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Explain Cloud Queues in detail.

How to create and process queues and messages in azure cloud?

Marks:10

Year:May 2012, Dec 2014, Dec 2011, May 2015

Windows Azure Queues, which are part of the Windows Azure storage infrastructure, feature a simple REST-based Get/Put/Peek interface, providing reliable, persistent messaging within and between services.

Limitations of Windows Azure Queues:

• A single Windows Azure Queue can process up to 2,000 transactions per second. A transaction is either a Put, Get, or Delete operation. Sending a single message to a queue (Put) is counted as one transaction, but receiving a message is often a two-step process involving the retrieval (Get), followed by a request to remove the message from the queue (Delete). As a result, a successful dequeue operation usually involves two transactions.

• The Windows Azure Active Directory Access Control (also known as Access Control Service or ACS) supported by the Service Bus offers three distinct roles: Admin, Sender, and Receiver, which is not supported at this time for Windows Azure Queues.

• The latency of Windows Azure Queues is 10 milliseconds on average when handling small messages (less than 10 KB) from a hosted service located in the same location (region) as the storage account compared to Service Bus Queues which is 100 ms.

• Windows Azure Queues impose certain restrictions on queue names.

• Service Bus Queues support the At-Least-Once delivery guarantee. In addition, the At-Most-Once semantic can be supported by using session state to store the application state and by using transactions to atomically receive messages and update the session state. The Windows Azure Workflow Service uses this technique to guarantee At-Most-Once delivery.

Programming with Queue message operations:

• Create a queue

  A CloudQueueClient object lets you get reference objects for queues. The following code creates a CloudQueueClient object.

  // Retrieve storage account from connection string

  CloudStorageAccount storageAccount = CloudStorageAccount.Parse(

  CloudConfigurationManager.GetSetting("StorageConnectionString"));

  // Create the queue client

  CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();

  Use the queueClient object to get a reference to the queue you want to use. You can create the queue if it doesn't exist.

  // Retrieve a reference to a queue

  CloudQueue queue = queueClient.GetQueueReference("myqueue");

  // Create the queue if it doesn't already exist

  queue.CreateIfNotExists();

• Insert a message into a queue

  To insert a message into an existing queue, first create a new CloudQueueMessage. Next, call the AddMessage method.

  A CloudQueueMessage can be created from either a string (in UTF-8 format) or a byte array. Here is code which creates a queue (if it doesn't exist) and inserts the message 'Hello, World':

  // Retrieve storage account from connection string.

  CloudStorageAccount storageAccount = CloudStorageAccount.Parse(

  CloudConfigurationManager.GetSetting("StorageConnectionString"));

  // Create the queue client.

  CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();

  // Retrieve a reference to a queue.

  CloudQueue queue = queueClient.GetQueueReference("myqueue");

  // Create the queue if it doesn't already exist.

  queue.CreateIfNotExists();

  // Create a message and add it to the queue.

  CloudQueueMessage message = new CloudQueueMessage("Hello, World");

  queue.AddMessage(message);

• Peek at the next message

  You can peek at the message in the front of a queue without removing it from the queue by calling the PeekMessage method.

  // Retrieve storage account from connection string

  CloudStorageAccount storageAccount = CloudStorageAccount.Parse(

  CloudConfigurationManager.GetSetting("StorageConnectionString"));

  // Create the queue client

  CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();

  // Retrieve a reference to a queue

  CloudQueue queue = queueClient.GetQueueReference("myqueue");

  // Peek at the next message CloudQueueMessage peekedMessage = queue.PeekMessage();

  // Display message.

  Console.WriteLine(peekedMessage.AsString);

• Change the contents of a queued message

  You can change the contents of a message in-place in the queue. If the message represents a work task, you could use this feature to update the status of the work task. The following code updates the queue message with new contents, and sets the visibility timeout to extend another 60 seconds. This saves the state of work associated with the message, and gives the client another minute to continue working on the message.

  // Retrieve storage account from connection string.

  CloudStorageAccount storageAccount = CloudStorageAccount.Parse(

  CloudConfigurationManager.GetSetting("StorageConnectionString"));

  // Create the queue client.

  CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();

  // Retrieve a reference to a queue.

  CloudQueue queue = queueClient.GetQueueReference("myqueue");

  // Get the message from the queue and update the message contents.

  CloudQueueMessage message = queue.GetMessage();

  message.SetMessageContent("Updated contents.") ;

  queue.UpdateMessage(message,

  TimeSpan.FromSeconds(0.0), // Make it visible immediately.

  MessageUpdateFields.Content | MessageUpdateFields.Visibility);

• De-queue the next message

  Your code de-queues a message from a queue in two steps. When you call GetMessage, you get the next message in a queue. A message returned from GetMessage becomes invisible to any other code reading messages from this queue. By default, this message stays invisible for 30 seconds. To finish removing the message from the queue, you must also call DeleteMessage.

  // Retrieve storage account from connection string

  CloudStorageAccount storageAccount = CloudStorageAccount.Parse(

  CloudConfigurationManager.GetSetting("StorageConnectionString"));

  // Create the queue client

  CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();

  // Retrieve a reference to a queue

  CloudQueue queue = queueClient.GetQueueReference("myqueue");

  // Get the next message

  CloudQueueMessage retrievedMessage = queue.GetMessage();

  //Process the message in less than 30 seconds, and then delete the message

  queue.DeleteMessage(retrievedMessage);

  How to: Leverage additional options for de-queuing messages

  There are two ways you can customize message retrieval from a queue. First, you can get a batch of messages (up to 32). Second, you can set a longer or shorter invisibility timeout, allowing your code more or less time to fully process each message. The following code example uses the GetMessages method to get 20 messages in one call. Then it processes each message using a for each loop. It also sets the invisibility timeout to five minutes for each message.

// Retrieve storage account from connection string. CloudStorageAccount storageAccount = CloudStorageAccount.Parse( CloudConfigurationManager.GetSetting("StorageConnectionString"));

// Create the queue client.

CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();

// Retrieve a reference to a queue.

CloudQueue queue = queueClient.GetQueueReference("myqueue");

foreach (CloudQueueMessage message in queue.GetMessages(20, TimeSpan.FromMinutes(5)))

{

// Process all messages in less than 5 minutes, deleting each message after processing.

queue.DeleteMessage(message);

}

• Get the queue length

  You can get an estimate of the number of messages in a queue. The FetchAttributes method asks the Queue service to retrieve the queue attributes, including the message count. The ApproximateMethodCount property returns the last value retrieved by the FetchAttributes method, without calling the Queue service.

  // Retrieve storage account from connection string.

  CloudStorageAccount storageAccount = CloudStorageAccount.Parse(

  CloudConfigurationManager.GetSetting("StorageConnectionString"));

  // Create the queue client.

  CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();

  // Retrieve a reference to a queue.

CloudQueue queue = queueClient.GetQueueReference("myqueue");

// Fetch the queue attributes.

queue.FetchAttributes();

// Retrieve the cached approximate message count.

int? cachedMessageCount = queue.ApproximateMessageCount;

// Display number of messages.

Console.WriteLine("Number of messages in queue: " + cachedMessageCount);

• Delete a queue

  To delete a queue and all the messages contained in it, call the Delete method on the queue object.

  // Retrieve storage account from connection string.

  CloudStorageAccount storageAccount = CloudStorageAccount.Parse(

  CloudConfigurationManager.GetSetting("StorageConnectionString"));

  // Create the queue client.

  CloudQueueClient queueClient = storageAccount.CreateCloudQueueClient();

  // Retrieve a reference to a queue.

  CloudQueue queue = queueClient.GetQueueReference("myqueue");

  // Delete the queue.

  queue.Delete();