Using ftp for downloading in several threads at same time.

Question

Hi, I'm writing a simple program that are downloading several files at the same time in different threads and It seems that as soon as the first file is finished all other file downloads ends with this exception: error in thread = 4 filename = movie1.wmv, error = An existing connection was forcibly closed by the remote host System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host.

It works fine before the first file is ready and I have five files downloading at same time.

Anyone have an idea what the problem can be? the server being used is Titan.

The code looks like this when starting the download for every file.

try
{
    Ftp client = new Ftp();
    client.Connect(hostname, port);
    client.Login(username, password);
    long fileSize = client.GetFile(fileName, fileNameWithPath);
}
catch (Exception ex)
{
    ...
}

Answer 1

Hi,

the FTP protocol does not support multiple file transfers in the same time.

If you want to download several files at once you have to maintaint multiple connections to the FTP server. This can cause problems with some FTP servers because they like to limit number of concurent connections from one user or from one IP address. The most common limit is 2 concurent transfers per user.

Pseudo code for downloading the file in two threads would look like this:

FileNameBuffer buffer = GetFileListFromFtpServer();
StartFirstDownloadThread(files);
StartSecondDownloadThread(files);

Pseudo code for each download thread would be:

ConnectToFtp();
while (file = GetNextFileFromBuffer())
{
   DownloadFile();
}
Disconnect();

You can download the fully functional code for downloading all files from FTP folder in two threads from http://www.rebex.net/extras/FtpsMultiDownloader.zip

If you want to include a support for simulataneous transfers directly to the FTP component please consider voting on the following feature requests:

Using ftp for downloading in several threads at same time.

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