S3 – Use using block to get the stream

When you’re using the Amazon S3 client, have you come across the occasional exception that says something like one of these exception messages:

“The request was aborted: The connection was closed unexpectedly”

“Unable to read data from the transport connection: A blocking operation was interrupted by a call to WSACancelBlockingCall”

“Unable to read data from the transport connection: An established connection was aborted by the software in your host machine “

If you do, then you’re probably attempting to return the response stream directly back to the rest of your application with something like this:

   1: var response = _s3Client.GetObject(request);

   2: return response.ResponseStream;

However, because the stream is coming from the Amazon S3 service and is fed to your code in chunks, your code needs to ensure that the connection to S3 stays open until all the data has been received. So as mentioned in the S3 documentation (which incidentally, most of us don’t read in great details…) here, you should be wrapping the response you get from the GetObject method in a using clause.

Depends on what it is you want to do with the stream, you might have to handle it differently. For instance, if you just want to read the string content of a text file, you might want to do this:

   1: using (var response = _s3Client.GetObject(request))

   2: {

   3:     using (var reader = new StreamReader(response.ResponseStream))

   4:     {

   5:         return reader.ReadToEnd();

   6:     }

   7: }

Alternatively, if you want to return the response stream itself, you’ll need to first load the stream in its entirety and return the loaded stream. Unfortunately, at the time of this writing, the AWSSDK library still hasn’t been migrated to .Net 4 and therefore doesn’t have the uber useful CopyTo method added in .Net 4, so you will most likely have to do the heavy lifting yourself and read the data out manually into a memory stream:

   1: using (var response = _s3Client.GetObject(request))

   2: {

   3:     var binaryData = ReadFully(response.ResponseStream);

   4:     return new MemoryStream(binaryData);

   5: }

   6:

   7: /// <summary>

   8: /// See Jon Skeet's article on reading binary data:

   9: /// http://www.yoda.arachsys.com/csharp/readbinary.html

  10: /// </summary>

  11: public static byte[] ReadFully (Stream stream, int initialLength = -1)

  12: {

  13:     // If we've been passed an unhelpful initial length, just

  14:     // use 32K.

  15:     if (initialLength < 1)

  16:     {

  17:         initialLength = 32768;

  18:     }

  19:

  20:     byte[] buffer = new byte[initialLength];

  21:     int read=0;

  22:

  23:     int chunk;

  24:     while ( (chunk = stream.Read(buffer, read, buffer.Length-read)) > 0)

  25:     {

  26:         read += chunk;

  27:

  28:         // If we've reached the end of our buffer, check to see if there's

  29:         // any more information

  30:         if (read == buffer.Length)

  31:         {

  32:             int nextByte = stream.ReadByte();

  33:

  34:             // End of stream? If so, we're done

  35:             if (nextByte==-1)

  36:             {

  37:                 return buffer;

  38:             }

  39:

  40:             // Nope. Resize the buffer, put in the byte we've just

  41:             // read, and continue

  42:             byte[] newBuffer = new byte[buffer.Length*2];

  43:             Array.Copy(buffer, newBuffer, buffer.Length);

  44:             newBuffer[read]=(byte)nextByte;

  45:             buffer = newBuffer;

  46:             read++;

  47:         }

  48:     }

  49:     // Buffer is now too big. Shrink it.

  50:     byte[] ret = new byte[read];

  51:     Array.Copy(buffer, ret, read);

  52:     return ret;

  53: }

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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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