Checking if a queue exists - azure

Question

I have a very basic question about Windows Azure Storage Queue errors/access.
I am trying to find out if the given storage account already contains a queue by the given name - say "queue1". I do not want to create the queue if it does not exist, and so am not keen on using the CreateIfNotExist method. The permissions I have given to the SAS token are - processing and Add (since all I want to do is to add a new message to the queue only if it already exists, and throw an error otherwise)
The problem is that when I try to get reference to a fake named queue, and add a message to it, I get a 403. 403 can also occur when the SAS token does not have permissions, so I cannot be sure what is causing the error.
Is there a way I could explicitly know if the queue exists or not?
I have tried the BeginExist, and EndExist methods but they always return false even when I can see the queue being there.
Any suggestions?

Answer 1

The Get Queue Metadata REST API operation will return status code 200 if the queue exists or a Queue Service Error Code otherwise.
Regarding to authorization,
This operation can be performed by the account owner and by anyone with a shared access signature that has permission to perform this operation.
A GET request to
https://myaccount.queue.core.windows.net/myqueue?comp=metadata
Will return a response like:
Response Status:
HTTP/1.1 200 OK
Response Headers:
Transfer-Encoding: chunked
x-ms-approximate-messages-count: 0
Date: Fri, 16 Sep 2011 01:27:38 GMT
Server: Windows-Azure-Queue/1.0 Microsoft-HTTPAPI/2.0

Answer 2

Are you sure you're getting a 403 error even if the queue does not exist. Based on what you described above, I created a simple console app. The queue does not exist in my storage account. When I try to add a message with valid SAS token, I get a 404 error:
CloudStorageAccount storageAccount = new CloudStorageAccount(new StorageCredentials("account", "key"), false);
CloudQueueClient client = storageAccount.CreateCloudQueueClient();
CloudQueue queue = client.GetQueueReference("non-existent-queue");
var queuePolicy = new SharedAccessQueuePolicy();
var sas = queue.GetSharedAccessSignature(new SharedAccessQueuePolicy()
{
SharedAccessExpiryTime = DateTime.UtcNow.AddMinutes(30),
Permissions = SharedAccessQueuePermissions.Add | SharedAccessQueuePermissions.ProcessMessages | SharedAccessQueuePermissions.Update
}, null);
StorageCredentials creds = new StorageCredentials(sas);
var queue1 = new CloudQueue(queue.Uri, creds);
try
{
queue1.AddMessage(new CloudQueueMessage("This is a test message"));
}
catch (StorageException excep)
{
//Get 404 error here
}
Next, I made the SAS token invalid by setting it's expiry to 30 minutes before current time. Now when I run the application, I get 403 error as expected.
CloudStorageAccount storageAccount = new CloudStorageAccount(new StorageCredentials("account", "key"), false);
CloudQueueClient client = storageAccount.CreateCloudQueueClient();
CloudQueue queue = client.GetQueueReference("non-existent-queue");
var queuePolicy = new SharedAccessQueuePolicy();
var sas = queue.GetSharedAccessSignature(new SharedAccessQueuePolicy()
{
SharedAccessExpiryTime = DateTime.UtcNow.AddMinutes(-30),//-30 to ensure SAS is invalid
Permissions = SharedAccessQueuePermissions.Add | SharedAccessQueuePermissions.ProcessMessages | SharedAccessQueuePermissions.Update
}, null);
StorageCredentials creds = new StorageCredentials(sas);
var queue1 = new CloudQueue(queue.Uri, creds);
try
{
queue1.AddMessage(new CloudQueueMessage("This is a test message"));
}
catch (StorageException excep)
{
//Get 403 error here
}

Answer 3

There is now an Exists and ExistsAsync (with various overloads).
Example of the former in use:
CloudStorageAccount storageAccount = CloudStorageAccount.Parse(connectionString);
CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();
CloudQueue queue = queueClient.GetQueueReference(queueName);
bool doesExist = queue.Exists();
You will want a reference to Microsoft.Azure.Storage.Queue (I believe older 'cloud' assemblies may not have had these properties - initially I could only access ExistsAsync before I had reference the right package, once I had added the above via Nuget Exists also was available)
For more details see the following links:
Exists
ExistsAsync

Answer 4

There is no Exists method in the v12 as well. Wrote a simple helper method to do the check:
private async Task<bool> QueueExistsAsync(QueueClient queue)
{
try
{
await queue.GetPropertiesAsync();
return true;
}
catch (RequestFailedException ex)
{
if (ex.Status == (int) HttpStatusCode.NotFound)
{
return false;
}
throw;
}
}