"The specified block list is invalid" while uploading blobs in parallel

Question

I've a (fairly large) Azure application that uploads (fairly large) files in parallel to Azure blob storage.

In a few percent of uploads I get an exception:

The specified block list is invalid.

System.Net.WebException: The remote server returned an error: (400) Bad Request.

This is when we run a fairly innocuous looking bit of code to upload a blob in parallel to Azure storage:

    public static void UploadBlobBlocksInParallel(this CloudBlockBlob blob, FileInfo file) 
    {
        blob.DeleteIfExists();
        blob.Properties.ContentType = file.GetContentType();
        blob.Metadata["Extension"] = file.Extension;

        byte[] data = File.ReadAllBytes(file.FullName);

        int numberOfBlocks = (data.Length / BlockLength) + 1;
        string[] blockIds = new string[numberOfBlocks];

        Parallel.For(
            0, 
            numberOfBlocks, 
            x =>
        {
            string blockId = Convert.ToBase64String(Guid.NewGuid().ToByteArray());
            int currentLength = Math.Min(BlockLength, data.Length - (x * BlockLength));

            using (var memStream = new MemoryStream(data, x * BlockLength, currentLength))
            {
                var blockData = memStream.ToArray();
                var md5Check = System.Security.Cryptography.MD5.Create();
                var md5Hash = md5Check.ComputeHash(blockData, 0, blockData.Length);

                blob.PutBlock(blockId, memStream, Convert.ToBase64String(md5Hash));
            }

            blockIds[x] = blockId;
        });

        byte[] fileHash  = _md5Check.ComputeHash(data, 0, data.Length);
        blob.Metadata["Checksum"] = BitConverter.ToString(fileHash).Replace("-", string.Empty);
        blob.Properties.ContentMD5 = Convert.ToBase64String(fileHash);

        data = null;
        blob.PutBlockList(blockIds);
        blob.SetMetadata();
        blob.SetProperties();
    }

All very mysterious; I'd think the algorithm we're using to calculate the block list should produce strings that are all the same length...

Answer 1

We ran into a similar issue, however we were not specifying any block ID or even using the block ID anywhere. In our case, we were using:

using (CloudBlobStream stream = blob.OpenWrite(condition))
{
   //// [write data to stream]

   stream.Flush();
   stream.Commit();
}

This would cause The specified block list is invalid. errors under parallelized load. Switching this code to use the UploadFromStream(…) method while buffering the data into memory fixed the issue:

using (MemoryStream stream = new MemoryStream())
{
   //// [write data to stream]

   stream.Seek(0, SeekOrigin.Begin);
   blob.UploadFromStream(stream, condition);
}

Obviously this could have negative memory ramifications if too much data is buffered into memory, but this is a simplification. One thing to note is that UploadFromStream(...) uses Commit() in some cases, but checks additional conditions to determine the best method to use.

Answer 2

This exception can happen also when multiple threads open stream into a blob with the same file name and try to write into this blob simultaneously.