Checking if a blob exists in Azure Storage

Question

I've got a very simple question (I hope!) - I just want to find out if a blob (with a name I've defined) exists in a particular container. I'll be downloading it if it does exist, and if it doesn't then I'll do something else.

I've done some searching on the intertubes and apparently there used to be a function called DoesExist or something similar... but as with so many of the Azure APIs, this no longer seems to be there (or if it is, has a very cleverly disguised name).

Answer 1

The new API has the .Exists() function call. Just make sure that you use the GetBlockBlobReference, which doesn't perform the call to the server. It makes the function as easy as:

public static bool BlobExistsOnCloud(CloudBlobClient client, 
    string containerName, string key)
{
     return client.GetContainerReference(containerName)
                  .GetBlockBlobReference(key)
                  .Exists();  
}

Answer 2

Note: This answer is out of date now. Please see Richard's answer for an easy way to check for existence

No, you're not missing something simple... we did a good job of hiding this method in the new StorageClient library. :)

I just wrote a blog post to answer your question: http://blog.smarx.com/posts/testing-existence-of-a-windows-azure-blob.

The short answer is: use CloudBlob.FetchAttributes(), which does a HEAD request against the blob.

Answer 3

Seem lame that you need to catch an exception to test it the blob exists.

public static bool Exists(this CloudBlob blob)
{
    try
    {
        blob.FetchAttributes();
        return true;
    }
    catch (StorageClientException e)
    {
        if (e.ErrorCode == StorageErrorCode.ResourceNotFound)
        {
            return false;
        }
        else
        {
            throw;
        }
    }
}

Answer 4

If the blob is public you can, of course, just send an HTTP HEAD request -- from any of the zillions of languages/environments/platforms that know how do that -- and check the response.

The core Azure APIs are RESTful XML-based HTTP interfaces. The StorageClient library is one of many possible wrappers around them. Here's another that Sriram Krishnan did in Python:

http://www.sriramkrishnan.com/blog/2008/11/python-wrapper-for-windows-azure.html

It also shows how to authenticate at the HTTP level.

I've done a similar thing for myself in C#, because I prefer to see Azure through the lens of HTTP/REST rather than through the lens of the StorageClient library. For quite a while I hadn't even bothered to implement an ExistsBlob method. All my blobs were public, and it was trivial to do HTTP HEAD.

Answer 5

The new Windows Azure Storage Library already contains the Exist() method. It´s in the Microsoft.WindowsAzure.Storage.dll.

Available as NuGet Package
Created by: Microsoft
Id: WindowsAzure.Storage
Version: 2.0.5.1

See also msdn

Answer 6

If you don't like using the exception method then the basic c# version of what judell suggests is below. Beware though that you really ought to handle other possible responses too.

HttpWebRequest myReq = (HttpWebRequest)WebRequest.Create(url);
myReq.Method = "HEAD";
HttpWebResponse myResp = (HttpWebResponse)myReq.GetResponse();
if (myResp.StatusCode == HttpStatusCode.OK)
{
    return true;
}
else
{
    return false;
}

Answer 7

With the updated SDK, once you have the CloudBlobReference you can call Exists() on your reference.

See http://msdn.microsoft.com/en-us/library/microsoft.windowsazure.storage.blob.cloudblockblob.exists.aspx

Answer 8

This is the way I do it. Showing full code for those who need it.

        // Parse the connection string and return a reference to the storage account.
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(CloudConfigurationManager.GetSetting("AzureBlobConnectionString"));

        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();

        // Retrieve reference to a previously created container.
        CloudBlobContainer container = blobClient.GetContainerReference("ContainerName");

        // Retrieve reference to a blob named "test.csv"
        CloudBlockBlob blockBlob = container.GetBlockBlobReference("test.csv");

        if (blockBlob.Exists())
        {
          //Do your logic here.
        }

Answer 9

Although most answers here are technically correct, most code samples are making synchronous/blocking calls. Unless you're bound by a very old platform or code base, HTTP calls should always be done asynchonously, and the SDK fully supports it in this case. Just use ExistsAsync() instead of Exists().

bool exists = await client.GetContainerReference(containerName)
    .GetBlockBlobReference(key)
    .ExistsAsync();

Answer 10

If your blob is public and you need just metadata:

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "HEAD";
        string code = "";
        try
        {
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            code = response.StatusCode.ToString();
        }
        catch 
        {
        }

        return code; // if "OK" blob exists