What is the point of the from service when sending a message?

Question

I'm looking into SQL Server Service Broker and I'm trying to understand the point of the from service and the send queue. Many of the examples I've seen look like this.

-- Create Message Type
CREATE MESSAGE TYPE TestMessage
    VALIDATION = NONE
GO
-- Create Contract
CREATE CONTRACT TestContract
    (TestMessage SENT BY INITIATOR)
GO
-- Create Send Queue
CREATE QUEUE TestSendQueue
GO
-- Create Receive Queue
CREATE QUEUE TestReceiveQueue
GO
-- Create Send Service on Send Queue
CREATE SERVICE TestSendService
    ON QUEUE TestSendQueue (TestContract)
GO
-- Create Receive Service on Recieve Queue
CREATE SERVICE TestReceiveService
ON QUEUE TestReceiveQueue (TestContract)
GO

DECLARE @dialog uniqueidentifier
DECLARE @message NVARCHAR(128)
BEGIN DIALOG CONVERSATION @dialog
    FROM SERVICE TestSendService
    TO SERVICE 'TestReceiveService'
    ON CONTRACT TestContract
    WITH ENCRYPTION = OFF
-- Send messages on Dialog
SET @message = N'Very First Message';
SEND ON CONVERSATION @dialog
    MESSAGE TYPE TestMessage (@message)
GO
-- View messages from Receive Queue
SELECT CONVERT(NVARCHAR(MAX), message_body) AS Message
    FROM TestReceiveQueue
GO
-- Receive messages from Receive Queue
RECEIVE TOP(1) CONVERT(NVARCHAR(MAX), message_body) AS Message
    FROM TestReceiveQueue
GO

So basically in this example what is the point of the TestSendQueue and TestSendService when the message I send goes into the TestReceiveQueue and that's where I view and receive it from? Additionally why is the FROM SERVICE in the BEGIN DIALOG a string value while the TO SERVICE is not?

Answer 1

You need a FROM SERVICE because replies sent by 'TestReceiveService' need an addressee. Where would the application look for a response?

The FROM service is a symbol because it must be a local, existing, service. Its presence can be and will be validated during execuiton/compile.

The TO service is a string because is a remote name, not necessarily present locally. Its location is resolved via routing. It cannot be validated during execution/compile. Your example, when the TO service is local, is a degenerate example. To better understand some of the requirements/syntax and apparent anomalies, try to think at every example as a remote example (TO service is on a different machine).