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A closer look at the BLOB storage service

3/2/2011 10:27:33 PM
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You have an idea how the BLOB storage service is hosted in Windows Azure. Let’s look at how files are stored in the service. In this section, we’ll look at the three layers of BLOB storage:
  • The account

  • The container

  • The BLOB

To help explain these concepts, we’ll use figure 1 as a reference. Figure 1 shows how an MP3 file might be stored in BLOB storage.

Figure 1. Podcast01.mp3 is stored in the ChrisOriginals container in the silverlightukstorage account

Before we get all technical about accounts, containers, and BLOBs, keep this in mind: an account is simply your account. Dave has an account, Jim has an account, and you have an account. An account is about ownership. A container is somewhere you can store your BLOBs. Containers are about access control (public or private access) and some level of organization.

With that in mind, let’s look at some of the specifics.

1. Accessing the BLOB (file)

In figure 1, you can see how files (otherwise known as BLOBs) are stored in BLOB storage. The BLOB Podcast01.wma resides in the container ChrisConverted, which resides in the storage account silverlightukstorage. A BLOB can’t directly reside in a storage account and must live in a storage container. If you do need to make the BLOB available as if it’s at the top level of the account (as if it doesn’t have a container), you can store the BLOB in the root container.

Because storage services use a REST-based architecture, you can retrieve a file from BLOB storage by performing an HTTP GET request to the URI for the BLOB. The following URI would let you retrieve Podcast01.wma from the ChrisConverted container (held in the silverlightukstorage storage account) from the live BLOB storage service: http://silverlightukstorage.blob.core.windows.net/ChrisConverted/Podcast01.wma.

We could formalize the URI for the live storage account as follows: http://<storageAccount>.blob.core.windows.net/<Container>/<BlobName>.

Let’s now take a closer look at accounts, containers, and BLOBs to get a clearer understanding of these components.

2. Setting up a storage account

When you sign up for Windows Azure, you can create a storage account in the Azure portal. The storage account is the top level for all storage services (BLOBs, queues, and tables) that reside under it.

When you create your storage account, you’ll be assigned a subdomain for each storage service. The following three domains are for the storage services:

  • http://<storageAccountName>.blob.core.windows.net/

  • http://<storageAccountName>.queue.core.windows.net/

  • http://<storageAccountName>.table.core.windows.net/

In our previous example, the name of the storage account was silverlightukstorage, which means that the top-level URI for each service in our account would be as follows:

How do you break up your storage accounts?

There are a couple of things to consider about your storage account, the major one being this: do you have one large account, or a separate account for each application? Although this is ultimately up to you, a good guide would be access control. If you’re a small shop, then one overall account is probably suitable; however, a single account wouldn’t work so well in, say, Microsoft or IBM. In these larger organizations, separating by application is probably a more suitable approach.

If you don’t like the beautiful subdomain assigned to you for BLOB storage (xxxxx.blob.core.windows.net) then you can always assign your own domain name.

3. Registering custom domain names

What we’ll do now is step through the process of associating your own domain name with the BLOB storage service. You’ll be able to access your WMA file using this URI: http://blobs.chrishayuk.com/ChrisConverted/Podcast01.wma.

To register a custom domain name with a BLOB storage account, you have to do the following:

  1. Register a suitable domain with your domain provider.

  2. Set up a domain to point at Windows Azure.

  3. Validate that you own the domain.

  4. Set up the subdomain to point at BLOB storage.

We’re going to skip the registering a suitable domain step. If you don’t know how to do that, then I’m sure GoDaddy (or some other provider) will happily provide some instructions so they can extract some lovely dollar bills (or British Pounds, or Euro Euros) from your pocket.

Set Up a Suitable Domain

After you’ve registered your domain (for example, chrishayuk.com), you need to let Windows Azure know that you want to point a suitable subdomain at it. To do that, log in to the Azure portal. Select your storage account (silverlightukstorage), and then click the Manage Domains button. You’ll be faced with the page shown in figure 2.

Figure 2. Validating in the Azure portal that you’re the owner of the domain that you want to point to the BLOB storage account

After you’ve entered the name of the domain (including the subdomain) that you want to point to the BLOB storage account, you need to validate the domain.

Validating that you Own the Domain

Validate the domain by clicking the Generate Key button. After you click the button, you’ll be presented with the screen shown in figure 3.

Figure 3. Receiving the domain validation CNAME GUID

The window in figure 3 indicates that you need to perform two actions:

  • Add a new CNAME for the GUID (fb160. . .) that points to verify.windowsazure.com.

  • Add a new CNAME for the subdomain (blobs.chrishayuk.com) that points to your BLOB storage account (silverlightukstorage.blob.core.windows.net).

Whichever company you used to register your domain probably manages the DNS for your domain name. Using their web control panel, you should be able to create the subdomain using a CNAME. Figure 4 shows the CNAMEs for chrishayuk.com in the GoDaddy Domain Manager.

Figure 4. The CNAME entries for chrishayuk.com; notice that both the domain verification CNAME and the BLOB storage CNAME are listed

If you manage your own DNS server, you already know how to set up a CNAME; if not, your system administrator will certainly be able to (although he might not be very pleased that you’re looking to replace him with an automated system).

After you’ve set up your CNAMEs, return to the Windows Azure portal a little later to validate the domain (click the Validate button shown in figure 3). As soon as the domain has been validated, you’ll be able to use your custom domain name.

Why do you need to come back later? Funnily enough, this is all to do with replication. After you’ve updated the DNS details on the server that’s responsible for maintaining your domain records, this update needs to be replicated to all the other DNS servers in the world. This replication delay is the reason that you’ll have to come back later (usually 10 minutes to an hour); it’ll take a little time for the Windows Azure DNS servers to receive that update. Perhaps the world’s DNS servers should use Windows Azure instead.

OK, you’ve got your custom domains set up and you understand containers; let’s look at how you can use them to store BLOBs.

4. Using containers to store BLOBs

In BLOB storage, you can’t store BLOBs directly in a storage account because every BLOB must live in a container. A container is really a top-level folder. Although you can set permissions directly on a BLOB, this can be a pain with a large number of BLOBs. To alleviate that administrative headache, you might want to group similar BLOBs that have similar access levels in the same container. Then you can set permissions at the container level rather than at the individual BLOB level.

In BLOB storage, there are two levels of access that you can set on a container: private and public.

Private Containers

BLOBs in a private container are restricted to the owner of the account. If you need to list the contents or download a BLOB stored in a private container, you need to make a request signed with your shared authentication key .

In figure 1, the container ChrisOriginals is a private container. If you wanted to access the BLOB podcast01.mp3, you would make a GET request to the following URI (this request must be signed with either your account master key or a pregenerated shared key; we’ll explain this later): http://silverlightukstorage.blob.core.windows.net/ChrisOriginals/Podcast01.mp3.

Full Public Read Access and Public Read-Only Access For BLOBS

If the container is set to full public read access, then you can retrieve any BLOB held in the container over HTTP without providing authentication credentials. Not only that, you can list all the BLOBs in that container and query data about the container.

With public read-only access for BLOBs, anonymous requests will only be able to read a BLOB (you won’t be able to read container data or list the BLOBs in the container).

In figure 4, the container ChrisConverted is a public container; anyone on the internet would be able to download the file podcast01.wma by making an HTTP GEThttp://silverlightukstorage.blob.core.windows.net/ChrisConverted/Podcast01.wma. request to

If you need to perform any operations beyond the container permission level (for example, if you need to upload or modify a BLOB), you need to provide authentication credentials (account owner or shared access) because these operations are restricted operations.

So far we’ve talked only about the live BLOB storage service. Now we’ll take some time to look at how you can develop against the BLOB storage service by using a development version of the BLOB service that’s in the development storage service.

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