Azure Blob storage: DownloadToByteArray VS DownloadToStream

Question

I have been playing with the Azure Blob Storage service to save/recover files in a context of a web page to be hosted in Azure Web Pages.

During the learning process I have come with two solutions; the first basically uses DownloadToStream which does the same but with a FileStream. In this case I have to write the file in the server prior to return it to the user.

public static Stream GetFileContent(string fileName, HttpContextBase context)
{
      CloudBlobContainer container = GetBlobContainer();    
      CloudBlockBlob blockBlob = container.GetBlockBlobReference(fileName);                                       
      Stream fileStream = new FileStream(
          context.Server.MapPath("~/App_Data/files/" + fileName), FileMode.Create);   
      blockBlob.DownloadToStream(fileStream);
      fileStream.Close();    
      return File.OpenRead(context.Server.MapPath("~/App_Data/files/" + fileName));
}

public ActionResult Download(string fileName)
{
    byte[] fileContent = MyFileContext.GetFileContent(fileName);
    return File(fileContent, "application/zip", fileName);        
}

On the other hand I used the DownloadToByteArray function with writes the content of the Blob in an array of bytes initialized with the size of the Blob file.

public static byte[] GetFileContent(string fileName)
{
    CloudBlobContainer container = GetBlobContainer();           
    CloudBlockBlob blockBlob = container.GetBlockBlobReference(fileName);
    blockBlob.FetchAttributes();
    long fileByteLength = blockBlob.Properties.Length;
    byte[] fileContent = new byte[fileByteLength];
    for (int i = 0; i < fileByteLength; i++)
    {
        fileContent[i] = 0x20;
    }
    blockBlob.DownloadToByteArray(fileContent,0);
    return fileContent;
}

public ActionResult Download(string fileName)
{   
   byte[] fileContent = MyFileContext.GetFileStream(fileName);
   return File(fileContent, "application/zip", fileName);
}

When I look at both options I see the first needs to create a file in the server's disk whereas the second stores the data from the Blob in a byte array consuming memory. In my particular case I am going to handle file sizes of ~150 MB.

Given the circumstances (environment, file sizes...) which approach do you think is best?

Is your objective always downloading the files on user's computer? Or in other words, do you want to process this data before streaming it to user's browser or will it be OK for you to directly download the file on user's computer from Azure storage? — Gaurav Mantri
Just to not lose sight of something: DownloadToStream requires a Stream but FileStream is not the only stream type around. — CyberDude
@GauravMantri yes, the idea is just to donwload the file without performing any operation with it. — Julen
@CyberDude thanks, just thought that if I was handling files FileStream should be appropriate, or there is a better alternative? — Julen
See this similar question: stackoverflow.com/questions/6752000/… A solution is given where the file is directly sent to the Response output stream without an intermediate saving to disk. Alternatively, if the blobs are public and your business logic allows it, you can always just give out a direct http link to the blob. — CyberDude

Gaurav Mantri Gaurav Mantri · Accepted Answer · 2014-06-20T18:51:22

Instead of streaming the blob through your server, you could download it directly from the blob storage. My answer is built on top of Steve's response here: Downloading Azure Blob files in MVC3. For downloading a blob directly from the storage, you would utilize Shared Access Signature (SAS). Recently Azure Storage has introduced an enhancement, which allows you to specify Content-Disposition header in SAS. See this modified code.

    public static string GetDownloadLink(string fileName)
    {
        CloudBlobContainer container = GetBlobContainer();
        CloudBlockBlob blockBlob = container.GetBlockBlobReference(fileName);
        //Create an ad-hoc Shared Access Policy with read permissions which will expire in 12 hours
        SharedAccessBlobPolicy policy = new SharedAccessBlobPolicy()
        {
            Permissions = SharedAccessBlobPermissions.Read,
            SharedAccessExpiryTime = DateTime.UtcNow.AddHours(12),
        };
        //Set content-disposition header for force download
        SharedAccessBlobHeaders headers = new SharedAccessBlobHeaders()
        {
            ContentDisposition = string.Format("attachment;filename=\"{0}\"", fileName),
        };
        var sasToken = blockBlob.GetSharedAccessSignature(policy, headers);
        return blockBlob.Uri.AbsoluteUri + sasToken;
    }

    public ActionResult Download(string fileName)
    {
        var sasUrl = GetDownloadLink(fileName);
        //Redirect to SAS URL ... file will now be downloaded directly from blob storage.
        Redirect(sasUrl);
    }

Azure Blob storage: DownloadToByteArray VS DownloadToStream

3 Answers