Append to CloudBlockBlob stream - azure

Question

We have a file system abstraction that allows us to easily switch between local and cloud (Azure) storage.
For reading and writing files we have the following members:
Stream OpenRead();
Stream OpenWrite();
Part of our application "bundles" documents into one file. For our local storage provider OpenWrite returns an appendable stream:
public Stream OpenWrite()
{
return new FileStream(fileInfo.FullName, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite, BufferSize, useAsync: true);
}
For Azure blob storage we do the following:
public Stream OpenWrite()
{
return blob.OpenWrite();
}
Unfortunately this overrides the blob contents each time. Is it possible to return a writable stream that can be appended to?

Answer 1

Based on the documentation for OpenWrite here http://msdn.microsoft.com/en-us/library/microsoft.windowsazure.storage.blob.cloudblockblob.openwrite.aspx, The OpenWrite method will overwrite an existing blob unless explicitly prevented using the accessCondition parameter.
One thing you could do is read the blob data in a stream and return that stream to your calling application and let that application append data to that stream. For example, see the code below:
static void BlobStreamTest()
{
storageAccount = CloudStorageAccount.DevelopmentStorageAccount;
CloudBlobContainer container = storageAccount.CreateCloudBlobClient().GetContainerReference("temp");
container.CreateIfNotExists();
CloudBlockBlob blob = container.GetBlockBlobReference("test.txt");
blob.UploadFromStream(new MemoryStream());//Let's just create an empty blob for the sake of demonstration.
for (int i = 0; i < 10; i++)
{
try
{
using (MemoryStream ms = new MemoryStream())
{
blob.DownloadToStream(ms);//Read blob data in a stream.
byte[] dataToWrite = Encoding.UTF8.GetBytes("This is line # " + (i + 1) + "\r\n");
ms.Write(dataToWrite, 0, dataToWrite.Length);
ms.Position = 0;
blob.UploadFromStream(ms);
}
}
catch (StorageException excep)
{
if (excep.RequestInformation.HttpStatusCode != 404)
{
throw;
}
}
}
}

Answer 2

There is now a CloudAppendBlob class that allows you to add content to an existing blob :
var account = CloudStorageAccount.Parse("storage account connectionstring");
var client = account.CreateCloudBlobClient();
var container = client.GetContainerReference("container name");
var blob = container.GetAppendBlobReference("blob name");
In your case you want to append from a stream:
await blob.AppendFromStreamAsync(new MemoryStream());
But you can append from text, byte array, file. Check the documentation.