Lesson 4: Beginning a Conversation and Transmitting Messages

New: 15 September 2007

In this lesson, you will learn to start a conversation that spans two databases in the same instance of the Database Engine. You will also learn how to complete a simple request-reply message cycle, and then end the conversation.

Switch to the InitiatorDB database
  1. Copy and paste the following code into a Query Editor window. Then, run it to switch context to the InitiatorDB database where you will initiate the conversation.

    USE InitiatorDB;
    GO
    
Start a conversation and send a request message
  1. Copy and paste the following code into a Query Editor window. Then, run it to start a conversation and send a request message to the //TgtDB/2DBSample/TargetService in the TargetDB. The code must be run in one block because a variable is used to pass a dialog handle from BEGIN DIALOG to the SEND statement. The batch runs the BEGIN DIALOG statement to begin the conversation and build a request message. Then, it uses the dialog handle in a SEND statement to send the request message on that conversation. The last SELECT statement displays the text of the message that was sent.

    DECLARE @InitDlgHandle UNIQUEIDENTIFIER;
    DECLARE @RequestMsg NVARCHAR(100);
    
    BEGIN TRANSACTION;
    
    BEGIN DIALOG @InitDlgHandle
         FROM SERVICE [//InitDB/2DBSample/InitiatorService]
         TO SERVICE N'//TgtDB/2DBSample/TargetService'
         ON CONTRACT [//BothDB/2DBSample/SimpleContract]
         WITH
             ENCRYPTION = OFF;
    
    SELECT @RequestMsg =
       N'<RequestMsg>Message for Target service.</RequestMsg>';
    
    SEND ON CONVERSATION @InitDlgHandle
         MESSAGE TYPE [//BothDB/2DBSample/RequestMessage]
          (@RequestMsg);
    
    SELECT @RequestMsg AS SentRequestMsg;
    
    COMMIT TRANSACTION;
    GO
    
Switch to the TargetDB database
  1. Copy and paste the following code into a Query Editor window. Then, run it to switch context to the TargetDB database where you will receive the request message and send a reply message back to the InitiatorDB.

    USE TargetDB;
    GO
    
Receive the request and send a reply
  1. Copy and paste the following code into a Query Editor window. Then, run it to receive the reply message from the TargetQueue2DB and send a reply message back to the initiator. The RECEIVE statement retrieves the request message. Then, the following SELECT statement displays the text so that you can verify that it is the same message that was sent in the previous step. The IF statement tests whether the received message is a request message type, and if a SEND statement is used to send a reply message back to the initiator. It also tests whether the END CONVERSATION statement is used to end the target side of the conversation. The final SELECT statement displays the text of the reply message.

    DECLARE @RecvReqDlgHandle UNIQUEIDENTIFIER;
    DECLARE @RecvReqMsg NVARCHAR(100);
    DECLARE @RecvReqMsgName sysname;
    
    BEGIN TRANSACTION;
    
    RECEIVE TOP(1)
        @RecvReqDlgHandle = conversation_handle,
        @RecvReqMsg = message_body,
        @RecvReqMsgName = message_type_name
    FROM TargetQueue2DB;
    
    SELECT @RecvReqMsg AS ReceivedRequestMsg;
    
    IF @RecvReqMsgName =
       N'//BothDB/2DBSample/RequestMessage'
    BEGIN
         DECLARE @ReplyMsg NVARCHAR(100);
         SELECT @ReplyMsg =
            N'<ReplyMsg>Message for Initiator service.</ReplyMsg>';
     
         SEND ON CONVERSATION @RecvReqDlgHandle
              MESSAGE TYPE
                [//BothDB/2DBSample/ReplyMessage] (@ReplyMsg);
    
         END CONVERSATION @RecvReqDlgHandle;
    END
    
    SELECT @ReplyMsg AS SentReplyMsg;
    
    COMMIT TRANSACTION;
    GO
    
Switch to the InitiatorDB database
  1. Copy and paste the following code into a Query Editor window. Then, run it to switch context back to the InitiatorDB database where you will receive the reply message and end the conversation.

    USE InitiatorDB;
    GO
    
Receive the reply and end the conversation
  1. Copy and paste the following code into a Query Editor window. Then, run it to receive the reply message and end the conversation. The RECEIVE statement retrieves the reply message from the InitiatorQueue2DB. The END CONVERSATION statement ends the initiator side of the conversation. The last SELECT statement displays the text of the reply message so that you can confirm it is the same as what was sent in the previous step.

    DECLARE @RecvReplyMsg NVARCHAR(100);
    DECLARE @RecvReplyDlgHandle UNIQUEIDENTIFIER;
    
    BEGIN TRANSACTION;
    
    RECEIVE TOP(1)
        @RecvReplyDlgHandle = conversation_handle,
        @RecvReplyMsg = message_body
        FROM InitiatorQueue2DB;
    
    END CONVERSATION @RecvReplyDlgHandle;
    
    -- Display recieved request.
    SELECT @RecvReplyMsg AS ReceivedReplyMsg;
    
    COMMIT TRANSACTION;
    GO
    

This concludes the tutorial. Tutorials are brief overviews and do not describe all available options. Tutorials have simplified logic and error handling to better focus on fundamental operations. To create efficient, reliable, and robust conversations, you need more complex code than the example in this tutorial.

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