Building Applications With Service Broker
Any program that can run Transact-SQL statements can use Service Broker. A Service Broker application can be implemented as a program running outside of SQL Server, or as a stored procedure written in Transact-SQL or a .NET language.
A program that uses Service Broker is typically composed of a number of components working together to accomplish a task. A program that initiates a conversation creates and sends a message to another service. That program may wait for a response, or exit immediately and rely on another program to process the response. For a service that is the target of a conversation, the program receives an incoming message from the queue for the service, reads the message data, does any necessary processing, and then creates and sends a response message if appropriate.
Service Broker extends Transact-SQL. An application does not need a special object model or library to work with Service Broker. Instead, programs send Transact-SQL commands to SQL Server and process the results of those commands. An application can be activated by Service Broker, can run as a background service, can run as a scheduled job, or can be started in response to an event. For more information on strategies for starting an application that uses Service Broker, see Choosing a Startup Strategy.
For information on creating applications with Service Broker, see Introduction to Service Broker Programming.
Service Broker Application Overview
The following illustration shows the interaction in an application that uses Service Broker:
As shown in the illustration, the SubmitExpense, AcceptDenyExpense, and ReimbursementIssued message types are created first. The ProcessExpenses contract is created based on these message types and provides a schema for having a conversation to complete an expense reimbursement task. The ProcessExpenses contract governs all conversations between the ProcessExpense service and the SubmitExpense service. The ProcessExpenses contract and the message types that it uses must exist in the databases of all services that have conversations based on this contract.
Service Broker stores messages sent to the SubmitExpense service on the queue for that service. The ExpenseSubmission stored procedure receives messages from this queue, processes them, and sends messages to another service if a reply is necessary.
Service Broker stores messages sent to the ProcessExpense service on the queue for that service. The ExpenseProcessing stored procedure receives messages from this queue, processes them, and sends messages to another service if a reply is necessary.
A conversation between these two services would be structured as follows:
- A user submits an expense reimbursement request through a user interface. The application runs the ExpenseSubmission stored procedure, which creates a SubmitExpense message. The SubmitExpense service starts a conversation with the ProcessExpense service, then sends the SubmitExpense message to the ProcessExpense service.
- Service Broker receives the SubmitExpense message for the ProcessExpense service and puts the message on the ExpenseQueue queue. The ExpenseQueue queue activates the ProcessExpense stored procedure, which dequeues and processes the SubmitExpense message. The ProcessExpense stored procedure then creates an AcceptDenyExpense message and sends this message to the SubmitExpense service. If the expense is denied, the ProcessExpense stored procedure ends the conversation.
- Service Broker puts the AcceptDenyExpense message for the SubmitExpense service on the queue for the service. If the ProcessExpense procedure ended the conversation, Service Broker puts an EndDialog message on the Expenses queue. The queue activates the ExpenseSubmission stored procedure, which dequeues and processes the AcceptDenyExpense message. If the ExpenseSubmission stored procedure finds an EndDialog message on the queue, the procedure ends the conversation.
- If the expense was accepted, the ProcessExpense service creates and sends a ReimbursementIssued message confirming that the expense payment has been issued, and then ends the conversation. Service Broker puts these messages on the queue for the service. The queue activates the ExpenseSubmission procedure, and the procedure processes the ReimbursementIssued message. The procedure then processes the EndDialog message and ends the conversation.