Service Bus Brokered Messaging
Service Bus brokered messaging provides enterprise class
asynchronous messaging capabilities hosted in Windows Azure datacenters. Asynchronous
queue-based communication is not new to Windows Azure, the Windows Azure
storage services have provided queues alongside table and blob storage since
the release of Windows Azure. What is new is the broad range of capacities that
Service Bus brokered messaging provides.
The following concepts are used in Service Bus brokered
messaging.
Messages
All data communicated through the Service Bus brokered messaging
services is encapsulated in messages. Messages contain the data that is being
transmitted in the message body, and contextual information in a message
header.
Queues
Service bus queues provide a FIFO queuing model similar to
MSMQ and the queues in Windows Azure storage services. Messages are placed on a
queue by a sending application and received by a receiving application. Service
Bus queues are provide durability for messages, meaning that the messages are
stored in a durable storage system and are resilient to system failures. Service
Bus queue provide options for deadlettering messages and message duplicate
detection as well as sessions and correlation.
Topics
and Subscriptions
Topics and subscriptions provide a publish/subscribe model
for exchanging messages between applications. A topic acts as the enqueueing
end of a queue, whilst subscriptions act as the dequeueing end of a queue.
Subscriptions are created within a topic, and a topic can contain zero or more
subscriptions.
A sending application sends messages to a topic, these
messages are then routed to zero, one or more subscriptions based on a set of
rules. The receiving application can then receive messages from the
subscriptions.
Subscriptions can subscribe to all messages in a topic, or
use a filter to subscribe messages based on values in the message properties.
Subscriptions can also use actions to modify the message property collection.