To instantiate a QueueService adapter, use the static method Zend_Cloud_QueueService_Factory::getAdapter(), which accepts either an array or a Zend_Config object. Three parameters apply to all adapters, while the remaining parameters are adapter-specific properties; these adapter-specific properties often contain access details.

The general parameters are as follows:

• queue_adapter specifies the concrete adapter class;

• message_class specifies the class to use to represent queue messages; defaults to Zend_Cloud_QueueService_Message; and

• messageset_class specifies the class to use to represent collections of queue messages; defaults to Zend_Cloud_QueueService_MesageSet.

Every queue service typically offers one or more queues. Each queue can store zero or more messages. A process can send a message to a queue, and another process can remove it. Usually processes remove the oldest message in the queue, observing a first in, first out (FIFO) queue-style interface.

If some error occurs inside the storage service, a Zend_Cloud_QueueService_Exception is thrown. If the exception was caused by underlying service driver, you can use the getClientException() method to retrieve the original exception.

Since different cloud providers implement different sets of services, some adapters do not implement certain features. In this case, the Zend_Cloud_OperationNotAvailableException exception is thrown.

The createQueue() method creates a message queue with the given name. It returns a queue identifier, the format of which is service-dependent. Some services return a URL for the queue identifier, while others return a GUID to use in future operations.

The deleteQueue() method removes the queue from the service. You must use the identifier received from createQueue() when calling deleteQueue().

Note: Deleting a queue can take significant time for some services. Typically, you cannot re-create a queue with the same name until the original queue is fully removed.

To retrieve the list of all queues in the system, use the listQueues() method.

In some services, you can associate a set of key-value pairs with the queue as queue metadata. To set queue metadata, use the storeQueueMetadata() method:

To retrieve queue metadata, use the fetchQueueMetadata() method.

To add a message to a queue, use the sendMessage() method. The message is passed as an unstructured string.

To receive one or more messages from the queue, use the receiveMessages() method. This method returns a Zend_Cloud_QueueService_MessageSet instance by default, unless configured otherwise. Each element of the MessageSet is an instance of Zend_Cloud_QueueService_Message by default, unless configuired otherwise.

When a message is received, it is not visible to other clients. It is not deleted from the queue, however, until the client that has received the message calls the deleteMessage() method. If it is not deleted during the specfied visibility timeout, it will become visible to all other clients again. In other words, all clients will be able to retrieve the message with the receiveMessages() method if the visibility timeout is exceeded.

In order to delete the message from the queue, use the deleteMessage() method. This method deletes the specified message.

Sometimes it is necessary to retrieve the concrete adapter for the service that the Queue API is working with. This can be achieved by using the getAdapter() method.

Note: Accessing the underlying adapter breaks portability among services, so it should be reserved for exceptional circumstances only.