How to Use Azure Blob Storage with Azure Web Sites

Introduction

The Windows Azure platform offers a wide variety of cloud hosted services, including "Azure Web Sites" (which is still in "preview") and Azure BLOB Storage. Since most BLOB storage tutorials assume you are working with "Azure Web Roles", their instructions aren't always necessarily applicable for Azure Web Sites. In this tutorial I will show how to use Azure BLOB storage to store images uploaded from an ASP.NET MVC 4 website, hosted on Azure Web Sites.

Step 1 – Set up an Azure Storage Account

This is quite straightforward in the Azure portal. Just create a storage account. You do need to provide an account name. Each storage account can have many “containers” so you can share the same storage account between several sites if you want. 

Step 2 – Install the Azure SDK 

This is done using the Web Platform Installer. I installed the 1.8 version for VS 2012.

Step 3 – Setup the Azure Storage Emulator

It seems that with Azure web role projects, you can configure Visual Studio to auto-launch the Azure Storage emulator, but I don’t think that option is available for regular ASP.NET MVC projects hosted on Azure Web Sites. The emulator is csrun.exe and it took some tracking down as Microsoft seems to move it with every version of the SDK. It needs to be run with the /devstore command line parameter to tell it to start the storage emulator: 

C:\Program Files\Microsoft SDKs\Windows Azure\Emulator\csrun.exe /devstore

To make life easy for me, I added this command as an option to my Eternal Tools list in Visual Studio so I could quickly launch it. Once it starts up, a new icon appears in the system tray, giving you access to the UI, which shows you what ports it is running on:

Step 4 – Set up a Development Connection String

While we are in development, we want to use the emulator, and this requires a connection string. For ASP.NET MVC sites, we need to go directly to our web.config and enter a new connection string ourselves. The connection string for the emulator is very simple: 

<connectionStrings>
  <add name="StorageConnection" connectionString="UseDevelopmentStorage=true"/>
</connectionStrings>

Step 5 – Upload an Image in ASP.NET MVC 4 

This is probably very basic stuff to most web developers, but it took me a while to find a good tutorial. This is how to make a basic form in Razor syntax to let the user select and upload a file: 

@using (Html.BeginForm("ImageUpload", "Admin", 
      FormMethod.Post, new { enctype = "multipart/form-data" }))
{ 
    <div>Please select an image to upload</div>
    <input name="image" type="file">
    <input type="submit" value="Upload Image" />
}

And now in my AdminController’s ImageUpload method, I can access details of the uploaded file using the Request.Files accessor which returns an instance of HttpPostedFileBase: 

[HttpPost]
public ActionResult ImageUpload()
{
    string path = @"D:\Temp\";
 
    var image = Request.Files["image"];
    if (image == null)
    {
        ViewBag.UploadMessage = "Failed to upload image";
    }
    else
    {
        ViewBag.UploadMessage = String.Format("Got image {0} of type {1} and size {2}",
            image.FileName, image.ContentType, image.ContentLength);
        // TODO: actually save the image to Azure blob storage
    }
    return View();
}

Step 6 – Add Azure References

Now we need to add a project reference to Microsoft.WindowsAzure.StorageClient, which gives us access to the Microsoft.WindowsAzure and Microsoft.WindowsAzure.StorageClient namespaces.

Step 7 – Connect to Cloud Storage Account

Most tutorials will tell you to connect to your storage account by simply passing in the name of the connection string: 

var storageAccount = CloudStorageAccount.FromConfigurationSetting("StorageConnection");

However, because we are using an Azure web site and not a Web Role, this throws an exception ("SetConfigurationSettingPublisher needs to be called before FromConfigurationSetting can be used"). There are a few ways to fix this, but I think the simplest is to call Parse and pass in your connection string from Web.Config directly: 

var storageAccount = CloudStorageAccount.Parse(
    ConfigurationManager.ConnectionStrings["StorageConnection"].ConnectionString);

Step 8 – Create a Container 

Our storage account can have many “containers”, so we need to provide a container name. For this example, I’ll call it “productimages” and give it public access.

blobStorage = storageAccount.CreateCloudBlobClient();
CloudBlobContainer container = blobStorage.GetContainerReference("productimages");
if (container.CreateIfNotExist())
{
    // configure container for public access
    var permissions = container.GetPermissions();
    permissions.PublicAccess = BlobContainerPublicAccessType.Container;
    container.SetPermissions(permissions);
}

The name you select for your container actually has to be a valid DSN name, or you’ll get a strange “One of the request inputs is out of range” error. This means no capital letters, so "ProductImages" won't work.

Note: the code I used as the basis for this part (the Introduction to Cloud Services lab from the Windows Azure Training Kit) holds CloudBlobClient as a static variable, and has the code to initialize the container in a lock. I don’t know if this is to avoid a race condition of trying to create the container twice, or if creating a CloudBlobClient is expensive and should only be done once if possible. Other accesses to CloudBlobClient are not done within the lock, so it appears to be thread-safe.

Step 9 – Save the Image to a BLOB

Finally we are ready to actually save our image. We need to give it a unique name, for which we will use a GUID, followed by the original extension, but you can use whatever naming strategy you like. Including the container name in the BLOB name here saves us an extra call to blobStorage.GetContainer. As well as naming it, we must set its ContentType (also available on our HttpPostedFileBase) and upload the data which HttpPostedFileBase makes available as a stream.

string uniqueBlobName = string.Format("productimages/image_{0}{1}", 
   Guid.NewGuid().ToString(), Path.GetExtension(image.FileName));
CloudBlockBlob blob = blobStorage.GetBlockBlobReference(uniqueBlobName);
blob.Properties.ContentType = image.ContentType;
blob.UploadFromStream(image.InputStream);

Note: One slightly confusing choice you must make is whether to create a block BLOB or a page BLOB. Page BLOBs seem to be targeted at BLOBs that you need random access read or write (maybe video files for example), which we don’t need for serving images, so block BLOB seems to be the best choice.

Step 10 – Finding the BLOB URI

Now our image is in BLOB storage, but where is it? We can find out after creating it, with a call to blob.Uri:

blob.Uri.ToString();

In our Azure storage emulator environment, this returns something like:

http://127.0.0.1:10000/devstoreaccount1/productimages/image_ab16e2d7-5cec-40c9-8683-e3b9650776b3.jpg

Step 11 – Querying the Container Contents

How can we keep track of what we have put into the container? From within Visual Studio, in the Server Explorer tool window, there should be a node for Windows Azure Storage, which lets you see what containers and BLOBs are on the emulator. You can also delete BLOBs from there if you don’t want to do it in code.  

The Azure portal has similar capabilities allowing you to manage your BLOB containers, view their contents, and delete BLOBs.

If you want to query all the BLOBs in your container from code, all you need is the following:

var imagesContainer = blobStorage.GetContainerReference("productimages");
var blobs = imagesContainer.ListBlobs();

Step 12 – Create the Real Connection String

So far we’ve done everything against the storage emulator. Now we need to actually connect to our Azure storage. For this we need a real connection string, which looks like this:

DefaultEndpointsProtocol=https;AccountName=YourAccountName;AccountKey=YourAccountKey

The account name is the one you entered in the first step, when you created your Azure storage account. The account key is available in the Azure portal by clicking the “Manage Keys” link at the bottom. If you are wondering why there are two keys, and which to use, it is simply so you can change your keys without downtime, so you can use either.

Note: most examples show DefaultEndpointsProtocol as https, which as far as I can tell, simply means that by default the URI it returns starts with https. This doesn’t stop you getting at the same image with HTTP. You can change this value in your connection string any time according to your preference.

Step 13 – Create a Release Web.config Transform

To make sure our live site is running against our Azure storage account, we’ll need to create a web.config transform as the Web Deploy wizard doesn’t seem to know about Azure storage accounts and so can’t offer to do this automatically like it can with SQL connection strings.

Here’s my transform in Web.Release.config: 

<connectionStrings>
  <add name="StorageConnection"
    connectionString="DefaultEndpointsProtocol=https;AccountName=YourAccountName;AccountKey=YourAccountKey"
    xdt:Transform="SetAttributes" xdt:Locator="Match(name)"/>
</connectionStrings>

Step 14 – Link Your Storage Account to Your Web Site

Finally, in the Azure portal, we need to ensure that our web site is allowed to access our storage account. Go to your websites, select “Links”, and add a link you your Storage Account, which will set up the necessary firewall permissions.

Now you're ready to deploy your site and use Azure BLOB storage with an Azure Web Site.