EXTENDING SQL SERVER INTO THE PLATFORM messages must be sent to another SQL Server, but the messages can be sent to the same database, between databases in the same instance, or to a different instance on the same machine or different machines. The asynchronous messages work similarly to a message queuing system (they even use queues), but because SQL Server is controlling both the database data and the messages, messages can participate in a local database transaction. The Transact-SQL language is being extended to support the handling of queued messages that can be retrieved asynchronously. SQL Server Service Broker uses dialogs to achieve coordinated messaging with multimessage correlation semantics. A dialog is a bidirectional series of messages between two endpoints, in which each message contains a conversation ID to enable the other side to correlate messages, as shown in Figure 1-5. One of the main advantages of a dialog is that it maintains message order across transactions, threads, applications, and database restarts, something no other messaging system does. A dialog also allows guaranteed delivery of messages to a number of different subscribers. Multiple dialogs can be grouped into a single application known as a Conversation Group. A Conversation Group can maintain its state and share state between multiple dialogs. This goes beyond the concept of simple correlation ID message queuing systems, and it s handled automatically by Service Broker, rather than manually by the application programmer. Using asynchronous messaging inside the database enables building scalable systems, because work that had to be handled serially in the past can now be multiplexed in the same transaction. In addition, Service Broker guarantees that the messages will arrive in order, even across transactions, and guarantees once-only delivery. Messages Awaiting Processing by Endpoint Correlated by Conversation Endpoint Endpoint Endpoint Conversation 2 Conversation 1 Conversation 2 Conversation 1 Conversation 1 Conversation 2 Figure 1-5: Messages and Dialogs

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