the data in buffer.
                    length = iStream.Read(buffer, 0, 10000);

                    // Write the data to the current output stream.
                    Response.OutputStream.Write(buffer, 0, length);

                    // Flush the data to the output.
                    Response.Flush();

                    buffer = new Byte[10000];
                    dataToRead = dataToRead - length;
                }
                else
                {
                    //prevent infinite loop if user disconnects
                    dataToRead = -1;
                }
            }
        }
        catch (Exception ex)
        {
            throw new ApplicationException(ex.Message);
        }
        finally
        {
            if (iStream != null)
            {
                //Close the file.
                iStream.Close();
            }
            Response.Close();
        }
}

It is strange that if I use the same code as above on a brand new Web application and download a large file (around 1GB), that application doesn't hang as I can switch between Views & trigger javascript alert function during download process.

But with my real project, application hangs until download is completed unfortunately.

Is there anything that I should consider in configs or anything else?

Any debugging tips to see how it is working on a brand new application but not on my actual project?

You need to post the code of GetLargeFile if you want any help — Camilo Terevinto, May 23 '17 at 11:45
`Some code to read file & write it to response stream in loop` Using async IO? Show us the code. — spender, May 23 '17 at 11:59
You aren't doing anything async in your async method. Simply marking a method as `async` will do **absolutely nothing** if the async method never awaits, and the method will run synchronously. This is bad juju on a webserver. Use the Task based async methods with `await` for all your IO interactions. — spender, May 23 '17 at 12:12
I am a newbie to the async world. Do you mind updating 'GetLargeFile' method if that doesn't take your time please? — Raghu, May 23 '17 at 12:16
@Raghu Why don't you give it a shot yourself and if you run into more trouble, ask a new question? To get you started, your first read operation might be `length = await iStream.ReadAsync(buffer, 0, 10000);`. — spender, May 23 '17 at 12:21
@spender: Thanks for your inputs. I am currently stuck at making 'Response.Flush()' as async, looks like this is blocking/hanging my application after making its above lines async. Any ideas how to make it asyn or any other better way? — Raghu, May 23 '17 at 13:22
google it [FlushAsync](https://msdn.microsoft.com/en-us/library/hh193384(v=vs.110).aspx) — bradgonesurfing, May 24 '17 at 14:05

score 0 · Answer 1 · answered May 23 '17 at 14:13

In the context of a web application, async merely allows the thread serving the request to be returned to the thread pool, when it enters a wait-state. If the thread is actively doing work, such as in spooling a file into the response, then it's effectively the same as running sync.

Again, in the context of a web application, there's a request and a response. The client makes a request and the server responds to that request. Here, that response is a file, so async or not, the thread will be tied up until the response is complete. You cannot "return" from the action until the response has been sent.

Why does my async action hang while downloading large files?

1 Answers1